Improving potential strategies for biological activities of phlorotannins derived from seaweeds

Abstract

Seaweeds have garnered considerable attention due to their capacity to serve as exceptional reservoirs of numerous bioactive metabolites possessing substantial chemical and biological significance. .Phlorotannins constitute a significant class of natural polyphenols originating from brown seaweeds, featuring a broad spectrum of bioactive attributes and demonstrating potential applicability across various sectors. The potential health advantages associated with phlorotannins, particularly concerning the prevention of conditions linked to oxidative stress, such as inflammation, diabetes, and allergies, have generated substantial interest within the food and pharmaceutical industries. Nevertheless, current research remains insufficient in providing a comprehensive understanding of their absorption, as comparisons drawn with their terrestrial counterparts remain speculative. It is commonly presumed that phenolic compounds, including phlorotannins, face challenges due to their limited solubility, instability, and extensive metabolism, all of which restrict their bioavailability. In order to circumvent these limitations and amplify their utility as components of medicinal formulations or healthcare products, researchers have explored various strategies, including the encapsulation or integration of phlorotannins into nano-/micro-particles or advanced drug delivery systems. This review offers a thorough exploration of the structural and biological attributes of phlorotannins and furnishes insights into potential strategies showing promise for their effective utilization in preclinical and clinical applications.

Keywords:

• Bioactivities
• bioavailability
• encapsulation
• phlorotannins
• surface modification

Introduction

The marine environment, widely recognized as a vast and unique repository, houses a multitude of organisms, including macroalgae (seaweeds), microalgae and other diverse life forms such as tunicates, sponges, soft corals, and mollusks (Victor and Sharma 2015). These organisms play a vital role within the aquatic ecosystem by providing nourishment and shelter to fish and other marine life. Furthermore, they hold significant promise as resources for the exploration of novel bioactive compounds with potential utility for human health (Ghosh et al. 2022). These bioactive compounds derived from natural sources are highly prized for their favorable characteristics, including lower toxicity and enhanced effectiveness, making them valuable for a range of therapeutic purposes (Aung et al. 2017). In contrast to terrestrial environments, the distinctive environmental factors in the marine system, such as fluctuating temperature, pressure, and light conditions, likely contribute to the production of unique metabolites (including proteins, peptides, amino acids, fatty acids, sterols, polysaccharides, oligosaccharides, phenolic compounds, photosynthetic pigments, vitamins, and minerals) by certain marine organisms (Karthikeyan, Joseph, and Nair 2022).

Phlorotannins, a distinct class of secondary metabolites belonging to polyphenolic compounds, are particularly derived from brown seaweed (Phaeophyta), primarily from the orders Laminariales (Laminariaceae) and Fucales (Fucaceae and Sargassaceae) (Nazzaro et al. 2019). Although some reviews have suggested the presence of trace amounts of phlorotannins in red algae (Pradhan et al. 2022), these claims lack support from published research articles or specific algal species. Phlorotannins are oligomers or polymers composed of the monomeric unit “phloroglucinol” (1,3,5-trihydroxybenzene) and exhibit structural dissimilarity compared to terrestrial plant polyphenols like phenolic acids or flavonoids (Shrestha, Zhang, and Smid 2021). Due to variations in size, degree of polymerization, and types of subunits, phlorotannins display structural diversity with molecular weights ranging from 126 Da to 650 kDa (Catarino et al. 2022). Moreover, different extraction and analytical techniques used in research may yield different phlorotannin profiles, adding to the complexity of identifying and quantifying specific compounds. To date, approximately 150 phlorotannins have been reported from brown seaweeds, with research into these compounds continuing to grow exponentially (Rajan et al. 2021).

Reported biological properties of phlorotannins encompass antioxidant (Kord et al. 2021), anti-inflammatory (Raja et al. 2023), and anti-diabetic (Moheimanian et al. 2023) potentials. Nevertheless, despite the increasing research interest in the therapeutic applications of phlorotannins, their limited bioavailability in the digestive tract and circulatory system remains an area requiring improvements (Corona et al. 2016). Bioavailability is a critical concept within the realm of bio-efficacy, serving as a fundamental indicator for assessing the efficacy of ingested nutrients. It pertains to the degree to which absorbed nutrient components successfully enter the systemic circulation, thereby enabling the accomplishment of highly efficient biological activities (Shahidi and Peng 2018). Like other polyphenols, phlorotannins are commonly believed to possess low bioaccessibility, ranging from approximately 2%–15% (Zhao et al. 2023). These compounds readily form crosslinks with other macromolecules, such as carbohydrates or proteins. Additionally, high molecular weight phlorotannins (up to 100 kDa) have demonstrated a propensity to interact with dietary and endogenous proteins, resulting in poor absorption in the upper gastrointestinal tract (Corona et al. 2016). Furthermore, phlorotannins are highly unstable in the acid-base environment encountered during the transition from the small intestine to the gut, undergoing rapid transformation and/or metabolism by digestive enzymes (Tong, Liu, and Yu 2021).

Given these properties, further research is warranted to explore strategies that maximize the biological activity of phlorotannins and their incorporation into clinical treatments. In recent years, encapsulation techniques, which enclose targeted materials, have gained widespread application for delivery into the system. Various carriers have been utilized for encapsulation, including microspheres, hydrogels, and nanoparticle-based drug carriers such as liposomes, niosomes, polymeric nanoparticles, and solid lipid nanoparticles (Mirchandani, Patravale, and Brijesh 2021). These carriers have demonstrated remarkable capabilities in enhancing nutrient/drug entrapment, improving bioavailability, and optimizing the pharmacokinetic profiles. However, to the best of our knowledge, there is currently no comprehensive insight into the specific digestive process of phlorotannins or the strategies to improve their bioavailability. Therefore, this review aims to shed light on the possibilities of improving the delivery and therapeutic potential of phlorotannins by exploring various strategies and considering plausible perspectives. Additionally, it offers promising opportunities for maximizing their bioavailability and harnessing their beneficial properties for targeted and controlled drug release.

Overview of seaweeds and their global utilization

Importance of seaweeds

Seaweeds, also known as marine macroalgae, constitute a foundational botanical category within marine ecology, comprising an estimated ∼24,000 species, of which ∼12,000 have been described (Guiry 2012). Anchoring themselves to shallow-water rocky substrates or aggregating in shallow marine waters, seaweeds are taxonomically categorized into three groups based on pigmentation: Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae (Xie et al. 2023). Seaweeds are renowned for their ecological significance, giving rise to benthic seascape formations that, in terms of ecological importance, can be likened to terrestrial forests, such as primarily production construction, contribution to nitrogen fixation, and cementation of reef framework, facilitation of coral settlement and creation of habitats for other reef species (El-Manaway and Rashedy 2022). Within submerged ecosystems, numerous herbivorous organisms sustain themselves by relying on seaweeds, thus facilitating energy transfer to higher trophic levels. The central role played by seaweeds in the marine food web is highlighted by their ability to capture solar energy for autotrophic nutrition, simultaneously releasing oxygen through photosynthesis. This pivotal role is exemplified by their contribution to approximately half of the total primary production, despite their representation comprising less than 1% of Earth’s total photosynthetic biomass (Falkowski 2012). Beyond this, seaweeds fulfill a critical ecological function by serving as agents of nutrient and toxin regulation, actively sequestering substances deleterious to marine organisms. In this capacity, they function as a natural filtration system within the vast underwater environment, entrapping heavy metals and chemical agents (Znad, Awual, and Martini 2022).

Undoubtedly, seaweeds possess an inherent appeal due to their culinary and edible attributes. However, while over 200 species of seaweeds are globally cultivated or wild-harvested for various industries, only 21 of these have received official authorization for use as vegetables and condiments (Pereira 2016). Upon closer examination, seaweeds reveal a wealth of functional ingredients and bioactive compounds that can be harnessed for the development of food supplements and dietary health products. From a nutritional perspective, seaweeds excel in their capacity to accumulate saccharides, proteins, phenolic compounds, unsaturated fatty acids, vitamins, and minerals. Specifically, saccharides are the most prevalent constituents within seaweed species, constituting a range of 4%–76% of their dry mass (DW) (Shannon and Abu-Ghannam 2019). Primary polysaccharides found in brown algae include alginate, fucoidan, and laminarin, while red algae contain carrageenans and agarans, and green algae contain ulvans (Patel et al. 2023). Although protein, polyphenol, and lipid contents are generally modest across all species, they play indispensable roles, with proteins aiding in the synthesis of hormones, enzymes, and hemoglobin, lipids mitigating the risk of several chronic diseases, and polyphenols acting as potent antioxidants (Subbiah et al. 2023). Notably, the nutritional profiles of seaweeds exhibit multifaceted variability influenced by factors such as species, age, size, reproductive status, ecological context, and a constellation of environmental parameters, including seawater pH, depth, temperature, salinity, nutrient enrichment, oxygen levels, ultraviolet radiation exposure, light intensity, and the extent of herbivore activity (Lobine, Rengasamy, and Mahomoodally 2022; E. Park et al. 2023).

Global market of seaweeds

The global cultivation and production of seaweed have undergone significant growth in recent decades, witnessing a remarkable increase from 0.56 million tons in 1950 to 35.82 million tons in 2019 (Junning and Giulia 2021). According to the estimations by Food and Agriculture Organization (FAO), the global seaweed industry was valued at over USD $6 billion annually in 2018 (FAO 2018). This value is projected to continue its upward trajectory and reach USD $30.2 billion by 2025 (Deepika et al. 2022).

As documented by FAO (2022), Asia overwhelmingly dominates global seaweed production, accounting for ∼99.54% of the total output. In contrast, Africa contributes 0.3%, America 0.07%, Europe 0.06% and Oceania 0.03% to a much lesser extent. Within Asian seaweed production, China stands as the foremost contributor, yielding over 20 million tons annually, constituting ∼59.46% of the global aggregate (FAO 2022). Notable contributors following China include Indonesia, South Korea, the Philippines, North Korea, and Japan. Japan, in particular, with its archipelagic geography, has a longstanding culinary tradition rooted in the utilization of various seaweed species for sustenance. The harvesting of Pyropia sp. from wild populations in Japan can be traced back to the seventeenth century. Additionally, additional seaweed species such as Undaria, Saccharina, Monostroma, Ulva, and Cladosiphon have also undergone significant cultivation and development (Tanaka, Ohno, and Largo 2020). Likewise, Australia, benefiting from its geographical location amidst the convergence of three distinct oceanic currents, showcases a rich tapestry of algal diversity, although it represents only 0.03% of the world market. For example, native seaweeds of the Chlorophyta phylum thrive in the pristine ecosystems of the Great Barrier Reef (Deepika et al. 2022).

In the broader context of global seaweed production, brown seaweeds hold a significant share, accounting for ∼62.37% in the year 2019. Within this category, noteworthy brown seaweed species such as Laminaria, Saccharina, and Lessonia emerge as preeminent contributors, collectively constituting ∼57.06% of the total brown seaweed production worldwide. Additional species, including Undaria, Sargassum, Macrocystis, and various miscellaneous varieties, make more modest contributions to this sector (Junning and Giulia 2021).

Utilizations of seaweeds

Seaweeds possess a rich history of utilization, extending into both culinary and medicinal domains. They have been integral to human diets, with particularly notable prominence in the Asian region, and have also been highly regarded for their ability to yield valuable compounds (Chopin and Tacon 2021). The global seaweed industry comprises 85% of food products for human consumption, and seaweed-derived extracts make up almost 40% of the world’s hydrocolloids market (Debbarma et al. 2022). For instance, agars and carrageenans extracted from red seaweeds, as well as alginates extracted from brown seaweeds, have found applications as thickening and gelling agents in various food products such as ice creams, jellies and shakes (Deepika et al. 2022). Furthermore, due to their macro- and micro-nutritional constituents, seaweeds are seen as a promising strategy to address the growing challenges associated with food supply stress resulting from the increasing global population (Duarte, Bruhn, and Krause-Jensen 2021). In addition to their culinary uses, some commercially significant seaweed species are also employed in the production of fertilizers, feed additives, biofuels, and various environmental applications, although to a lesser extent (Hessami, Rao, and Ravishankar 2019). For example, certain quaternary cations, such as NH₄⁺ found in seaweed extracts from Laminaria sp., provide buffering capacity for the roots while acting as osmoprotectants, which are particularly beneficial in agriculture, especially in marshy lands (Blanz et al. 2019).

Brown seaweed-derived phlorotannins

Generalized structural diversity

Phlorotannins are found in various brown seaweed species, including but not limited to Ecklonia, Eisenia, and Ishige species (Erpel et al. 2020). They are known for their chain- and/or net-like structures with diverse molecular weights. Phlorotannins can be further classified into six subclasses based on their linkage polymorphism: phlorethols, fuhalols, fucols, fucophloroethols, eckols, and carmalols (as shown in Figure 1). These subclasses are characterized by various types of covalent bonds, including:

Figure 1. The example structures of phlorotannins, divided by their linkages and functional groups.

• Aryl-ether bonds and ether bridges (para- or ortho-arranged).

• Aryl-aryl bonds.

• Phenyl bonds.

• Dibenzo-1,4-dioxin bonds (Charoensiddhi et al. 2020; Kumar et al. 2022).

The presence of additional hydroxyl groups (–OH) accounts for the differences between fuhalols and phlorethols (Y. Li et al. 2017), as well as between eckols and carmalols (Catarino, Amarante, et al. 2021).

In addition to nonhalogenated phlorotannins, halogenated phlorotannins have also been reported. For instance, monobromo-/monoiodophloroglucinol and 4′-bromo/4′-idodoeckol were identified several decades ago by Glombitza and Gerstberger (1985) from the brown alga Eisenia arborea. Subsequently, four other halogenated phlorotannins were discovered and reported in 1999 from another species, Carpophyllum angustifolium, after per-acetylation. These include 2-chlorophloroglucinol triacetate, 2[D’] iododiphlorethol pentaacetate, 3[A] chlorobifuhalol hexaacetate, and 3[A4] chlorodifucol hexaacetate (Glombitza and Schmidt 1999). In the same year, Sailler and Glombitza (1999) isolated and characterized no fewer than 17 halogenated (brominated, chlorinated and iodinated) phlorotannins, along with 30 non-halogenated forms from the Sargassaceae Cystophora retroflexa.

Biosynthesis of phlorotannins in brown algae

The biosynthesis of phlorotannins involves multiple steps, and although consensus on the exact mechanisms varies among scientists, a general outline of the biosynthesis process can be summarized as follows: (A) phloroglucinol formulation, (B) oxidative coupling and enzymatic modification, and (C) subcellular localization. For a more detailed understanding of the potential biosynthetic pathway, refer to Figure 2.

Figure 2. Potential biosynthesis pathway of phlorotannin in brown seaweed cells (Gómez and Huovinen 2020; Phang et al. 2023). PKS: polyketide synthase; CoA: coenzyme A.

The biosynthesis of phlorotannins, which are considered the fundamental building blocks of this class of compounds, is thought to commence with the production of phloroglucinol. Three potential pathways for phloroglucinol production have been proposed, namely the shikimic acid pathway, the phenylpropanoid pathway, and/or a polyketide synthase pathway (PKS), based on existing knowledge (Pelletreau 2008). However, Siriwardhana, Lee, and Jeon (2005) have suggested that the shikimic acid pathway is more likely to serve as the metabolic pathway for condensed tannins and hydrolyzable tannins within the polyphenol category, with limited evidence specifically addressing phlorotannins. Conversely, due to the involvement of type III polyketide synthase (PKS III), the PKS pathway has gained stronger support as a more plausible and widely accepted route for phlorotannin biosynthesis, as corroborated by research studies (Bertoni 2013; Subbiah et al. 2023).

This hypothesis received support from Meslet-Cladière et al. (2013), whose genome investigation of the brown alga Ectocarpus siliculosus led to the discovery of a predicted key enzyme, PKS1, capable of facilitating the consecutive condensation of acetate and malonate units. In the first step, two molecules of acetyl coenzyme A (acetyl-CoA) are converted into malonyl-CoA in the presence of carbon dioxide, catalyzed by PKS III. The resulting polyketide chain undergoes Claisen-type cyclization, generating a hexacyclic triketide. Subsequently, tautomerization and aromatic stabilization processes occur, resulting in the production of thermodynamically stable phloroglucinols (Fernando, Lee, and Ahn 2022).

Once phloroglucinol is synthesized, it undergoes oxidative coupling reactions facilitated by peroxidases, notably vanadium-dependent haloperoxidases, or phenoloxidases, as well as heme-containing glycoproteins. These enzymes play a catalytic role in the formation of end-radicals within phloroglucinol oligomers, achieved through the dehydrogenation of hydrogen atoms on –OH bonds, utilizing hydrogen peroxide (H₂O₂) as a reactant (Berglin et al. 2004; Lemesheva et al. 2020). The coupled C–C and/or C–O–C linkages contribute to the structural diversity observed among the six classes of phlorotannins mentioned above.

Regarding the localization of phlorotannin biosynthesis, it has been proposed that they are predominantly found within the chloroplast or its membrane, with potential involvement of the endoplasmic reticulum and Golgi bodies (Schoenwaelder and Clayton 2000). In general, based on their interactions with algal cells, phlorotannins exist in two distinct sub-cellular fractions: intracellular (cytoplasm) and cell wall-binding fractions (Birkemeyer et al. 2020). Polar and/or reactive-formed phlorotannins are initially sequestered in physodes, specialized secretion vesicles, which have the ability to be secreted into the extracellular compartment to form insoluble cell-wall phlorotannins that become embedded in the alginate matrix and other components of the cell wall through oxidative cross-linking (Arnold and Targett 2003; Emeline et al. 2021). The second fraction of phlorotannins is predominantly associated with carbohydrates or proteins, such as alginates, and they exhibit characteristics akin to conventional tannins, contributing to the integrity of the cell wall (Birkemeyer et al. 2020).

Quantification of phlorotannins

Quantifying phlorotannins holds significant importance across various research and industrial purposes. Researchers, for instance, employ phlorotannin quantification to enhance their understanding of the ecological roles played by brown seaweeds in marine ecosystems or to assess potential applications in the food, pharmaceutical, or cosmetic industries, along with determining the nutritional values of seaweed-based products. Common methodologies utilized for the quantification of phlorotannins encompass spectrophotometry assays (e.g., Folin-Ciocalteu assay), high-performance liquid chromatography (HPLC), and nuclear magnetic resonance (NMR).

Two spectrophotometry assays have been commonly employed to quantitatively assess the phlorotannin contents in seaweeds, including the Folin-Ciocalteu assay and the 2,4-dimethoxybenzaldehyde (DMBA) assay, which have garnered recognition due to their applicability, stemming from their distinctive interactions with the target compounds (Erpel et al. 2023; Gümüş Yılmaz et al. 2019). However, reports indicate that the Folin-Ciocalteu method has limited sensitivity. There are also concerns about its accuracy due to potential interference from other constituents in the plant matrix. Soluble sugars and proteins, for instance, can react with the Folin-Ciocalteu reagents, leading to artificially elevated values (Castro-Alves and Cordenunsi 2015). Similarly, although the DMBA assay demonstrates greater specificity as it selectively reacts with 1,3- and 1,3,5-trihydroxybenzenes, making it a more discerning option for assessing the phlorotannin structures within seaweed, some phlorotannins may exhibit branching or feature aryl linkages and fuhalols with additional –OH groups at positions 2, 4, or 6. In cases where substitutions occur at these positions, the absence of a color change may impede the detection of these particular phlorotannins (Ford et al. 2019).

The Prussian blue assay (PBA), in addition, has seen limited application in a few studies and may exhibit somewhat lower specificity compared to the other two methods (Margraf et al. 2015; Subbiah et al. 2023). Given that phlorotannins are classified as tannins, the butanol-HCl-iron method may also represent a potential means of quantifying their content, although there is currently no confirmed research validating this approach.

Proton (¹H) NMR spectroscopy indeed stands as another valuable approach for quantifying phlorotannin contents when utilizing phloroglucinol as an internal standard. However, it is essential to emphasize that NMR directly measures molar concentrations, and as such, the results obtained through NMR cannot be directly compared with those determined using spectrophotometric assays. Moreover, the quantification of phlorotannins using HPLC is impeded by the scarcity of commercially standardized reference compounds. In other words, the calibration of the sample content can only be achieved through quantification using known standards (Ford et al. 2019). Table 1 presents the phlorotannin contents as reported in various scholarly sources through the aforementioned methodologies.

Table 1. Phlorotannin content among different seaweed species by different determination methods.

Seaweed species	Collection spots	Collection time	Extraction method	Contents (mg PGE/g DW)	References
Spectrophotometry assays
Folin–Ciocalteu assay
Fucus vesiculosus L.	Oskar Bay, Zelenetskaya Gulf, Barents Sea, Russia	August 2017	Extracted by NADES, dissolved (50–70%) in H2O and solvents of various polarities that were used to extract the phenolic compounds (40 and 96% ethanol, Me2CO, 70% aqueous Me2CO) for 2 h at 50 °C	8.7–61.2	(Obluchinskaya et al. 2019)
Ascophyllum nodosum (L.) Le Jolis				7.8–50.3
F. vesiculosus	Donegal, Ireland	(Not Specified)	30%, 50%, 70% ethanol with a sample-to-solvent ratio of 1:10 (w/v) for 4 h at 20 °C with pretreated by 0, 35 and 130 kHz ultrasonic for 10 and 30 min	382.1–568.9	(Ummat et al. 2020)
F. vesiculosus	Coastal zone of the Artic: Irminger Sea, Norwegian Sea, Barents Sea, and White Sea	Fertile and sterile phases in 2019	60% methanol with a sample-to-solvent ratio of 1:25 (w/v) for 24 h at 200 rpm	72.4–158.1	(Obluchinskaya et al. 2022)
F. vesiculosus	Donegal, Ireland	November 2017	Applied UAE, MAE, UMAE, HAE and HPAE with 50% ethanol at a sample/solvent ratio of 1:10 (w/v) by with post-treatment of 0 h and 24 h	238.7–362.9	(Garcia-Vaquero et al. 2021)
Pelvetia canaliculata				128.3–203.9
Polycladia myrica	Hurghada, Red Sea	Winter 2020	Pre-removed lipids by adding hexane with a sample-to-solvent ratio of 1:5 (w/v), and add 50 mL of 70% acetone containing 0.3% w/v ascorbic acid to the sample for 1 h	57.52 ± 1.2	(Ismail et al. 2023)
Sirophysalis trinodis				55.58 ± 1.6
Dictyota spiralis				48.75 ± 2.1
Sargassum euryphyllum				56.13 ± 1.4
Turbinaria decurrens				43.75 ± 1.2
2,4-Dimethoxybenzaldehyde assay
Phyllospora comosa	Queenscliff Harbor, Victoria, Australia	February (summer)	Conventional extracted and using UAE with 70% methanol, 70% ethanol, 70% acetone, and absolute ethyl acetate	0.33–6.65	(Subbiah, Ebrahimi, et al. 2023)
Ecklonia radiata				0.59–8.29
Durvillaea sp.				0.29–8.22
Sargassum sp.				0.54–6.32
Cystophora sp.				1.02–4.15
P. comosa	Queenscliff Harbor, Victoria, Australia	February 2022	70% ethanol (1:20, w/v) for 16 h at 120 rpm at 10 °C	0.03–0.38	(Subbiah, Duan, et al. 2023)
E. radiata				0.18–0.65
Durvillaea sp.				0.04–0.40
Sargassum sp.				0.11–0.46
Cystophora sp.				0.63–2.86
Sargassum vulgare	Stora, Skikda, northeastern Algeria	February 2018	70% acetone solution (1:70, w/v) for 3 h at room temperature	0.4–9.4	(Chouh et al. 2022)
P. myrica	Hurghada, Red Sea	Winter 2020	Pre-removed lipids by adding hexane with a sample-to-solvent ratio of 1:5 (w/v), and add 50 mL of 70% acetone containing 0.3% w/v ascorbic acid to the sample for 1 h	13.44 ± 0.4	(Ismail et al. 2023)
S. trinodis				11.22 ± 0.6
D. spiralis				9.05 ± 0.3
S. euryphyllum				10.92 ± 0.2
T. decurrens				9.94 ± 0.4
C. compressa	Cleopatra coast, Marsa Matruh city, Mediterranean Sea, Egypt	April 2017	Pre-removed lipids by adding hexane with a sample-to-solvent ratio of 1:5 (w/v), and add 50 mL of 70% acetone containing 0.3% w/v ascorbic acid to the sample for 1 h	140.5	(Gheda et al. 2021)
Prussian blue assay
P. comosa	Queenscliff Harbor, Victoria, Australia	February (summer)	70% methanol, 70% ethanol, 70% acetone, and absolute ethyl acetate by conventional extraction and UAE	0.13–12.62	(Subbiah, Ebrahimi, et al. 2023)
E. radiata				0.46–8.00
Durvillaea sp.				0.48–6.63
Sargassum sp.				0.12–7.77
Cystophora sp.				0.48–8.61
P. comosa	Queenscliff Harbor, Victoria, Australia	February 2022	70% ethanol (1:20, w/v) for 16 h at 120 rpm at 10 °C	0.17–25.35	(Subbiah, Duan, et al. 2023)
E. radiata				0.06–3.33
Durvillaea sp.				0.20–5.31
Sargassum sp.				0.34–6.01
Cystophora sp.				0.56–28.77
Nuclear magnetic resonance (NMR)
Cystoseira tamariscifolia	Penmarc’h, Brittany, France	From February 2009 to July 2010	50% methanol at 40 °C	0.07%–0.46% DW	(Jégou et al. 2015)
Laminaria digitata	Scotland	2015	80% methanol for 1.5 h under 20 °C	4.3% DW	(Vissers et al. 2017)
Laminaria hyperborea	Haugesund, Norway	March 2020, September 2020, and August 2021	50% ethanol for 30 min with pretreated by 35 kHz ultrasonic for 30 min	4.08–6.17	(Wekre, Hellesen Brunvoll, and Jordheim 2022)
L. digitata	Southern Australian coast, Melbourne, Victoria, Australia	August 2019		5.09 ± 0.01
F. vesiculosus	Storåkervika, Bergen, Norway	August 2019		11.0 ± 0.0
Saccharina latissima	Trollsøy, Vestland	June 2021		12.4 ± 0.0
A. nodosum	Eidsvåg, Bergen	September 2017		8.57 ± 0.0
High-performance liquid chromatographic (HPLC)
F. vesiculosus	Archipelago Sea, SW Finland	(Not Specified)	Soluble fraction: 70% acetone (1:50, w/v) for 1 h; Insoluble fraction: soluble fractions was further suspended in 8 mL of 1M NaOH solution (80 °C) and stirred for 2.5 h at room temperature	6.01–7.47	(Koivikko et al. 2008)
Ecklonia cava	Aewol-eup, Jeju-island, South Korea	(Not Specified)	50% ethanol (1:10, w/v) for 6 h at 60 °C	48.08 ± 0.67 mg dieckol/g	(Petchidurai et al. 2019)
E. cava Kjellman	Busan, South Korea	2009	Methanol (1:4, w/v) for 2 h at room temperature	7.25–24.15 µg dieckol/mL 5.82–19.41 µg phlorofucofuroeckol-A/mL	(Chowdhury et al. 2011)

Note: NADES: natural deep eutectic solvents; PGE: phloroglucinol equivalents; DW: dry weight of seaweed biomass; UAE: ultrasound-assisted extraction; MAE: microwave-assisted extraction; UMAE: ultrasound–microwave-assisted extraction; HAE: hydrothermal-assisted extraction; HPAE: high-pressure-assisted extraction.

Indeed, the quantification of phlorotannin content can vary significantly based on the chosen extraction method, including the selection of organic solvents or solvents ratios. In the case of brown algae, specifically Sargassum angustifolium, researchers have employed three distinct ratios of ethanol-aqueous solvents (30%, 50% and 70%, v/v), resulting in a range of phlorotannin content from 0.4 to 3.4 mg phloroglucinol equivalents (PGE)/g (Hodhodi, Babakhani, and Rostamzad 2022). It is well-established that brown algae can adjust their production of these metabolites in response to environmental cues and stress conditions. Typically, phlorotannin levels in brown algae surpass 2% DW in temperate and tropical Atlantic areas, as well as temperate Pacific regions. As noted by Ank, da Gama, and Pereira (2019), brown seaweeds from higher latitudes have demonstrated the capacity to produce phlorotannins in a broader range, often exceeding 4% DW, likely in response to environmental factors or stimuli, compared to algae found in lower latitudes, which generally contain less than 2% DW.

Furthermore, different seaweed parts and growth conditions also affect their phlorotannin contents. For instance, Iken et al. (2007) reported the presence of 0.5% DW of phlorotannins in the pneumatocysts of Cystosphaera jacquinotii (Montagne) Skottsberg and 9% DW of phlorotannins in the branches of Desmarestia antárctica. Singh and Sidana (2013) reported that phlorotannin concentrations can reach notable levels, sometimes as high as 25–30% DW in various algal thalli. However, it is a limitation that the specific seaweed species associated with these high phlorotannin concentrations are not specified in the provided information. This lack of information may suggest the possibility of referring to hypothetical or unidentified species, or it could indicate a potential misunderstanding or misinterpretation of the literature, thus warranting further research and investigation.

Biological activities and bioavailability of phlorotannins

Fascinating biological activities

Due to their diverse chemical structures and the presence of polyhydroxy groups, phlorotannins exhibit a wide range of biological activities. These activities, summarized in Table 2, highlight their promising potential in functional foods, nutraceuticals, and the pharmaceutical industry. These biological properties include antioxidative, anti-inflammatory, anti-allergic/histamine, anti-diabetic, and anti-bacterial/microbial effects, among others.

Table 2. A brief summary of phlorotannin biological activities.

Bioactivities	Seaweeds	Phlorotannins	Models	Outcomes	References
Antioxidant	Undaria pinnatifida sporophyll (China)	Crude extracts	(Not Specified)	Approximately 40 µg/mL of phlorotannin extract showed similar antioxidant capacity to positive control (80 µg/mL of epigallocatechin gallate.	(Dong et al. 2019)
	Sargassum vulgare (Algeria)	Crude extracts and liquid-liquid purification fractions	(Not Specified)	DPPH: IC50 values ranged from 6.14 to 97.41 µg/mL. ABTS: IC50 values ranged from 1.29 to 93.76 µg/mL.	(Chouh et al. 2022)
	Fucus vesiculosus (Denmark)	Phlorotannin-rich extracts	(Not Specified)	EC50 of DPPH: 16.3–76.3 µg/mL.	(Hermund et al. 2022)
Anti-inflammatory	Ascophyllum nodosum (France)	Phlorotannin-rich extracts	Human epithelial cells (ARPE-19 and WKD cell lines)	A concentration of 0.2% effectively prevent the overproduction of ROS. A concentration of 0.05%, it reduced TNF-α release. A concentration of 0.2%, it reduced TNF-α and IL-6 release.	(Dutot et al. 2012)
	U. pinnatifida Sporophyll (China)	Phlorotannin-rich extracts	RAW 264.7 Macrophages	More efficient than epigallocatechin gallate at decreasing NO production.	(Dong et al. 2019)
	F. vesiculosus (Portugal)	Crude extracts & ethanolic extracts	RAW 264.7 Macrophages	Inhibit the NO production in a dose-dependent manner, decreasing to 14.08 ± 3.68 and 16.80 ± 6.32% in 100 µg/mL crude and ethanolic extract, respectively. However, at 200 µg/mL a decrease in cell viability indicates a cytotoxic effect. Down-regulation of iNOS expression and reduce the expression of COX-2 and IL-1β.	(Catarino et al. 2020)
	Eisenia bicyclis (Portugal)	Crude extracts	THP1 human monocytes	Reduce 43.5% of nitrite in the medium. Reduce 1.2-fold in IL-8 mRNA expression, and approximately 3.7-6-fold in the expression of TNF-α, IL-1β, IL-8, and IL-6. Increase 4-fold anti-inflammatory cytokine IL10 expression.	(Raja et al. 2023)
	Eisenia arborea (Japan)	6,6′-bieckol and 6,8′-bieckol	Mice	Significantly suppress mice ear swelling, with 35.7%–61.8% in 6,6′-bieckol and 55.7%–77.6% in 6,8′-bieckol.	(Sugiura et al. 2017)
Anti-allergy/ histamine	E. arborea (Japan)	Eckol, 6,6′-bieckol, 6,8′-bieckol, 8,8′-bieckol, phlorofucofuroeckol-A/-B	Rat mast cell line (RBL-2H3 cells)	Suppressed histamine release at a concentration of 100 mM, while phlorofucofuroeckol-B showing a suppressive ratio at 200 mM of 37.1%. Suppress PGE2 release at 10, 100, 200 mM.	(Sugiura et al. 2018)
	Eckolonia cava (South Korea)	Fucodiphloroethol G, eckol, phlorofucofuroeckol-A	Human leukemia cell line (KU812) & rat basophilic leukemia cell line (RBL-2H3)	KU812 – Reduced IgE induced stimulated histamine release to 23.97, 44.26, and 34.54% at the concentration of 100 mM. Reduced A23187 stimulated histamine release to 27.40, 61.15, and 37.05%, respectively. RBL-2H3 – A23187 stimulated: 27.73, 50.63, and 34.18%, respectively. Reduced β-hexosaminidase release to 18.02, 31.67, and 20.80%, respectively.	(Y. Li et al. 2008)
Anti-diabetic	E. bicyclis (Portugal)	Crude extracts	Mouse myoblast cell line (C2C12 cells)	Superiorly inhibit α-glucosidase inhibitory action with an IC50 of 31.7 ± 0.84 mg/mL, comparable to acarbose. Significantly inhibit glycation, PTP1B, α-glucosidase, and α-amylase activity. Increase glucose uptake by 51.09% (p < 0.05) under insulin-stimulated conditions.	(Raja et al. 2023)
	Sargassum linifolium (Egypt)	Phlorotannin-rich extracts	White albino male Wistar rats	Significant decrease in serum glucose levels. Promote the β-cell in the pancreas to secret insulin. Reduce glucosidase, α-amylase, and CAT levels. Lower the urea, creatinine, cholesterol and triglyceride levels. Activate of AMPK pathway and inhibit adipogenesis.	(Gheda et al. 2023)
	Cystoseira compressa (Egypt)	Phlorotannin-rich extracts	White albino Wistar rats	Significant reduce the blood glucose, urea, creatinine, cholesterol, and triglyceride levels compared to the diabetic control. Increase the insulin levels. Reduce the α-amylase and glucosidase activities. Increase CAT activity to decrease the hepatic lipid peroxidation.	(Gheda et al. 2021)
	Sargassum tenerrimum (India)	Phlorotannin-rich extracts	Wistar albino rats	Elevate insulin levels after 21 days of treatment (p < 0.001). Considerably improve oral glucose tolerance by 52% and 63% at 200 mg/kg and 400 mg/kg (p < 0.001). Reduced the production of the lipid oxidation product MDA level but increase SOD and CAT concentrations. Alleviate the TNF-α level.	(Vijay and Vellapandian 2023)
Anti-bacterial/microbial	Halidrys siliquosa (France)	Phlorotannin-rich extracts	Pseudomonas aeruginosa (ATCC 9027), Staphylococcus aureus (ATCC 6538), and E. coli (ATCC 8739)	Reduce higher than 5 log of the initial bacterial concentration with Pseudomonas aeruginosa and E. coli (<1 CFU/mL). 4.5 log reduction of the initial bacterial concentration of Staphylococcus aureus (75 CFU/mL).	(Le Lann et al. 2016)
	Hizikia fusiforme (China)	Phlorotannin-rich extracts	S. aureus (ATCC 25923), E. coli (ATCC 25922), P. aeruginosa (PAO1), Aeromonas hydrophila (YJ-1), and Vibrio parahaemolyticus (X-1)	The MIC of phlorotannins against the selected bacteria was: 0.1943 g/ml for C. violaceum and E. coli, and 0.0972 g/ml for S. aureus, A. hydrophilia, V. parahaemolyticus, and P. aeruginosa. Inhibit the formation of biofilm.	(Tang, Wang, and Chu 2020)
	F. vesiculosus & Pelvetia canaliculate (Russia)	Cell wall extracts & intracellular phlorotannin-rich extracts	E. coli (ATCC 25922), P. aeruginosa (ATCC 27853), Listeria monocytogenes (EGD), S. aureus (ATCC 25923), S. aureus (SG-511), methicillin resistant S. aureus (ATCC 33591), Micrococcus luteus (CIP A270)	Low or moderate antimicrobial activity for all extracts. Cell wall extracts exhibited higher antimicrobial activity in comparison to the intracellular phlorotannin-rich extracts. For E. coli and S. aureus (ATCC 25923) –Cell wall fraction (MIC = 31.2 µg/mL), intracellular fraction (MIC = 62.5 µg/mL), For methicillin resistant S. aureus–Cell wall fraction (MIC = 125 µg/mL), intracellular fraction (MIC = 500 µg/mL)	(Meshalkina et al. 2023)

Note: DPPH: 2-diphenyl1-picryl-hydrazine-hydrate; ABTS: 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid; EC50: half maximal effective concentration; ROS: reactive oxygen species; NO: nitric oxide; iNOS: inducible nitric oxide synthase; COX-2: cycloocygenase-2; IL: interleukin; PGE2: prostaglandin E2; Ig: immunoglobulin; IC50: half maximal inhibitory concentration; PTP1B: protein tyrosine phosphatase 1B; AMPK: AMP-activated protein kinase; MDA: malondialdehyde; SOD: superoxide dismutase; CAT: catalase; TNF-α: tumor necrosis factor alpha; MIC: minimum inhibitory concentration.

Antioxidant

Phlorotannins have been affirmed as excellent exogenous antioxidants that exhibit their antioxidant properties through three main mechanisms, including direct reactive oxygen species (ROS) scavenging, nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway and the upregulation of antioxidant enzyme activities.

The ROS are commonly generated as natural byproducts of various metabolic processes in living organisms. When overproduced, they can lead to a state of oxidative stress, which can, in turn, cause damage to lipids, proteins, and DNA, and is associated with various diseases. The ability of phlorotannins to defend against ROS is linked to their intricately branched molecular structure, where phenol rings donate their electrons to these oxidative agents, resulting in the formation of an intermediate phenoxyl radical species that stabilizes unpaired electrons and establishes hydrogen bonds with neighboring –OH groups or engages in dimerization (phenol coupling) to create C–O or C–C linkages (Phang et al. 2023). These mechanisms have been corroborated by numerous previous studies employing colorimetric assays focused on determining radical scavenging activity. These assays encompass the assessment of radical scavenging activity against various radicals, including the 2,2-diphenyl-1-picryl-hydrazine-hydrate (DPPH) and 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radicals. For example, phlorotannins derived from Fucus vesiculosus and Ascophyllum nodosum have exhibited comparable or even superior DPPH radical scavenging activity when compared to commercially available antioxidants such as butylated hydroxytoluene and ascorbic acid (Liu et al. 2017). Similarly, stronger ABTS inhibitory effects than those observed with ascorbic acid were found in the ethyl acetate fractions of Sargassum hemiphyllum and Sargassum pallidum, with the half maximal inhibitory concentration (IC50) values of 0.11 ± 0.01 mg/mL and 0.02 ± 0.00 mg/mL, respectively, in contrast to 0.27 ± 0.00 mg/mL (Chen et al. 2023). Additionally, they include assessments of reducing power via ferric ion reducing antioxidant power and phosphomolybdate assay, as well as metal chelating activity. The ferrous chelating properties of phlorotannins obtained from F. vesiculosus were further investigated, revealing up to 95% chelation in water extracts and 55% in acetone aqueous (v/v, 7:3) extracts (Wang, Jonsdottir, and Ólafsdóttir 2009).

Another mechanism modulated by phlorotannins involves the activation of Nrf2, a signaling pathway that leads to the downstream production of antioxidants. Phlorotannins have been found to form quinones that act as electrophiles, interacting with the thiol group in Kelch-like ECH-associated protein 1, a crucial sensor of oxidative and electrophilic stress. With further activating quinone reductase activity that led to the reduction and detoxification of electrophilic quinones and quinone derivatives, phlorotannins help result in the formation of Michael adducts and induce a conformational change that releases the Nrf2 protein, which, in turn, promotes the translocation of Nrf2 from the cytoplasm into the nucleus, upregulating the production of antioxidant enzymes such as the heme oxygenase-1 (Hmox-1) (Jun et al. 2014). Phlorotannins can also activate the Nrf2 pathway through other mechanisms, such as the phosphorylation of several downstream pathways. Notably, the differential activation of various signaling pathways in response to phlorotannin administration, such as the induction of phosphorylated c-Jun N-terminal kinase (JNK) and phosphorylated protein kinase B (PKB) on HepG2 hepatocytes and the activation of p-Akt and phosphorylated extracellular signal-regulated kinase (ERK) pathways on V79-4 lung fibroblasts by eckol (Jun et al. 2014; K. C. Kim et al. 2010), is contingent on several factors, including the specific types of phlorotannins, the cell types involved, and the nature of the oxidative stimuli applied.

In addition to its impact on Hmox-1, phlorotannins have also been observed to reduce the expression of NADPH oxidase 2 (NOX-2), a significant source of ROS production induced by lipopolysaccharide (LPS). This findings are supported with an increased expression of glycoprotein 91^phox, a catalytic substrates of NOX complex (Cui et al. 2015). Meanwhile, A. D. Kim et al. (2014) reported that eckol derived from Ecklonia cava could enhance the expression of manganese superoxide dismutase, which serves as the primary defense mechanism against mitochondrial ROS, safeguarding cells from oxidative stress.

Anti-inflammatory

Inflammation, whether acute or chronic, can give rise to a range of health complications, including autoimmune diseases like rheumatoid arthritis (Shrivastava and Pandey 2013), cardiovascular diseases such as high blood pressure (Botelho et al. 2021), and gastrointestinal disorders like inflammatory bowel disease (Yao et al. 2019). Researchers have posited that phlorotannins exert significant effects on three pivotal mediators, including cytokines, enzymes, and signaling pathways, drawing from observations in cell cultures and animal studies.

Besides the previously mentioned antioxidant mechanisms, phlorotannins alleviate inflammation, often associated with oxidative stress, by modulating the production of inflammation-related cytokines. For instance, treatment with eckol isolated from E. cava at concentrations of 50 µg/mL and 100 µg/mL in a mouse model of anaphylaxis resulted in the reduction of mRNA expression levels of proinflammatory cytokines such as interleukin 1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α) (Han et al. 2020). They also observed the downregulation of Th2-type cytokines, specifically IL-4, IL-5, and IL-13, which led to a reduction in immunoglobulin (Ig) E and eosinophilic responses. Another study reported an amelioration of endothelial dysfunction in mouse models through the use of pyrogallol-phloroglucinol-6,6-bieckol extracted from the same species, which was attributed to the reduced expression of TNF-α and IL-6 (Son et al. 2019). The observed enhancement of transforming growth factor beta can inhibit the activation and function of various immune cells, such as T cells and macrophages, reducing their capacity to produce pro-inflammatory cytokines and mediators. Simultaneously, it promotes the development and differentiation of regulatory T cells, which are essential in maintaining immune tolerance and preventing excessive inflammation (Bierie and Moses 2010).

Several enzymes implicated in the processes of inflammation are subject to regulation by phlorotannins. Notably, inducible nitric oxide synthase (iNOS) is among the enzymes responsible for the production of nitric oxide (NO), a pivotal signaling molecule associated with a range of physiological processes, including inflammation, immune responses, and tissue damage (Spiller et al. 2019). Recent investigations have revealed that trifuhalol A, extracted from Agarum cribrosum, exerts a significant downregulatory effect on iNOS expression in LPS-stimulated RAW264.7 cell models, resulting in a notable decrease in NO production, estimated at 36.6%–56.8% (Phasanasophon and Kim 2019). Lopes et al. (2012) proposed that phlorotannins have the capacity to regulate NO levels not only by inhibiting related enzymes but also through direct scavenging and/or by reducing NO production via their influence on the inflammatory signaling cascade. Moreover, Kurihara, Konno, and Takahashi (2015) reported that fucophlorethol C, isolated from the brown seaweed Colpomenia bullosa, effectively inhibited lipoxygenases (LOX) to a similar extent as the known inhibitor nordihydroguaiaretic acid. LOX plays a crucial role as a rate-limiting enzyme in the conversion of arachidonic acid into leukotrienes, known mediators of inflammation (Hu and Ma 2018); therefore, compounds that inhibit LOX have the potential to decelerate this process and exert anti-inflammatory effects.

In terms of signaling pathways, Catarino et al. (2020) reported that phlorotannin derivatives from F. vesiculosus effectively inhibited the phosphorylation and degradation of inhibitory protein κBα, an intracellular protein that functions as a primary inhibitor of the proinflammatory transcription factor nuclear factor kappa B (NF-κB). This inhibition, in turn, prevents the activation of the NF-κB signaling pathway, thereby locking the inflammatory cascade at the transcriptional level. In line with the findings of S. Kim et al. (2019), they observed that the addition of an ethanolic extract of E. cava, primarily containing dieckol, induced the translocation of NF-κB to the nucleus while preventing its binding to the cell DNA, along with a dose-dependent reduction in the levels of cytokines and various enzymes. Trifuhalol A was found to significantly inhibit the expression of JNK and ERK1/2 at concentrations ranging from 5 to 20 µg/mL (Phasanasophon and Kim 2019). This inhibition further attenuated mitogen-activated protein kinases (MAPKs), serving as crucial components of a series of vital signal transduction pathways regulating processes such as cell proliferation, cell differentiation, and cell death in eukaryotes (Morrison 2012).

Anti-diabetic

Diabetes is a global health issue affecting millions of individuals, with its incidence projected to increase in the coming years. This chronic condition is associated with severe complications, including cardiovascular diseases, kidney damage, and nerve damage (Dal Canto et al. 2019). Recent advances in the study of plant polyphenols have brought attention to phlorotannins as potential candidates for modulating key enzymes, specifically α-glucosidase and α-amylase (Lopes et al. 2019), which are responsible for the elevation of postprandial glycemia. Several phlorotannins have demonstrated lower IC50 values than acarbose, a clinically approved inhibitor for type 2 diabetes treatment, indicating greater inhibitory potential. For example, fucophloroethol structures obtained from Fucus distichus exhibited an IC50 of 0.89 µg/mL and 13.9 µg/mL against α-glucosidase and α-amylase, respectively. In contrast, acarbose, the pharmaceutical inhibitor, showed IC50 values of 112.0 µg/mL and 137.8 µg/mL for the same enzymes (Kellogg, Grace, and Lila 2014). Consistent with the findings of H.-A. Lee, Lee, and Han (2017), oral administration of 2,7″-phloroglucinol-6,6′-bieckol led to a significant reduction in postprandial blood glucose levels in diabetic mouse models induced by streptozotocin, attributed to the delayed absorption of dietary carbohydrates through the inhibition of the related enzymes. The compound exhibited IC50 values of 23.35 µM against α-glucosidases and 6.94 µM against α-amylase, in contrast to acarbose at 130.04 and 165.12 µM, respectively. Lo Piparo et al. (2008) hypothesized that the inhibitory activity of phlorotannins is influenced by the formation of hydrogen bonds between their hydroxyl groups and the carboxylate groups of Asp₁₉₇ and Glu₂₃₃, two active sites of α-amylase. This impedes the enzymatic breakdown of complex carbohydrates into simple sugars like glucose. Moreover, similar polyphenol categories, such as flavonoids and phenolic acids, offer insight into possible inhibitory mechanisms influenced by methylation and methoxylation processes on phlorotannin structures (Guan et al. 2022). The quantity, position, and substitution pattern of these hydroxyl and methoxy groups are presumed to participate in various chemical interactions with the active sites of α-glucosidase and α-amylase, including hydrogen bonding and electrostatic interactions, further influencing binding affinity and inhibitory potency (J. Zhang et al. 2022).

Hepatic gluconeogenesis represents another critical target for diabetes modulation, involving the regulation of insulin-related enzymes, such as glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase (PEPCK), and glucokinase. Octaphlorethol A, derived from Ishige foliacea, was identified as an inhibitor of hepatic gluconeogenesis in a study by S.-H. Lee et al. (2016). It significantly suppressed the upregulation of hepatic mRNA expression levels of gluconeogenic enzymes, particularly PEPCK and G6Pase, leading to a reduced hepatic glucose production and potential benefits in diabetes management. Murray et al. (2021) provided compelling evidence through a placebo-controlled randomized clinical trial, demonstrating that the administration of a polyphenol-rich seaweed extract containing 8,8′-bieckol, 6,6′-bieckol, and phlorofurofucoeckol A for 12 weeks resulted in a greater reduction in postprandial glucose levels compared to fasting glucose levels, with further support for the regulation of G6Pase and PEPCK. Conversely, S. Zhang et al. (2020) indicated that certain phlorotannins, notably trifuhalol A, did not exhibit an inhibitory effect on G6Pase, even at increased concentrations. In a randomized crossover trial, the administration of polyphenols extracted from F. vesiculosus demonstrated the potential to moderate glycemic responses in females by reducing peak postprandial glucose concentration (Murray et al. 2019). However, this effect was not observed in males, suggesting a gender-specific response that requires further investigation to understand the underlying mechanisms and implications of these gender differences in response to polyphenol treatment.

Oxidative stress plays a pivotal role in the development of type 2 diabetes mellitus (T2DM), resulting from an imbalance between the production of ROS and antioxidant defense. In T2DM, the decreased levels of enzymatic antioxidants contribute to the development of diabetic complications. According to a meta-analysis of 22 studies conducted by Banik and Ghosh (2021), the association between T2DM and biomarkers was emphasized, with eight studies focused on superoxide dismutase (SOD) levels and four on glutathione peroxidase levels. Administration of dieckol through intraperitoneal injections (10–20 mg/kg for 14 days) in diabetic mice resulted in a notable reduction in blood glucose and serum insulin levels, with a slight increase in the activity of the antioxidative enzyme, SOD, in the liver tissues (Kang et al. 2013). Similarly, eckol, 7-phloroeckol, and phlorofucofuroeckol-A have been identified as inhibitors of protein tyrosine phosphatase 1B (PTP1B) (Gunathilaka, Keertihirathna, and Peiris 2021), an enzyme critical for regulating insulin signaling. PTP1B overactivity can result in insulin resistance; thus, the inhibition of PTP1B is a potential therapeutic strategy to enhance efficient insulin signaling by preventing the dephosphorylation of tyrosine residues and promoting glucose uptake into cells (Koren and Fantus 2007).

Anti-allergy/histamine

Histamine, a bioactive amine released by immune cells, serves as a signaling molecule and neurotransmitter in response to allergic reactions (Mandola, Nozawa, and Eiwegger 2019). The prevalence and severity of allergic diseases have exhibited a significant rise over the past decade worldwide, particularly affecting approximately 20% of the population in developed countries (Y. Li et al. 2008). Two bioactive phloroglucinol derivatives, fucodiphloroethol G and phlorofucofuroeckol A, were initially isolated from E. cava by Y. Li et al. (2008), who reported their notable inhibitory activity against histamine release. This inhibition was demonstrated in human basophilic leukemia (KU812) and rat basophilic leukemia (RBL-2H3) cell lines using a histamine release assay. To support their results, Le et al. (2009) suggested that these active compounds may be useful in preventing allergen-IgE-induced allergic reactions. Likewise, G. Ahn et al. (2015) highlighted the inhibitory effects of dieckol on IgE-mediated degranulation observed in LAD2 cells. An insightful hypothesis posits that phlorotannins have the capacity to stabilize the cell membrane by reducing intracellular Ca²⁺ levels, thereby inhibiting the binding activity between IgE and its high-affinity receptor. This inhibition, in turn, disrupts downstream signaling cascades, including the activation of protein tyrosine kinases (Syk, Lyn), calcium mobilization, and protein kinase C translocation, all of which are essential for degranulation (Shim, Lee, and Lee 2016; Yoshioka et al. 2014).

Sugiura et al. (2021) discovered that the phlorotannin dieckol exhibited significant anti-allergic effects in an ear swelling test conducted on mice sensitized to arachidonic acid, 12-O-tetradecanoylphorbol-13-acetate and oxazolone. These effects were attributed to the suppression of degranulation, the release of chemical mediators, and the expression of mRNA, including cyclooxygenase-2 (COX-2), IL-6, and TNF-α, in rat basophilic leukemia-2H3 cells. In another study, 6,6′-bieckol and 6,8′-bieckol were found to inhibit COX-2 activity at a concentration of 500 µM, with inhibition ratios of 9.8% and 40.4%, respectively. Notably, 8,8′-bieckol (96.6%) and phlorofucofuroeckol A (96.8%) exhibited inhibitory effects comparable to that of epigallocatechin gallate (95.8%) (Sugiura et al. 2018). These findings align with the explanation that the bioactivity of phlorotannins is strongly influenced by their molecular size or the number of phenol groups. The structures of eckols and the bieckols, as presented by Sugiura et al. (2018), suggest that the positioning of the hydroxyl group at either C-4 or C-4′ in these compounds may play a crucial role in their ability to suppress degranulation and COX-2 mRNA expression.

In addition, a recent study isolated three phlorotannins from Sargassum carpophyllum, including 2-[2-(3,5-dihydroxyphenoxy)-3,5-dihydroxyphenoxy]-1,3,5-benzenetriol, 2,2′-[[2-(3,5 dihydroxyphenoxy)-5-hydroxy-1,3-phenylene]bis(oxy)]bis(1,3,5-benzenetriol), and 2-[2-[4-[2-(3,5-dihydroxyphenoxy)-3,5-dihydroxyphenoxy]-3,5-dihydroxyphenoxy]-3,5 dihydroxyphenoxy]-1,3,5-benzenetriol (Matsui et al. 2022). These compounds were found to prevent immediate hypersensitivity by exhibiting a dose-dependent reduction in β-hexosaminidase secretion in vitro similar to other phlorotannins such as eckol (Han et al. 2020), with IC50 values of 50.7, 35.9, and 43.5 µM, respectively. Meanwhile, these phlorotannins exhibited a significant reduction of approximately 50% in prostaglandin D2 (PGD2) levels compared to the positive control group. PGD2 is a major prostanoid secreted by activated mast cells, and elevated PGD2 levels have been closely associated with medical conditions, including asthma and allergic reactions (Nakamura et al. 2015). However, their findings revealed no statistically significant effect on the expression of COX-2, the enzyme responsible for converting arachidonic acid to prostaglandin H2 (PGH2), the precursor of PGD2 synthesis. This contrasts with the conclusion previously reported by Sugiura et al. (2018), and as a result, they were unable to establish a definitive relationship between COX-2 expression and the levels of PGD2. It is plausible that the activity of downstream enzymes involved in PGD2 synthesis or the presence of other mediators in the cellular environment may have influenced the observed results.

Anti-microbial/viral

The hypothesis suggests that phlorotannins may exert antiviral effects through two primary mechanisms: by preventing the virus from entering host cells and by inhibiting viral replication once it has entered the host cell. J.-Y. Park et al. (2013) observed that the dieckol derived from E. cava competitively inhibits 3CLpro, a cysteine proteinase with similarities to chymotrypsin, which is crucial for viral replication. Other enzymes, such as reverse transcriptase and protease of human immunodeficiency virus type 1 (HIV-1), were reported to be potently inhibited by 8,4′′′-dieckol and 8,8′-bieckol (M.-J. Ahn et al. 2004). Likewise, it was observed that phlorotannins extracted from E. cava suppressed the production of surface proteins, such as hemagglutinin and neuraminidase, which are responsible for the initial attachment of the virus to host cells and the release of newly formed virions, respectively (Cho et al. 2019). Meanwhile, phlorotannins were found to increase the activity of mitochondrial dehydrogenases, causing membrane depolarization, as well as reducing the ergosterol content of the fungal cell wall, thus demonstrating efficacy against yeast and several dermatophytes (Lopes et al. 2013). A synergistic effect was reported when using phlorotannins in conjunction with antibiotics against the Listeria genus (H.-J. Kim et al. 2018).

Bioavailability of phlorotannins

The concept of bioavailability refers to the proportion of an administrated dose of nutrients that enters the systemic circulation (Price and Patel 2021), which is influenced by factors such as biological response (bioactivity), metabolism, absorption (bioaccessibility). While numerous biological activities of phlorotannins have been determined, research on their bioavailability is still ongoing. Scientists have hypothesized that they exhibit similar behaviors to other polyphenols, such as phenolic acids and flavonoids. In this regard, the overall bioavailability of phlorotannins is anticipated to be limited, as Catarino, Marçal, et al. (2021) reported only a small fraction (14.1%) of phlorotannins from F. vesiculosus is likely to undergo absorption in the upper gastrointestinal tract. This finding aligns with the results of Corona et al. (2016), who observed that, following the ingestion of a phlorotannin-rich capsule by healthy volunteers, most of the phlorotannin metabolites were subsequently detected in their urine and plasma, indicating a low rate of absorption in the small intestine.

In detail, the low absorption rate could probably be explained by the polymeric nature of phlorotannins, as well as their molecular weight and interactions with other dietary components (Meng et al. 2021). The initial challenge in the absorption of phlorotannins is to liberate them from the seaweed matrices, because they are typically embedded or bound within the cellular and structural components that hinder their release, especially polysaccharides and proteins (Ford et al. 2020). Phlorotannins, with higher molecular weights, have a greater capacity to form bonds with proteins, which reduces their solubility and makes it more challenging for the digestive system to break them down and absorb them in the small intestine (Cáceres-Jiménez et al. 2023). Also, it is hypothesized that enzymes responsible for metabolizing them may have reduced access to the bound phlorotannins, slowing down their transformation into low molecular weight derivatives. Catarino, Marçal, et al. (2021) observed a shielding effects between bound phlorotannins and gastric enzymes that prevent their degradation during the upper gastrointestinal and therefore forcing their transportation to the colon. On the other hand, some interactions between phlorotannins and cell wall polysaccharides, such as fucoidans and alginates, would also affects their release. For example, alginates can form a three-dimensional gel-like network that captures phlorotannins, similar to the patterns seen with condensed tannins-proanthocyanidins (Liu and Bourvellec 2023).

Moreover, phlorotannins undergo various metabolic processes. Similar to terrestrial polyphenols, these metabolism processes can be categorized into three phases. Phase I reactions primarily entail intramolecular modifications, including processes such as oxidation, reduction, and hydrolysis. Meanwhile, Phase II reactions involve the conjugation of endogenous substances with ingested compounds, transforming them into water-soluble substances that can be easily excreted from the body. This process is often referred to as detoxification and is achieved through main processes like acetylation, methylation, glucuronidation, and sulfation (also known as sulfonation) (Schaffenburg, Lockshin, and DeKlotz 2021). It is worth noting that recent emphasis has been placed on a term “Phase III reaction” in the context of drug or nutrient metabolism, which is responsible for the final elimination of toxins and metabolic products from cells (Phang-Lyn and Llerena 2019). Together, these processes influence their stability during transit. In their pioneering work, Corona et al. (2016) investigated the possibility of the relationship between ingested seaweed and their colonic biotransformation by gut microbiota. This study successfully identified a variety of unconjugated and conjugated metabolites of phlorotannins, including glucuronides and/or sulfates, in urine and plasma samples. During Phase II detoxification, phlorotannins undergo transformations that cause them to form larger and more polar conjugates, which hampers their passive diffusion across biological membranes, ultimately diminishing their bioavailability and facilitating their elimination from the body (Corona et al. 2017). This possibility found further support in a clinical experiment conducted on obese individuals, where the analysis of their urine revealed the presence of various phlorotannin conjugates, including phloroglucinol sulfate, hydroxytrifurahol A-sulfate, dioxinodehydroeckol, fucophloroethol glucuronide, diphlorethol sulfates, and dioxinodehydroeckol glucuronide (Baldrick et al. 2018).

Safety and stability of phlorotannins

When considering phlorotannins for medical or health-promoting purposes, addressing their safety concerns is imperative. However, the majority of the available studies are conducted in vitro based on cell models, with limited in vivo animal or human research. Indeed, phlorotannins have been shown to be nontoxic or not significantly toxic to several cell lines in most of the studies.

For example, diphlorethohydroxycarmalol from Ishige okamurae was reported to increase cell viability against H₂O₂-induced oxidative stress in murine hippocampal neuronal cells. The highest cell survival rate reached 77.21% at a dose of 50 µM, compared to the blank control, which only had a survival rate of 17.18% (Heo et al. 2012). This assessment was conducted using the 2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, a widely used test in cell biology and toxicology for evaluating cell viability and proliferation, enabling the indirect assessment of substance toxicity. However, there were some controversial results, such as dieckol (20–100 µg/mL) decreasing cell viability and increasing prostaglandin E2 (PGE2) release on HaCaT cells exposed to particulate matter (PM₁₀) (Ha et al. 2019), which could potentially lead to cardiovascular conditions, respiratory symptoms, chronic inflammation and tissue damage. These results suggest that the response patterns may vary depending on the cell line models and the specific doses applied.

In terms of animal studies, the safety and toxicity of phlorotannins have been extensively investigated using zebrafish, mice, rats and dogs. These studies have generally shown a 100% survival rate while exhibiting efficient biological activities. However, some slight symptoms have been reported, such as soft stool and diarrhea in Beagle dogs when given high-purity phlorotannin fractions at a dose of 750 mg/kg on days 3 and 13 (Yang et al. 2014). It is worth noting that these effects were only observed at high oral doses, and no adverse effects were found at doses of 250 and 500 mg/kg. Similar safety has also been demonstrated in human trials with phlorotannin-rich capsules, especially eckol from E. cava at a dose of 500 mg/day and treatment for one week (Um et al. 2018).

The European Food Safety Authority (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA) delivered a scientific report on the safety of E. cava products (SeaPolynol^TM) (Patent NO.: US10016471B2) as a phlorotannin-rich food supplements (Turck et al. 2017). They reported that there is no evidence of genotoxicity for the product and identified a low risk of allergic reactions. They suggested an intake of 163 mg/day for adolescents aged 12 to 14 years (mean body weight of 43.4 kg), 230 mg/day for adolescents above 14 years of age (mean body weight of 61.3 kg), and 263 mg/day for adults (default body weight of 70 kg).

Investigating strategies to improve bioavailability and bioactivities of phlorotannins

Research on phlorotannins is still in its early stages, and one commonly employed approach to overcome their inherent limitations involves the micro-/nano-encapsulation of phlorotannins within a carrier (Tong, Liu, and Yu 2021). Various carrier systems, such as nano-emulsions, liposomes, edible films, nanoparticles, nanogels, have been utilized for delivering terrestrial polyphenols (J. Zhang et al. 2023). In the case of phlorotannins, specific examples of targeted carriers can be observed.

Polymeric nanoparticles

To enhance the therapeutic effectiveness of phlorotannins in the human body, Bai et al. (2020) conducted a study on the encapsulation efficiency of polyvinylpyrrolidone (PVP) nanoparticles, employing various ratios in combination with phlorotannins. Among these ratios, the nanoparticles with a 1:8 ratio exhibited superior stability in water, remaining without precipitation for 5, 10, and 15 days at room temperature compared to free phlorotannins. Furthermore, the application of PVP nanoparticles at concentrations of 6.25 and 12.5 µg/mL in a HaCaT keratinocyte model resulted in a significant reduction of ROS production by 12% and 18%, respectively, without detectable irreversible cytotoxic effects. However, after discovering the carcinogenic nature of PVP, its use in health supplements and oral administration became limited. Consequently, Bai, Chen, and Qi (2022) developed an alternative approach by formulating improved protein-polysaccharide nanocomplexes using whey protein isolate and chitosan oligosaccharides, which are edible, cost-effective, and commercially available. In their experiment, Caco-2 cells exposed to hydrogen peroxide exhibited a decrease in cell viability to 68.79 ± 3.78%. However, treatment with phlorotannin-loaded nanoparticles at varying concentrations revealed a dose-dependent increase in cell viability. Notably, at a concentration of 40 µg/mL of phlorotannin-loaded nanoparticles, the cell viability significantly rose to 84.83 ± 1.92% (p < .05). Additionally, the administration of the nanoparticles at the aforementioned concentration effectively restrained ROS generation in H₂O₂-treated cells and NO production in LPS-induced cells, bringing them to levels comparable to the negative control group, which further validates the enhanced anti-inflammatory and antioxidant capacity achieved through nano-encapsulation.

Another investigation employed electrospinning to prepare zein nanofiber membranes loaded with ε-polylysine-polyphenol-polyaspartic acid for the controlled delivery of polyphenols, including phlorotannins (Yingying et al. 2022). In terms of their in vitro release profile, approximately 19.65% of the phlorotannins were released within the initial 2 h under acidic conditions (pH 1.2), thereby illustrating the protective effect of zein nanofiber membranes against the acidic gastric pH environment. Moreover, a higher release of 62.35% occurred over an 8-h period at pH 6.8, which was facilitated by the electrostatic interaction between ɛ-polylysine and polyaspartic acid, effectively enhancing system stability and preventing burst release. Consistent with the findings reported by Amjadi et al. (2022), the investigation revealed that zein nanofiber membranes effectively preserved the activity of polyphenols and displayed favorable biocompatibility, as well as induced a higher degree of stress and apoptosis in HT-29 cells, a human colorectal adenocarcinoma cell line with epithelial morphology. Consequently, electrospinning has been validated as a highly effective method for fabricating sub-micron or nano-scale polymer fibers, which can be tailored to serve as safe and nontoxic drug delivery systems (DDSs). Moreover, Surendhiran, Cui, and Lin (2019) suggested using various biodegradable polymer-based nanofibers for food preservation purposes. In their study, they fabricated sodium alginate and poly(ethylene oxide) (PEO) blended nanofibers encapsulated with phlorotannin (50:50:10, v/v/v) via electrospinning process, which exhibited temperature-dependent antimicrobial activity. The preservation capacity of active nanofibers was assessed against Salmonella enteritidis on chicken at 4 and 25 °C, with cell count drastically decreasing from 6.20 to 3.28 Log CFU/g at 4 °C and from 8.80 to 2.53 Log CFU/g at 25 °C. Due to the presence of 8 interconnected rings in marine algal polyphenols, the phlorotannin encapsulated nanofibers showed higher overall acceptability scores in sensory assessment, with food preservability almost equal to sodium nitrite, a widely used synthetic preservative in meat industries (Sathya et al. 2017). Similarly, a multifunctional wound dressing using gelatin/chitosan hybrid fibers embedded with a phlorotannin-enriched extract derived from the seaweed Undaria pinnatifida was prepared (Ferreira et al. 2021). Electrospun meshes were created by incorporating 1 or 2 weight (wt)% of the extract, resulting in enhanced structural stability over time, as indicated by reduced in vitro degradation rates under enzymatic conditions. Initial antimicrobial assessments demonstrated the effectiveness of mesh against Pseudomonas aeruginosa and Staphylococcus aureus, confirming its antimicrobial activity. Regarding biological characterization, none of the tested conditions showed cytotoxic effects on hDNF cells, allowing for the adhesion and proliferation of the cells over a span of 14 days, with the exception of the 2 wt% condition after 7 days exhibited some cytotoxicity effects.

Liposomes

Nano-liposomes, also known as submicron bilayer lipid vesicles, represent a novel technology for the efficient encapsulation and targeted delivery of bioactive agents, which enable the incorporation of a wide range of bioactive materials, including pharmaceuticals, cosmetics, and nutraceuticals (Mozafari 2010). This technology has opened up diverse opportunities for the development of advanced delivery systems in various fields, allowing for improved efficacy and controlled release of bioactive compounds. Savaghebi, Ghaderi-Ghahfarokhi, and Barzegar (2021) explored the potential of nano-liposomes encapsulated with brown algae extract (Sargassum boveanum) for their antioxidant and antimicrobial properties. Their research indicated that the ethyl acetate fraction demonstrated significant activity against gram-positive bacteria (Bacillus cereus), with minimum inhibitory concentration (MIC) values of 0.937 and 0.469 mg/mL before and after encapsulation in nano-liposomes. This finding was supported by the speculation made by Hierholtzer et al. (2013) that phlorotannins, present in the brown algae extract, exert antimicrobial effects by disrupting cell membranes and eliciting substantial extra- and intra-cellular responses. On the contrary, certain bacterial strains have developed resistance to a vast array of antibiotics, particularly among gram-negative species. This resistance can be attributed to the presence of an additional outer membrane within their cell wall structure, serving as a protective barrier that restricts the entry of numerous antibiotics into the bacterial cell (Breijyeh, Jubeh, and Karaman 2020). Phlorotannin extracts and their encapsulated forms exhibited MIC values of 1.875 and 0.937 mg/mL, respectively, against P. aeruginosa and Escherichia coli (Savaghebi, Ghaderi-Ghahfarokhi, and Barzegar 2021). Contrary to expectations, a previous study conducted by Savaghebi, Barzegar, and Mozafari (2020) on the same seaweed species revealed that nano-liposomes exhibited a lower antioxidant capacity compared to the free algal extract. The investigation specifically highlighted a reduction in both the radical scavenging ability and the reducing antioxidant power of the nano-liposomes when compared to the unencapsulated algal extract. Notably, the nano-liposomes displayed a notably diminished scavenging capacity on ABTS radicals, with half maximal effective concentration (EC50) values measured at 453.9 and 219.5 ppm, respectively. Zou et al. (2014) proposed that the observed phenomenon can be plausibly attributed to the structural interactions between the liposome matrix and the loaded compounds. In addition to the localization of phenolic compounds within the core of nano-liposomes, the interactions between the compounds and the lipid bilayers also play a significant role in enhancing their retention time and impeding the kinetics of their release (Karonen 2022). The balance between the biological activities of the encapsulated compounds and their release patterns within nano-liposomes introduces new challenges for their diverse applications. Undoubtedly, the attainment of an optimal release profile, which guarantees the desired biological effects while preserving stability and controlled release, constitutes a complex undertaking. Addressing these challenges holds the key to unlocking the successful utilization of nano-liposomes as efficacious delivery systems within a wide array of industries.

Composite scaffolds

The incorporation of specific compounds within a range of composite scaffolds, achieved through various mechanisms such as encapsulation, absorption, or entrapment, holds promise for advantageous utilization in osteoinductive applications within the realm of bone tissue engineering (Romagnoli, D’Asta, and Brandi 2013). Im et al. (2017) have introduced a biocomposite scaffold that incorporates collagen extracted from fish skin along with phlorotannin sourced from brown algae E. cava. Significantly, this composite scaffold exhibited a substantial increase in both proliferation and mineralization (p < .05) of osteoblast-like cells, in stark contrast to the control group lacking phlorotannins, as well as the group that was only exposed to phlorotannins. This notable enhancement can likely be attributed to the delayed and sustained release of phlorotannin facilitated by the coated components. Supported by Yeo, Jung, and Kim (2012), the utilization of phlorotannins as supplemental bioactive agents in conjunction with scaffolds has been shown to be advantageous for promoting bone tissue growth, attributed to the ability of phlorotannin to create pronounced hydrophilic sites on the composite materials, leading to enhanced wettability.

Future perspectives and possibilities

Apart from using different materials to embed phlorotannins, various chemical/physical modifications to the vehicle can achieve more targeted delivery and controlled release (Priya, Desai, and Singhvi 2023). Some of the most widely employed approaches can be summarized as follows: PEGylation (attachment or amalgamation of polyethylene glycol), surface modifications (grafting ligands, peptides and/or proteins), as well as the addition of co-solvents or co-surfactants. Moreover, significant attention has been devoted to the manipulation of microenvironments through the incorporation of magnetic fields (Hari et al. 2023; Sanadgol and Wackerlig 2020) and/or ultrasonic fields (Wei, Cornel, and Du 2021). These modifications alter the drug delivery environment and enhance the permeability of nutrients, ultimately leading to the desired outcomes of targeted or controlled release effects. However, it should be noted that although these strategies have been applied to several terrestrial polyphenols, no research has been conducted specifically on phlorotannins.

PEGylation

The term “PEGylation” was coined to describe the covalent and non-covalent processes that attach or merge PEG polymer chains with molecules and macrostructures (Gupta et al. 2019). PEGylation offers various benefits over non-PEGylated products in enhancing the drug circulation time and improving drug stability by reducing their immunogenicity and antigenicity (Yadav and Dewangan 2021). It has also been confirmed that PEGylation enhances the permeability and retention effects due to its unrecognizable property by the reticuloendothelial system, providing a unique opportunity to target tumors following systemic administration (Kalyane et al. 2019). Regarding specific applications of polyphenols, multifunctional PEG-mesoporous silica nanoparticles (MSNPs) loaded with curcumin (CUR), a polyphenol with a long history of use in Ayurveda medicines to treat various diseases, exhibited strong fluorescence in HepG2 and HeLa cells compared to non-PEGylated MSPNs, reflecting a high accumulation of nanocarriers within cancerous cells and an enhancement in the bioavailability of CUR (Elbialy et al. 2020). Likewise, liposomal formulations of luteolin, a naturally derived polyphenol, with or without PEG modification, were studied (Sinha and PK 2019). PEGylated liposomes carrying luteolin were the most effective in inducing caspase-3 and caspase-14 protein expressions, two proteins that modulate the cell death program of human keratinocytes. Similar results were observed with liposome containing polyphenols from the red seaweed Kappaphycus alvarezii, which not only increased cellular uptake efficiency but also offered site-specific targeted delivery to overexpressed folate receptors in human adenoma MCF-7 breast cancer cells (Baskararaj et al. 2020), suggesting a promising future for phlorotannins.

Recent scrutiny has raised concerns about PEGylation, making it a topic of controversy. While PEGylation reduces interactions with the mononuclear phagocyte system, it is also expected to impede interactions with tumor cells (e.g., via a receptor), potentially affecting uptake by the target tissue (Verhoef and Anchordoquy 2013). One critical step in DDSs is the internalization of nanoparticles by cells through endocytosis. Interestingly, evidence suggests that certain PEGylated nanocarriers may not undergo effective receptor-mediated endocytosis in cancer cells within carcinoma tissue (Magarkar et al. 2012). Through free energy analysis, Shen et al. (2018) discovered that the aggregation of PEG polymers during the membrane wrapping process of PEGylated carriers introduces a significant energy penalty of approximately 800kBT. The presence of such a substantial energy barrier and the formation of a ligand-free region on the surface of PEGylated nanoparticles can hinder the membrane-wrapping process and reduce the likelihood of internalization by tumor cells. This physically-based theory of free energy transformation was also emphasized by Y. Li, Kröger, and Liu (2014), who suggested that the grafting of PEG polymers before and after the endocytosis process had an effect comparable to or even greater than the bending energy of the membrane during endocytosis. Neglecting an investigation into this free energy gap, the delivery of PEGylated nanoparticles to diseased cells cannot be predicted. Apart from that, it has been observed that linear PEGylated nanocarriers can stimulate the production of anti-PEG IgM, resulting in significant alterations in the pharmacokinetics and biological distribution after several repeated injections, a phenomenon known as “accelerated blood clearance” (Kierstead et al. 2015).

Peptide-functionalization

Peptide-functionalization, as the name suggests, involves attaching specific peptides to the surface of co-polymer nanoparticles or micelles. This approach is commonly used in DDSs, particularly in gene therapy, to enhance various desirable characteristics such as superior colloidal stability, enhanced cellular uptake efficiency, effective escape from endo/lysosomes, efficient nuclear import, and successful unpacking of DNA (Muhammad et al. 2019). By designing peptides that can recognize specific receptors or biomarkers on target cells or tissues, these modified copolymer carriers can improve drug delivery efficiency and minimize off-target effects. Sun et al. (2017) demonstrated the effectiveness of this strategy by coating MSPNs encapsulating CUR and doxorubicin (DOX) with an azido-peptide sequence composed of glycine (G), phenylalanine (F), leucine (L), histidine (H-H-H), arginine (R-R), aspartic acid (D), and serine (S). These MSPNs exhibited high loading efficiency and content of CUR and DOX, and they were selectively taken up by DOX-resistant MCF-7/ADR cells due to specific recognition triggered by the attached tetrapeptide R-G-D-S, allowing them to enter lysosomes. Additionally, another part of this sequence, H-H-H-R, was found to induce effective lysosomal membrane permeability, which is beneficial for drug escape from lysosomes, CUR-mediated inhibition of P-glycoprotein 1 (P-gp), and nuclear import of DOX into cells. P-gp is a type of membrane transporter primarily responsible for pumping foreign substances out of cells, which can limit the entry of drugs, reduce their bioavailability, and contribute to multidrug resistance. Another example is the TAT peptide, a short sequence derived from HIV, which can facilitate the delivery of various cargoes, including drugs, proteins, and genetic material, into cells. Sancini et al. (2013) reported that CUR-decorated nanoliposomes gained the ability to cross the blood-brain barrier (BBB), a selective barrier that restricts the entry of various molecules (e.g., drugs, toxins, and pathogens), after functionalization with a modified cell-penetrating TAT peptide. They observed a three-fold increase in the uptake of intact nanoliposomes by human brain capillary endothelial cells, with no adverse effects on cell viability.

Magnetic drug targeting

Superparamagnetic iron oxide nanoparticles (SPIONs), like maghemite (γ-Fe₂O₃), magnetite (Fe₃O₄) or hematite (α-Fe₂O₃) particles, offer the potential to encapsulate drugs or therapeutic agents for controlled release, hyperthermia therapy and BBB crossing, and targeted delivery when subjected to an external magnetic field (Mittal, Roy, and Gandhi 2022). Amanzadeh et al. (2019) demonstrated the benefits of quercetin-SPIONs, which significantly enhanced learning and memory in rats compared to free quercetin. This enhancement is likely due to the prolonged circulation of quercetin in the bloodstream and its increased concentration in the brain, facilitated by the incorporation of SPIONs. In addition to these improvements, Ramya et al. (2021) proposed a pH-responsive release behavior with silibinin-loaded Fe₂O₃ nanoparticles. They observed that less than 25 ± 2.2% of silibinin was released within 48 h at pH 7.4 in phosphate-buffered saline. However, more than 54 ± 1.6% of entrapped silibinin was released after a 48-h incubation at pH 4.5. This behavior is attributed to the deformation of the linkage between silibinin and Fe₂O₃ nanoparticles induced by the acidic pH, facilitating their efficient delivery to the tumor environment (pH 4.0–5.0). Furthermore, C. Li et al. (2020) optimized and prepared tea polyphenol-alginate/chitosan magnetic microcapsules by incorporating nano-Fe₃O₄ particles and magnetofluid. These microcapsules demonstrated a good slow-release effect lasting up to 100 h and exhibited superparamagnetism with extremely low magnetic retention and coercivity, which means that there is no remanent magnetization when an external magnetic field is removed, making them excellent carriers for delivering tea polyphenols in tumor endovascular embolization therapy.

In the field of seaweed bioactive compounds, magnetic MSPNs coated with a Fe₃O₄ network have been synthesized for the delivery of fucoxanthin, a carotenoid pigment derived from brown seaweeds (Feng et al. 2022), and they can be effectively guided to the tumor site with no adverse effects. Indeed, while specific research on phlorotannins may be limited, the applications and strategies discussed in the context of other compounds provide a valuable foundation for exploring the potential delivery of phlorotannins through various nanocarriers and modifications.

Conclusion

Seaweeds exhibit remarkable versatility, holding significant importance across various sectors of the global market. They function as a valuable resource for environmental conservation, economic growth, and numerous industries. Capitalizing on their rich array of macro- and micro-nutrients, seaweeds present diverse and sustainable opportunities to enhance soil fertility, serve as constituents in animal feed, act as a renewable source of bioenergy, and contribute to the improvement of water quality. Moreover, beyond their pivotal role in environmental ecosystems, seaweeds merge as highly nutritious components, offering a broad spectrum of health benefits attributable to their distinctive composition of metabolites. This review has highlighted the generalized chemical properties of phlorotannins, a group of polyphenols derived from brown seaweeds. It covers their classification, biosynthesis pathway, and quantification among different seaweed species. Phlorotannins are promising candidates in the biomedical and nutraceutical sectors that provide safe and effective alternatives to synthetic counterparts, due to their remarkable biological activities, such as antioxidant, anti-inflammatory, anti-diabetic, anti-allergy/histamine, and anti-microbial/viral, which can be the basic mechanisms of further therapeutic applications. Despite their importance, few studies have focused on the absorption of phlorotannins. Current evidence suggests they have a bioavailability similar to terrestrial polyphenols, which hinders a comprehensive understanding of their interactions and underlying mechanisms within the human gastrointestinal tract and presenting a significant challenge for their prospective applications. To address these challenges, several innovative strategies, particularly encapsulation techniques, have been proposed to protect phlorotannins against gastrointestinal tract biotransformation and degradation. Additionally, phlorotannins can be released in a controlled and targeted manner through additional modifications, with the goal of maximizing their therapeutic potential. However, further in-depth research and exploration of these strategies are crucial to unlock the full potential of phlorotannins in various applications, including pharmaceuticals, functional foods, and nutraceuticals.

Abbreviations
ABTS	=	2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid
Acetyl-CoA	=	acetyl coenzyme A
AMPK	=	AMP-activated protein kinase
BBB	=	blood-brain barrier
CAT	=	catalase
COX-2	=	cycloocygenase-2
CUR	=	curcumin
DDS	=	drug delivery system
DMBA	=	2,4-dimethoxybenzaldehyde assay
DOX	=	doxorubicin
DPPH	=	2,2-diphenyl1-picryl-hydrazine-hydrate
DW	=	dry mass
EC50	=	half maximal effective concentration
EFSA	=	the European Food Safety Authority
ERK	=	extracellular signal-regulated kinase
FAO	=	Food and Agriculture Organization
G6Pase	=	glucosde-6-phosphatase
HAE	=	hydrothermal-assisted extraction
HIV-1	=	human immunodeficiency virus type 1
Hmox-1	=	heme oxygenase-1
HPAE	=	high-pressure-assisted extraction
HPLC	=	high-performance liquid chromatography
H2O2	=	hydrogen peroxide
IC50	=	half maximal inhibitory concentration
Ig	=	immunoglobulin
IL	=	interleukin
iNOS	=	inducible nitric oxide synthase
JNK	=	c-Jun N-terminal kinase
LOX	=	lipoxygenases
LPS	=	lipopolysaccharide
MAE	=	microwave-assisted extraction
MIC	=	minimum inhibitory concentration
MDA	=	malondialdehyde
MSPN	=	mesoporous silica nanoparticle
MTT	=	2,5-diphenyl-2H-tetrazolium bromide
NADES	=	natural deep eutectic solvents
NDA	=	the Panel on Dietetic Products, Nutrition and Allergies
NF-κB	=	nuclear factor kappa B
NMR	=	nuclear magnetic resonance
NO	=	nitric oxide
NOX	=	NADPH oxidase
Nrf2	=	nuclear factor erythroid 2-related factor 2
–OH	=	hydroxyl group
PBA	=	Prussian blue assay
PEG	=	polyethylene glycol
PEO	=	poly(ethylene oxide)
PEPCK	=	phosphoenolpyruvate carboxykinase
PGD2	=	prostaglandin D2
PGE2	=	prostaglandin E2
PGH2	=	prostaglandin H2
PGE	=	phloroglucinol equivalents
P-gp	=	P-glycoprotein 1
PKB	=	protein kinase B
PKS	=	polyketide synthase
PTP1B	=	protein tyrosine phosphatase 1B
PVP	=	polyvinylpyrrolidone
ROS	=	reactive oxygen species
SOD	=	superoxide dismutase
SPION	=	superparamagnetic iron oxide nanoparticle
T2DM	=	type 2 diabetes mellitus
TNF-α	=	tumor necrosis factor alpha
UAE	=	ultrasound-assisted extraction
UMAE	=	ultrasound–microwave-assisted extraction

Acknowledgments

We would like to express our gratitude to “The Future Food Hallmark Research Initiative” at the University of Melbourne, Australia. Furthermore, we extend our appreciation to the Honours, Master’s, PhD, and Postdoctoral researchers of Dr. Hafiz Suleria’s group from the School of Agriculture, Food and Ecosystem Sciences within the Faculty of Science at the University of Melbourne for their invaluable support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was supported by the Australian Research Council [Discovery Early Career Award ARC-DECRA-DE220100055]; InnoVactiv Inc., Rimouski, QC, Canada [Project ID: 302611]; University of Melbourne [Collaborative Research Development Grant UoM-21/23].