Development of halophytes as energy feedstock by applying genetic manipulations

ABSTRACT

Decreasing arable land and fresh water resources, and increasing soil salinization and production of energy from food crops pose a threat to plant productivity and caused several environmental problems. Using plant species which can grow on saline degraded soils for energy production can be a sustainable approach because they do not compete for the agricultural lands and availability of fresh water. These plants can be cultivated with seawater without compromising their biomass for commercializing purposes such as lignocellulosic content and seed yields and could strongly benefit as energy plants. Optimizing biomass production and its economic conversion to the end product are, however, of paramount importance. The biomass of halophytes can be improved through studying unexplored aspects of these plants related to genetic manipulations. Here, we recommended current advances and highlight the key genetic approaches required in halophytes for biofuel production. Genetic approaches might lead to desired alterations in the composition of halophytic lignocellulosic biomass, including higher quantity of cellulose and hemicelluloses and lower lignin. In summary, the genetic manipulations in halophytes might lead to improvement in biomass compositions hence can be used as a feedstock for the production of biofuel on nutrient poor degraded soils.

KEYWORDS:

• Bioenergy
• desalination
• drought
• genetic manipulation
• halophytes
• lignocellulose
• salinity

Abbreviations

EIA	=	Energy Information Agency
IPCC	=	Intergovernmental Panel on Climate Change

Creeping energy crisis

The world is experiencing the energy crisis because of consumption of the limited fossil fuel assets (Amaro et al. 2012). The utilization of fossil fuel as a primary energy source is currently perceived to be unsustainable in light of exhausting assets and environmental pollution (Khan et al. 2009). According to Energy Information Agency (EIA), the world energy requirements are expected to be raised 60% more than today till 2030. If this trend continues then the world overall fossil oil stores will be depleted in less than 45 years (Ahmad et al. 2011) that might cause a shortage of energy supply for economic activity. Thusly to unravel these essential issues the advancements in renewable energy resources are necessary to improve energy efficiency and to drop energy demand by offering more opportunity for daily energy consumption.

Shortage of fresh water

Fresh water provisions are running lower throughout the world and this water emergency would worsen as the population is increasing. Most of the world’s Availability of fresh water is limited because they are mainly located in the polar icecaps and underground stream frameworks which could only supply through wells and springs. In addition, irregular precipitation, periodic rise in temperature and change in land climate interactions also presented imbalance in water availability. Global climate models predict many dry periods in future that might experience greater water stress as these regions become warmer and drier (Ward et al. 2016). Uneven rain timing, decreased access to water supplies, winds and low relative humidity caused innate dry periods over a extended duration in many countries that affects millions of people of the world (Sun et al. 2006; Grainger 2013). The growing aridity and freshwater demands build a serious need to link scientific research with optimal water management that will assist to allocate water more efficiently in extremes water scarce areas. (Salinas et al. 2016). One solution is to use saline water in sub-tropical habitats for agricultural purposes instead of fresh water. Use of saline water impairs more than 45 million hectares of irrigated land (Munns and Tester 2008). Fresh water can be generated by using largely available saline water through the process of desalination. Numerous nuclear powered desalination plants are present; though the cost is higher especially for poor developing countries in the world. It will create the transporting cost to transfer huge amounts of desalinated seawater in to the plant (Hinrichsen and Tacio 2002). Zhoua and Tolb (2005) estimated that the cost of desalination is equal to the cost required to transport it over 1600 km or raise the water up to 2000 km. Somewhat high and far places have higher desalination costs while other places cost is higher due to transport rather than desalination. Desalinated water might be an answer for some water-push districts, however not designed for spots that are poorer, somewhere down in the inside of a landmass, or at elevated rise. Tragically, that incorporates a portion of the areas with greatest water issues. Another potential issue with desalination is the creation of salt water, which can be a noteworthy reason for marine contamination when drained once again into the sea at elevated temperature (Zhou and Tol 2005). Limitation of water or presence of salty water both can decrease biomass of economic important and might cause shortage in food supply.

Food security

Lack of affordable nutrition by conventional channels has been a serious issue of experience malnutrition and starvation on daily basis especially in third world countries. Many scientists recommended that energy made from edible plants for transport is not appropriate (Nouairi et al. 2006).

Food crops such as sugarcane, wheat, maize and corn are the most efficient energy crops in current scenario (Cherubini et al. 2009). These edible crops that are grown and processed for bioenergy will have huge impact on food supply and demand system. This is particularly fact that the cultivation of these dedicated food crops for the purpose of bioenergy rather than food purposes only. The utilization of prime agricultural lands which was patents to feed the peoples in previous decades sharply boasting the food prices up to the limit which can harm life of common people (Colombo and Onorati 2013). Growing food crops for bioenergy instead of those for food products really invented the huge contention between food and fuel. In order to represent and provides relaxation to extensive utilization for food-based feedstock crops to biofuel raises emerging technology that convert non-edible grasses and forest products. These are termed as second generation biofuel feedstocks plant productivity changes are still held to baseline levels. These second generation feedstocks for biofuel still can pop up the land compactions against food crops can harm food prices and food security (Flora et al. 2012).

Now the whole world is searching for more sustainable and environmental friendly crops for the biofuels otherwise the image of fuels production from plants can get bad views in the world regarding environmental and food shortage issues. The compatible solution for all the food curse issues may the plants which cannot compete to use the cultivated lands for their survival. Those plants which can grow and complete their life events on marginal soil, saline soil and other then the prime agricultural lands. On the basis of these plants, we may solve dilemma of food vs fuel.

Halophytic crops for a saline agriculture and bioenergy

A consistent supply of reliable feedstock regarding quantity and quality of plant biomass is required for the monetary viability of the biofuel industry. The plants specific to saline environments and can survive and complete their life cycles, no less than, 200 mM NaCl are called halophytes (Grigore et al. 2014). However, many can develop at salt concentration significantly higher than that of seawater (Rabie and Almadini 2005). Salt resistance plants cover around 0.25% of angiosperm species that represent approximately 600 taxa of plants consist of genera and families (Fita et al. 2015). Halophytes are highlighted as wild plants but huge numbers of halophytes can possibly be changed into valuable ‘new crops’, after extensive a domestication procedure. They are already salt resistant plants – which is the most essential and the most difficult quality to introduce and manipulate- it ought to be moderately simple to practice particular breeding programs to quickly enhance the required agronomic attributes of the most encouraging halophytic taxa. For specific development conditions, it might be important to choose the best genotypes, by killing or possibly decreasing the substance of dangerous mixes or hostile to supplements, by expanding yields, or by enhancing showcasing attributes (time span of usability, market accessibility, consistency of the item in size, shading, taste, and so forth), and to modify general farming techniques to specific crops (Baram and Bourrier 2011). Individuals of halophytes have been collected from nature for hundreds of years. Sometimes they are pickled or cooked, or their leaves are regularly eaten as raw vegetables and salad, or sold in local markets (Nyankanga et al. 2012). The conventional use as sustenance of these species will make them all the more effectively worthy by the overall population, with the goal that they are fitting possibility to be tamed and changed into verdant vegetable products for saline agribusiness. Halophytes are exceptionally nutritious alongside eatable trademark; for the most part they are rich in basic supplements – minerals, vitamins, amino acids, as well as unsaturated fats, protein and antioxidants (Fita et al. 2015). These plants can grow under seawater and many of them are cultivated and domesticated as vegetable crops due to their salt resistance ability. For instance Salicornia and Sarcocornia are two suitable halophytes attracted due to their utilization in food purposes (Fita et al. 2015). Numerous halophytes can be potential oilseed plant to their chemical composition (Abideen et al. 2015). For instance, Salicornia bigelovii is fascinating because of its seed production, approximately two tons per hectare every year, which are similar to those of oilseed harvests of conventional species such as soybean. The chemical composition of these halophytes are acceptable in term of their seed total protein (30%), oil contents (30%), and the oil contains of these plants composed of a high percentage of polyunsaturated fatty acids (70% of linoleic acid) so that it can be considered as promising edible oil. Chenopodium quinoa also need to be focused regarding food nutrient (Ladeiro 2012). Seeds of S. salsa can be used as a raw material to produce fatty acid methyl esters from their high oil contents, about 97% of S. salsa oil can be processed to fatty acid methyl esters (Piloto-Rodríguez et al. 2014). Substantial scale advancement and modern production of halophytes could be received as another agribusiness alternative by using reasonable areas along coastal areas. There is a requirement for the efficient investigation of halophytes to screen out high value industrial species and their rearing for desirable plant attributes Many halophytic species including Phragmites karka, Halopyrum mucronatum, Panicum antidotale and Desmostachya bipinnata has been reported for promising lignocellulosic content (Table 1) (Panta et al. 2014).

Halophytes are reported as a sustainable source of bioethanol production in many studies. The production of ethanol from plant biomass depends on cellulose, hemicelluose and lignin content of a potential fast growing species. Presence of higher lignin (30–40%) in energy feedstock ensure resistant to their sequential conversion from dry biomass to fermentable sugars and in to ethanol. It was reported earlier studies that five promising plants such as Phragmites karka, Panicum antidotale, Desmostchya bipinnata, Halophyrum mucronatum and Typha domengesis (Figure 1) showed exceptionally optimal ratios of cellulose, hemicellouse and lignin and in some cases better than our food crops. There are also some plant species those showed lower cellulose and hemicellulose content such as Dichanthium annulatum, Sporobolus ioclados, Aerva javanica and Arthrocnemum indicum (Tables 1 and 2). A recent study assessed Juncus maritimus a salt marsh plant that can be used for producing lignocellulosic biomass, because its total carbohydrate content can reach up to 73%, with cellulose and hemicellulose representing approximately 41% and 31%, respectively, of the lignocellulose biomass (Smichi et al. 2016). Tamarix aphylla plants were irrigated from domestic sewage (EC approximately 3 dS/m⁻¹) to different salinity levels or with brine (EC approximately 7–10 dS/m⁻¹), produced 52 and 26 t/ha, respectively, of organic biomass. Tamarix was selected as a biofuel plants for its higher cellulose and low hemicellulose and polyphenol contents, properties particularly suitable for bioethanol production, because the species of yeast commonly used for fermentation prefer C6 − sugars to C5 − sugars (Calvin 1980; Santi et al. 2015). The variation in lignocellulosic contents and oil yields may be due to the harvesting stage, different plant species, cultivation time, climate change, ripening stage of plant, and/or extraction method of chemical analysis. Considerable genetic variability exists among halophytic genotypes (Llanes et al. 2011) and genotypes of some populations may perform better in suboptimal conditions (Maron et al. 2004). Now a day’s modern genetic manipulation tools are also in operation to enhance biomass of salt and the drought resistance plants to produce higher biomass which can be useful for bioenergy.

Figure 1. Some halophytes that can be a potential candidate for biofuel production.

Table 1. Ligno-cellulosic contents of halophytic biomass (% dry weight).

	Species	Cellulose	Hemicellulose	Lignin
	Grass halophytes^a
1	Aeluropus lagopoides	26.67	29.33	7.67
2	Cenchrus ciliaris	22.67	23.17	7
3	Chloris baraeta	25.33	23	8.33
4	D. bipinnata	26.67	24.68	6.67
5	D. annulatum	19	24.33	7
6	Eleusine indica	22	29.67	7
7	H. mucronatum	37	28.67	5
8	Lasiurus scindicus	24.67	29.67	6
9	Panicum turgidum	28	27.97	6
10	Paspalum paspaloides	20.33	32	2.33
11	P. karka	26	29	10.33
12	S. ioclados	15.33	30.67	2
13	Urochondra setulosa	25.33	25	6.33
14	Typha domingensis	26.33	38.67	4.67
	Non grass halophytes^a
1	A. javanica	15.67	13.33	6.33
2	A. indicum	11.33	13	7
3	Calotropis procera	12.33	11	5
4	Ipomea pescaprea	12.67	17	5.33
5	Salsola imbricata	9	18.33	2.67
6	Salvadora persica	22	13.33	7
7	Suaeda fruticosa	8.67	21	4.67
8	Sueda monoica	10.67	11.33	2.33
9	Tamarix indica	12.17	24.67	3.33
	Conventional Species^b
1	Panicum virgatum	45	31.4	12
2	Popolar	40	23	20
3	Cynodon dactylon	25	35.7	6.4

^aAbideen et al. (2011).

^bKarp and Shield (2008), Reshamwala et al. (1995).

Table 2. Oil content, Saponification number (SN) Iodine value (IV) Cetane number (CN) of fatty acid methylesters of some halophytes seeds oils.

Name of species	Oil content %	SN	IV	CN
Alhagi marorum	21.9	202.31	97.31	51.38
Allenrolfea occidentialis	14	193.44	121.36	47.21
Arthrocnemum macrostachyum	25	205.66	115.5	46.86
Atriplex bunge	15.8	194.89	75.29	57.35
Atriplex rosea	12.9	194.05	115.5	49.33
Cressa cretica	23.3	203.5	114.27	47.41
Halogeton glomeratus	24.7	199.82	103.47	50.31
H. mucronatum	22.7	202.95	124.63	45.15
Haloxylon stocksii	23.3	203.27	121.71	45.77
Kochia scoparia	9.7	200.56	138.69	42.31
Kosteletzkya virginica	17.54	205.99	113.14	47.37
Sacrobatus vermiculatus	17.5	199.22	139.84	42.23
S. bigelovii	29.7	200.86	154.32	38.78
Salicornia brachiata	22.4	129.89	29.7	81.67
S. europaea	30	202.55	155.96	38.19
Salicornia fruticosa	25.98	204.38	93.68	51.96
Sarcocornia ambigua	13	187.19	106.66	51.49
Suaeda fruticosa	25.98	204.38	93.68	51.96
Suaeda aralocaspica	29.92	198.7	146.59	40.82
Suaeda salsa	25.99	171.1	133.9	48.1
Sueda torreyana	25.25	202.02	154.77	38.49

Source: Abideen et al. (2015).

Genetic manipulation of biosynthetic pathways of halophytes to enhance bioethanol production

It is essential to outline the most suitable approach to convert plant complex polysaccharides into the sugars and their conversion to ethanol by fermentation for biofuel production. If cell wall degrading enzymes (hemicellulase and cellulase) production can be induced inside the growing cell through bioengineering of the crop, then the reliance on microbial bioreactors for enzymes will be reduced and the production costs could be efficient (Balan 2014). Cost optimization can be accomplished by engineering of lignin biosynthetic pathway in plants. These measures can substantially reduce the cost of pretreatments but the plants those already have lower lignin may be more cost effective in hydrolysis of biomass. The yield of cellulosic biofuel can be enhance by up regulation of cellulose biosynthesis pathway enzymes to support higher polysaccharide production (Bita and Gerats 2013). It is reported earlier that adaptable polymerization is possible in plants and mono-lignins can be substituted by polyphenols (Wymelenberg et al. 2006). These above describe procedure may not suffer the advancement procedure of plants, but will increase extraction of important saccharides which is critical for the energy production from the plants. Similar approach can be applied to water hyacinth and many other bioenergy feedstocks which is composed of lower lignin content and might enhance extraction of saccharides (Lukuyu et al. 2014). Vanden Wymelenberg et al. (2006) revealed a substantial number of genes involved in breakdown of lignin from the fungus Phanerochaetechrysosporium genome. Accordingly, expression of such genes in water hyacinth may also reduce the cost of biofuel. The utilization of transgenes could support already existing breeding methods to enhance biofuel production through accessibility of suitable genes to modify synthesis of sugars, lignin and lipids in plants.

Finding an operative method for tissue culturing and transformation of water hyacinth cultivars is the initial step. Tissue culture specifies the strategies and procedures accessible for cultivating a huge number of cells in controlled and axenic environment. Cell totipotency that is to regenerate intact generally as fertile plants, that depends on the expression of developmental genes is the basic principle of tissue culturing. The last are typically heritable, in light of the tissue culture conditions. Hence, numerous permutations of plant growth regulators are needed to be evaluated for plant organogenesis and regeneration (Hassanein and Soltan 2000). In order to use the tissue culture as a baseline for genetic transformation, this is a crucial pre-requisite. In biolistic gene delivery and Agrobacterium tumefaciens-mediation tissue culturing techniques this is a key component needed (Bhatia et al. 2004). Mass multiplication of plants and micropropagation is a major application of tissue culturing. It offers another method for acquiring particular pathogen free plants from meristem culture, hereditary control, and the generation of haploid plants. In the propagation of various plant species the Micropropagation has been applied broadly. The components that are responsible for the success of a tissue culture system includes composition of the growth media, plant species, availability of utilizable carbon, growth regulators, the axenic culture conditions and callus induction response. Some additional factors are carbon source, trace elements, plant growth regulators, inorganic salts and organic compounds (Bhattacharya and Kumar 2010). Tissue culture of halophytes including S. bigelovii (Lee et al. 1992), Leymus chinensis T, Suaeda maritima has been explored recently. There are many halophytes that show potential in biofuel production but still needs careful studies to improve their yield by genetic transformation.

Genetic transformation can enhance oil content of halophyte

Knockout and overexpression strategies have been applied lately to clarify the genes role in lipid synthesis and accumulation in order to enhance the Arabidopsis thaliana, rapeseed (Brassica napus) and soy bean (Glycine max), plants lipid contents. These above described studies might improve quality and quantity triacylglycerols of seed oil and other plant organs. Ohlrogge and Jaworski (1997) that seed might be preprogrammed to produce the particulate amount of fatty acids (Ohlrogge and Jaworski 1997); substantial efforts have been made to make the expression of pathways of fatty acid synthesizing enzymes.

The initial step in synthesis of fatty acid in many organisms is catalyzed by acetyl-CoA carboxylase (ACCase) is the exchange of acetyl-coenzyme A (CoA) to malonyl-CoA. Nevertheless, significant efforts were made to enhance lipid content in a range of systems by utilizing ACCase overexpression which seems somehow disappointing. Insignificant increase in seed lipid content 6% (384 mg g⁻¹ and 408 mg g⁻¹ dry weight for wild-type and transgenic ACCase rapeseed lines, respectively) was noted by the overexpression of ACCase in the oleaginous seeds of B. napus. In lipid biosynthesis the ACCase levels are a restricting stride for the most part in cells that ordinarily don’t store a lot of lipid (Figure 2). Another endeavor to expand the lipid production involving unsaturated fat amalgamation through 3-ketoacyl-acyl-transporter protein synthase III (KASIII) was made but was not much effective (Dehesh et al. 2001). Many fascinating results have been performed through the overexpression of genes required in TAG assembly. A 40% increase in lipid content was achieved through the overexpression of cytosolic yeast, glycerol-3-phosphate dehydrogenase (G3PDH), in the seeds of B. napus (Vigeolas et al. 2007). Synthesis of glycerol-3-phosphate which is needed for TAG formation is catalyzed by the G₃PDH. Overall seed oil production thus is also dependent somewhat on genes involved in TAG assembly. This finding was further strengthening by studies on the correlation of TAG assembly genes overexpression and increase in seed oil content. Studies (Jain et al. 2000; Jako et al. 2001; Taylor et al. 2002; Lardizabal et al. 2008) reported significant increase in plant lipid productivity by overexpressing the TAG assembly genes (lysophosphatidic acid acyltransferase, glycerol-3-phosphate acyltransferase or diacylglycerol acyltransferase). Due to the fact that enzymes seem to be good candidates for overexpression strategies to increase storage lipid contents, an attempt has also been made to use directed evolution for increasing the efficiency of DAGAT enzymes (Siloto et al. 2009).

Figure 2. Mechanism pathways for drought tolerance in B. napus.

Genetic manipulation in halophytes for the biofuel productivity can be introduced for two purposes (a) to increase lipids contents (b) to improve oil quality. Recently researchers have observed that high levels of NaCl enhance membrane lipids’ unsaturated fatty acid in halophytes. α-linolenic acid 18:3 is the main fatty acid enhance under NaCl stress in halophytes.

Stress resistance ability of transgenic tobacco plant at water deficit and salinity was improved through x-3 desaturases (Sui and Han 2014; Zhang et al. 2005), which infers that the drought and salt resistance of plant is dependent on the unsaturated fatty acids (Berberich et al. 1998; Mikami and Murata 2003). This concept was further strengthen by the lower of the x-3 and x-6 desaturase activity in Synechocystis mutants (Allakhverdiev et al. 2001). In another study an increased tolerance to low temperature and salt stress was observed when x-6 desaturase sunflower gene were transformed in yeast cells. Three sorts of halophytes were noticed which increment their resilience to salt stress through expanding or keeping up their unsaturated fats composition. Possible clarification about role of higher unsaturated fatty acid is linked with the membrane (Na+ or K+) ion channels and Na+/H+ antiporter frameworks stability under stress situations. Higher quantity of unsaturated fatty acids robust the membrane fluidity, and triggers the activity of Na+/H+ antiporter and H+-ATPase to protect the photosynthesis apparatus and ultimately increase carboxylation efficiency. It was also reported that with the change in membrane fluidity can activate certain membrane bound enzymes which can alter physiological responses of plants under suboptimal conditions (Zhang et al. 2012). According to Sui and Han (Sui and Han 2014) to enhance membrane fluidity for the ion compartments, it’s probable that euhalophytes for example, S. salsa required additional un-saturated fatty acids. It is also pragmatic that at lower temperature and light intensity favors the synthesis of unsaturated fatty acids protect photosynthesis apparatus by reducing the photoinhibition and photodamage of PSII and PSI (Sui et al. 2007). Plant seedlings of Solanum tuberosum L. was transformed with the desA encoding gene Δ12 acyl-lipid desaturase in to another organism cyanobacterium Synechocystissp. PCC 6803. In this situation sequence of genes was translationally fuzed with the sequence of the reporter gene encoding to thermostable lichenase to analyze the efficiency of this gene expression in plant. Interestingly the lichenase retained the thermostability and its activity within the hybrid protein observed by the comparison of hybrid and native gene expression however another enzyme named as desaturase started inserting the double bond in fatty acid chains to amend their composition in membrane lipids. (Maali-Amiri et al. 2007). The shoots enclosed higher linoleic acid (39–73%) and linolenic acid (12–41%) in most transformed plants. In comparison to wild-type plants, the total absolute unsaturated FAs content was (20–42%) higher in transformants. When severely cooled to −7°C wild-type plants increased their membrane lipid substantially by 25% whereas in under such conditions the membrane lipid peroxidation rate was not increased in transformed plants. The reason for the higher resistance to lower temperatures and the oxidative injury could have induced by hypothermia (Maali-Amiri et al. 2007).

Gas–liquid chromatography (GLC) technique was used for the oil compositions (qualitative and quantitative) of esterified fatty acids (FAs) in the total fat content of halophytic plants such as Salicornia europaea, Artemisia lerchiana and Suaedaaltissima collected in their natural environments. GLC results showed that the vegetative tissues of these halophytes contained 16 very-long-chain FAs among total 24 FA species. Around four very-long-chain FAs groups were C20, C21, C22, and C23, each including saturated, mono-, and di-unsaturated components; C24 and C25 FAs were also present. The concentration of VLCFAs in the total FAs comprised 4–64%. In vegetative organs of higher plants not subjected to genetic transformation, such a high VLCFA content was found for the first time. Saturated and even-numbered components predominated among the VLCFAs, and the roots exceeded several fold the above-ground organs in the total VLCFA content. Possible pathways of VLCFA biosynthesis in plants, VLCFA content in the vegetative tissues, and the physiological role of membrane lipid FA composition in the plant salt metabolism are discussed (Ivanova et al. 2009).

The study by Ramani et al. (2004) focused on sulfolipids role and salt resistance mechanisms of halophytes including Sesuvium portulacastrum, Aster tripolium L., members of Compositae and L., Aizoaceae families, and glycophyte A. thaliana (L.) Heynh, Brassicaceae. The sulfolipid contents of Sesuvium and Aster increased significantly at 517 mM or 864 mM salt stress conditions. At up to 100 mM NaCl changes in sulfolipid contents were not observed in Arabidopsis. Whereas with an increase in NaCl concentration of Aster modified the fatty acid profile of sulfoquinovosyldiacyl glycerol. The presence of 16:0/18:3 and 18:3/18:3 molecules was confirmed through sulfolipids quantification performed by LC-MS from Aster and Sesuvium.

The sulfolipid content improved substantially in the presence of NaCl in Crithmum maritimum halophyte. Plants treated with salinity were similar in fatty acid composition of sulfolipids, except for linolenic acids and linoleic composition.

There was a significant decline in sulfolipids content under NaCl-treated plants in the unsaturated fatty acid (C18:3) composition as compared to the control plants, whereas the percentage of unsaturated fatty acids (C18:2) increase analogously (Ben Hamed et al. 2005). In a conclusion from the above studies it is found that the sulfolipds are an important aspect of strategy in salt tolerance of halophytes. Nouairi et al. (2006) carried out the experiments on young small-sized hydroponically grown S. portulacastrum and aseptically germinated seeds of Mesembryanthemum crystallinum. They exposed these halophytes to 0, 50, 100 and 200 μM of cadmium concentration for four weeks. The effect of cadmium on fatty acid composition and leaf lipid contents of S. portulacastrum and M. crystallinum were studied recently. It was found that the total lipids (TL) contents as well as lipid fractions such as neutral lipids (NL) galactolipids (GL) and phospholipids (PL) reduced more in M. crystallinum as compared to S. portulacastrum at 200 μM cadmium concentration. Additionally there was no noteworthy changes in the aggregate unsaturated fat composition of S. portulacastrum leaves. However, an increase in the di-unsaturated fatty acid (C18:2) and decrease of the tri-unsaturated fatty acid (C18:3) was observed in M. crystallinum leaves. These changes in fatty acid composition interestingly block the sugar transport to leaves and conversion pathways but resulted in higher oil production and accumulation in these plants (Zhai et al. 2017). Another future option would be to dissolve lignocellulose material from halophytes in saline ionic liquids, which are well established as alternative and ‘green’ solvents to be used in the pre-treatment of the walls of plant cells prior to enzymatic hydrolysis (Gunny et al. 2014). Lignocellulosic biomass from halophytes for ethanol production proved advantageous both in term of high net productivity and low maintenance costs (Fooladvand and Fazelinasab 2014). Considering the advantages of second-generation biofuels, it is recommended that biofuel production be increased up to 10–20 EJ a year by 2050 (Searle and Malins 2015) and the share of biofuels in the transport sector be increased from 3% to 8% worldwide between 2013 and 2035.

Conclusions

Halophytes have a strong potential of biofuel production as there are multiple feedstocks which could be exploited industrially at lower cost. Exploration of halophytes to produce biofuel can reduce the dependence on conventional fuels in the world and can discover some environmental benefits. However, biofuel productions from some halophytes stocks need special attention to improve their biomass for higher yields by using genetic manipulations. A suitable composition of lignocellulosic biomass (higher cellulose and hemi cellulose but lower lignin content) induce through genetic manipulations manifest the potential of halophytes as a compatible biofuel candidate to existing edible feedstock. These plant not only avoids competition with water and land meant for production of edible crops because of its ability to grow in soils with salinity but will also help in CO₂ sequestration and reclaiming degraded lands.

Disclosure statement

No potential conflict of interest was reported by the authors.