Photosynthetic microorganisms, an overview of their biostimulant effects on plants and perspectives for space agriculture

Figures & data

Table 1. Search and number of results in the PubMed, Google Scholar and Scopus databases on the 5th of July 2023.

Search terms	Number of Google scholar results	Number of Pubmed results	Number of scopus results
Biostimulant	29,300	2,719	2,165
Biostimulant microorganisms	18,100	434	231
Photosynthetic microorganisms biostimulant	14,800	15	16
Plant growth promoting bacteria	2,140,000	11,263	10,908
Plant growth promoting rhizobacteria	102,000	2,341	6,267
Cyanobacteria biostimulants	2,880	22	34
Bacteria biostimulant	19,400	1,119	293
Algae biostimulant	11,200	58	175

Figure 1. Biostimulant application impact on the plant depending on their nature and their application method. PGPB = Plant Growth Promoting Bacteria; PGPR = Plant Growth Promoting Rhizobacteria.

Table 2. Effects of different cyanobacterial metabolites on crop productions, from their cellular targets in plants to whole plant physiological functions.

Metabolites categories	Metabolites name	Cellular mechanism activated in plants	Physiological function on whole plant	Interest for crop performance (effects on yields)	References
Phytohormones	Auxin, gibberellins, cytokinins	Release and activation of growth hormones (e.g. auxin) pathways is responsible of roots and shoots morphogenesis	Increased lateral root density, surface of root hairs and shoot length	Increased root foraging capacity, enhancing nutrient use efficiency and then plant growth	(Dobbelaere et al. 1999; Rodríguez et al. 2006; du Jardin 2015; Santini et al. 2021)
Vitamins	Riboflavin (vitamin B2), ascorbic acid (vitamin C), thiamine (vitamin B1), cobalamin (vitamin B12), pyridoxine (vitamin B6)	Worked as signalling molecules to promote plant growth	Vitamins help reduce stress and increase the immunity of plants against disease and rough climate by activating various enzymatic pathways	Increase biotic and abiotic stress tolerance	(Rodríguez et al. 2006; Havaux et al. 2009; Goyer 2010; Singh 2014)
Volatile organic compound (VOC)	Dimethyl-sulfide, alkanes, alkenes, alcohols, aldehydes, ketones, furans, sulfides, and halocarbons, benzenoids, pyrazines, terpenes	Enhanced photosynthetic CO₂ fixation signalling mediated by ABA and cytokinin Phytohormone shifts are responsible in roots morphogenesis	Modulation of root system architecture, root hair formation and chlorophyll accumulation	Promote photosynthesis, induce systemic drought tolerance, improve water and nutrient acquisition	(Liu et al. 2021; Gámez-Arcas et al. 2022)
Photoprotective metabolites, antioxidants, pigments and carbohydrates	Mycosporine-like amino acids and scytonemin, exopolysaccharides	Activation of scavengers of reactive oxygen	Maintenance of leaf photosynthetic activity under light stress	Increased crop tolerance to abiotic stresses allowing a better growth under stressed conditions	(Kultschar et al. 2018)
Indole alkaloids	Ergot, iboga, pyrroloindole, carboline, aspidosperma and strychnos alkaloids, hapalindoles and fischerindoles	Production of natural toxins (e.g. saxitoxins, anatoxin-a, cylindrospermopsin, Cyanopeptinolin, microcystin-LR) Act as protease inhibitors, which are cyclic depsipeptide derivatives thought to be elastase inhibitors	Antimicrobial activities against gram negative and positive bacteria, insecticidal, antifungal and antialgae activities	Improve crop health, avoid infections from bacteria, fungi and insects’ attacks	(Kulik, 1995; Gademann and Portmann 2008; Walton and Berry 2016; Kultschar et al. 2018)
Fertilisors, NPK (Nitrogen, Phosphorus and Potassium) and iron chelators	Schizokinen, synechobactin and anachelin	Iron chelators form stable complexes with metal micronutrients, due to the presence in their structure of oxygen-, nitrogen- and sulfur-containing functional groups. Then responsible in maintaining micronutrients available for plants uptakes	Iron is necessary in trace quantity for photosynthesis, cell wall metabolism, and respiratory electron transport chain and provides protection to the plants against various oxidative stress	Improve crop health	(Kultschar et al. 2018; Zanin et al. 2019)

Figure 2. Schematic view of different types of indoor farming. (A) Hydroponics, (B) Aeroponics, (C) Aquaponics, (D) In soil growth with watering system on the soil or by spraying the leaves.

Figure 3. Impact of influencing parameters on plants on Earth and other planetary bodies. Impact is represented by a range of colour: green = positive impact; orange = mid negative impact; red = negative impact; grey = neutral, impact not applicable in this case (Space Settlement environment image from SpaceX (FECHT 2023)).

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