Algae-based extracts as a natural biostimulant for plant growth and development: Current and future prospects

A photobioreactor (PBR) is a bioreactor utilizing a light source to cultivate phototrophic microorganisms that produce biomass by using light and CO2; these phototrophic microorganisms include plants, micro- and macroalgae, mosses, cyanobacteria and purple bacteria. Micro- and macroalgae are grown naturally in sea water (an economical approach) or brackish water and in fresh water. Global production of marine algal monocultures is primarily restricted to a limited number of species, including rapidly growing plants (e.g., Chlorella, Nannochloropsis and Scenedesmus) and extremophiles (e.g., Arthrospira and Dunaliella). Given their innate resistance to competitors and predators, the above species can be cultivated naturally in open systems, such as ponds, bubble columns and stirred tanks. However, most of the species that comprise the bulk of microalgal biodiversity are not easy to farm because of slow growth rates and fragility of their cell wall. As a result, slow-growing species must be cultivated under specific conditions in closed systems to ensure that they would dominate, whereas cultivation of typical species requires unique precautions to elude forces that will possibly cause stress to cells, especially surface tension and centrifugal and shear forces. Researchers currently focus on the use of PBRs (e.g., open, selective or closed system) to produce marine algae by utilising a light source for abundant biomass accumulation. Exploitation of marine algae is a new offshoot of fields essential to humanity, particularly food, health, energy and environment. This study focuses on the current and future prospects in the application of marine algae or their extracts to promote growth and development of plants. This study shows that the algae extracts demonstrated characteristics similar to those of major plant growth stimulants, namely, auxins, cytokinins, gibberellins, abscisic acid, ethylene, secondary metabolites, brassinosteroids, jasmonates and strigolactones (recently identified). We examined the effects of various marine macroalgae extracts on seed germination and growth of Solanum trilobatum and Eleusine coracana, as well as assessed the benefits of hydroponic treatments, seed priming and soil drenches. The application of marine algal extracts during in vitro plant regeneration are beneficial in embryogenic callus induction, somatic embryogenesis, enhancement of regeneration followed by high frequency of rooting, resistance to freezing, enhancement of photosynthetic activity, increase in crop and fruit yields and resistance to biotic (bacteria, fungi and virus) and abiotic (drought and salt) factors. Marine algae or algae-based extracts remain predominantly unexploited worldwide, and this paper highlights a few of the potential approaches to be used in research and development in this field.