Plant growth-promoting properties of extracts produced by fermenting the freshwater macroalga, Oedogonium intermedium

Abstract Extracts produced from marine macroalgal biomass promote plant growth, improve yield, and increase the tolerance of plants to physiological stressors. Most of these extracts are produced with dried macroalgal biomass as their stock material, and use processing techniques that cause considerable modification to the original biomass and require a substantial investment in energy and equipment. As such, the aim of this study was to determine whether the freshwater macroalga Oedogonium intermedium could be successfully fermented using freshly harvested biomass, such that this biomass was transformed into a stable, non-offensive product that had plant growth-promoting properties. This was demonstrated by manipulating the starting conditions (presence or absence of salinity, molasses, and microbes) of the fermentations. The liquid extracts had a positive effect on the growth of mung bean plants compared with the positive controls – a soluble plant fertiliser and a commercially available fermented seaweed product. The most effective extract (Oedo 4), which was fermented in 20 ppt saltwater with the addition of molasses, demonstrated a 28.9–48.6% larger total plant dry weight than the commercial controls, respectively. Notably, extracts fermented in 20 ppt saltwater resulted in larger quantities of N, P, K, Ca, and Fe than those fermented in freshwater, while extracts fermented with molasses had the lowest final pH (