Selecting extraction conditions for the production of liquid biostimulants from the freshwater macroalga Oedogonium intermedium

Liquid biostimulants made from macroalgae have traditionally been based on the processing of brown seaweeds collected from the wild. In this study, liquid extracts were produced from the freshwater green macroalga Oedogonium intermedium, cultivated in a land-based system. The first part of the study was aimed at selecting extraction conditions to maximize the liquid yield and the recovery of soluble solids from Oedogonium biomass, which resulted in the selection of a biomass-to-solvent ratio of 5% (w/v), an extraction time of 4 h, and a temperature of 70 °C. These extraction conditions were used in the second part of the study to assess the effect of extraction solvent, across the whole pH range, on root growth of tomato and mung bean seedlings. Root growth of tomato seedlings was highest for the acid extract prepared with 0.05 M HCl, and the alkali extract prepared with 0.01 M KOH. Therefore, these two extracts were further tested in a mung bean assay to determine their auxin-like activity, and were also analyzed to determine their elemental and hormone profiles. The selected acid and alkali extracts, at concentrations of 10% and 2–10% (v/v), respectively, had an auxin-like activity equivalent to 5 × 10−5 M indole-3-butyric acid, with the alkali extract having the highest concentration of auxins (120 ng IAA g−1 dry weight). This study highlights the potential of O. intermedium for the production of liquid extracts with biostimulant activity