Biostimulant potential of three Chlorophyta and their consortium: application on tomato seeds

Biostimulants are a sustainable tool for enhancing crop productivity while reducing dependency on fertilizers. This study aims to investigate the biostimulant effect of algal cell extracts and spent growth media (supernatants post biomass separation) on tomato (Solanum lycopersicum L.) seeds. Both extracts and media were obtained from three chlorophytes (Tetradesmus obliquus, Chlamydomonas reinhardtii and Auxenochlorella protothecoides) and their microalgal consortium which were grown in replete standard medium. Additionally, the consortium was cultured in diluted digestate. Three concentrations of algal extract (0.05, 0.1, and 0.2 mg mL−1) and spent medium (10%, 50%, and 100%) were applied by seed priming. Results revealed concentration-dependent biostimulant effects, with the algal species contributing moderately to the observed variability. Seeds treated with 0.1 mg mL−1 algal extract or 50% spent medium achieved up to 100% germination compared to 86% when treated with water. Additionally, they exhibited significantly greater root and shoot growth, up to 40% longer than controls, despite a delay in germination. When treatments utilized T. obliquus extract or spent medium, the overall yields were the highest. Conversely, treatments utilizing extract or spent medium from the digestate grown-consortium resulted in the lowest yields, even below control levels. This highlights the critical role growth medium plays in determining the composition of algal biomass, which in turn affects the efficacy of biostimulants. The study underscores the potential of algal-derived biostimulants in enhancing seed germination and seedling development. It also emphasizes the critical role of selecting and optimizing growth media to fully harness the benefits of algae as biostimulants.