Chloroidium sp. and Tetradesmus obliquus biomass for plant biostimulants: Extraction, characterization and bioactivity on seeds development

Microalgal biomass has emerged as a promising feedstock for producing sustainable plant biostimulants (PBs), particularly when cultivating microalgae using residual nutrients and CO 2 sources. This study focuses on the extraction of PBs from Chloroidium sp. and Tetradesmus obliquus microalgae cultivated in anaerobic digestion liquid digestate and their impact on seeds development. Alcalase enzymes-assisted extraction at 50 °C emerged as the most efficient method, achieving protein extraction yields of 47.78 ± 1.94% (equivalent to 16.0 ± 0.6% of the biomass dry weight) for Chloroidium sp., and 16.24 ± 1.13% (equivalent to 8.1 ± 0.6% of the biomass dry weigh) for T. obliquus . The influence of protein or peptide size on the potential of the PBs was evaluated using three fractions of the extracts: < 3 kDa, 3-100 kDa, and >100 kDa. Optimized extracts were evaluated on mustard and wheat seeds, revealing a dose-dependent relationship between the potential of the PBs and their molecular sizes. Interestingly, the biostimulant activity of T. obliquus extracts on mustard was attributed to the medium-sized fraction. In contrast, Chloroidium sp. extracts required a synergistic interaction between different molecular sizes to enhance root growth across both seed species. This study reveals species-dependent biological activity of microalgae-based PBs.