Distinct biochemical signatures of microalgal extracts drive species-specific biostimulant activities in tomato and lettuce

Bio-based biostimulants, including those derived from microalgae, are central to sustainable agriculture, but their efficacy is influenced by environmental and biological factors, such as the target plant species and the microalgal source. In order to define more reliable biostimulant formulations, it is necessary to understand how the diversity of the biochemical composition of microalgae affects plant growth. This study examined whether different microalgal species, grown and tested under standardized conditions, yield extracts with distinct biochemical signatures that elicit predictable plant responses. Four species (i.e., Chlamydomonas pitschimanii, Chlorella vulgaris, Chromochloris zofingiensis, and Scenedesmus vacuolatus) were grown under controlled conditions, and their aqueous extracts were tested at three concentrations (1, 2, and 4 mL L−1) on lettuce (Lactuca sativa L.) and tomato (Solanum lycopersicum L.) in vitro. Metabolite profiling revealed distinct signatures: carbohydrate-rich C. zofingiensis consistently enhanced shoot biomass, while amino acid– and polyphenol–rich C. vulgaris induced dose-dependent phytotoxicity. C. pitschimanii exhibited hormesis, promoting root architecture at low doses but inhibiting growth at high doses, and S. vacuolatus acted as a potent lettuce biostimulant with negligible effect on tomatoes. These findings demonstrate that biostimulant efficacy is not generic but emerges from a three-way interaction between extract chemistry, dose, and crop species. This work contributes to moving biostimulant development from empirical trials towards a more predictive framework, supporting the design of tailored formulations that maximize benefits and minimize risk, an essential step towards next-generation solutions for sustainable horticulture.