Metabolic profiling reveals distinct modes of action of microalgal biostimulants in lettuce and tomato

Microalgal extracts are increasingly recognized as effective plant biostimulants, yet the physiological mechanisms driving their growth-promoting effects are only partially resolved. To characterize these modes of action, this study evaluated the metabolic reprogramming induced by four chemically distinct microalgal extracts in Lactuca sativa (lettuce) and Solanum lycopersicum (tomato). Plants were grown in vitro to isolate plant-extract effects from rhizosphere interactions. Targeted metabolic profiling revealed that the extracts acted as species-specific metabolic modulators rather than simple fertilizers. In lettuce, Chlorella vulgaris and Chlamydomonas pitschimanii elicited a defense-priming response characterized by significant H₂O₂ and stress metabolite accumulation. This physiological state coincided with reduced biomass accumulation, suggesting a diversion of metabolic trade-off. In contrast, in tomato, the growth-promoting extract from Chromochloris zofingiensis was associated with a “high-flux”-like metabolic phenotype. This state was defined by a significant depletion of starch reserves in leaves and soluble sugars in stems, coupled with the maintenance of nitrogen homeostasis and oxidative balance. These findings suggest that, in tomato, effective biostimulation functions by increasing carbon utilization relative to storage, thereby favoring the rapid conversion of photo-assimilates into structural biomass. Conversely, less effective treatments induced passive carbon accumulation (high starch/sugar levels), indicating low sink demand.