Phycobiostimulants 2.0: Integrating Algal Biomass Provenance, Processing, Molecular Mechanisms and Predictable Crop Performance

Phycobiostimulants derived from macroalgae, microalgae, and cyanobacteria are increasingly used to improve crop performance. However, their effects vary across products, crops, and environments. In this review, we propose a biomass–process–composition–mechanism–efficacy (BPCME) framework to systematically trace this variability. Biomass provenance, including taxonomic identity, cultivation conditions, physiological state, and harvest timing, defines the initial biochemical potential, whereas processing and formulation determine the molecular profile delivered to the plant. The current findings indicate that phycobiostimulants act mainly by modulating endogenous signaling networks, hormone crosstalk, redox regulation, metabolic reprogramming, and rhizosphere and plant–microbiome responses rather than by direct nutrient or hormone replacement. Agronomic performance is further moderated by the genotype, dose, application route, developmental stage, soil conditions, and environmental stress. To improve reproducibility, we propose a phycobiostimulant quality-by-design (QbD) framework that integrates candidate critical biomass attributes and process parameters, chemical fingerprints, biological potency, and batch equivalency. Cascading algal biorefineries and precision application strategies can improve resource efficiency while helping to define the conditions for reliable field performance. Progress towards precision phycobiostimulants will require the experimental linking of product identity and mechanism to reproducible potency across batches and agronomic environments.