A High-Throughput Screening Platform to Evaluate Biostimulant Activity of Five Microalgae in Arabidopsis thaliana

Microalgae are increasingly recognized as promising biostimulants for sustainable agriculture, yet their potential remains underexplored due to the complexity of biostimulant activity and the vast diversity of species. Efficient standardized screening approaches are therefore needed. In this study, a high-throughput screening platform assessed the biostimulant activity of five microalgal species (Limnospira platensis, Chlorella vulgaris, Dunaliella salina, Microchloropsis gaditana, and Isochrysis galbana) in Arabidopsis thaliana. The system enabled full life-cycle assessment of A. thaliana under optimal and drought stress conditions, incorporating three application methods (soil amendment, irrigation, foliar spray) and a wide concentration range of 0.01–0.5 g/L. Biostimulant efficacy depended strongly on concentration and application method. Irrigation-based applications generally enhanced drought tolerance but delayed bolting and flowering. The highest concentration inhibited germination and root elongation, likely due to bioactive compound toxicity rather than salinity or pH. L. platensis exhibited broad activity across environmental conditions, while I. galbana likewise showed wide-ranging effects, including enhanced generative growth. In contrast, D. salina and M. gaditana primarily improved drought tolerance, and C. vulgaris acted mainly under optimal conditions. These findings highlight the value of A. thaliana to accommodate rapid biostimulant screening and identify both novel and established microalgae for further validation in crops.