Strain-Specific Microalgal and Cyanobacterial Suspensions Modulate Germination Kinetics and Early Seedling Vigor in Cucumber

Microalgal and cyanobacterial biostimulants are increasingly recognized as sustainable tools for enhancing crop establishment and reducing dependence on synthetic agrochemicals. However, the strain-specific effects of many taxa on seed germination and early seedling development remain insufficiently characterized. This study evaluated the effects of seven microalgal and cyanobacterial suspensions on the germination kinetics and early seedling vigor of cucumber (Cucumis sativus L.). Several strains significantly accelerated germination and enhanced seedling performance relative to the control. Treatment with Coelastrella rubescens BCAC 301 S39, Scotinosphaera lemnae BCAC 113, Vischeria magna UTEX 2351, and Anabaena sp. IT4 significantly reduced mean germination time from 4.50 d to 2.23–2.29 d and advanced the time to 50% germination (T50) from 4.0 to 2.0–2.1 d. These treatments also increased the germination index from 48.32 to 78.17–100.67 and enhanced seedling traits, including root length (32–53%), shoot length (≈29%), leaf length (17–21%), and fresh (30–43%) and dry biomasses (12–22%). Correlation analysis revealed strong positive associations between germination indices and seedling vigor parameters, indicating the faster germination promotes early growth. In conclusion, the results demonstrate that specific microalgal strains can function as effective seed-phase biostimulants, offering a sustainable strategy to enhance germination uniformity, early seedling establishment, and crop productivity.