This study investigates the potential of microalgal extracts as natural biostimulants for scarlet runner bean, also known as ayocote bean (Phaseolus coccineus) germination. We evaluated the effects of different extract concentrations (0–100% v/v) over 120 hours on germination dynamics and the levels of bioactive compounds (total phenolics and antioxidant power, FRAP) in the resulting seedlings. While extract concentration did not significantly affect the final germination percentage (P=0.086), it had a pronounced effect on germination speed and seedling physiology. Higher concentrations generally suppressed germination, as measured by a decrease in the Continuous Germination Index (CGI, P=0.003), which is a novel index derived from the functional data analysis of the smoothed germination curve that describes the trajectories of said curves. The highest concentration (100%) even exhibited moderate toxicity (Seed Germination Index: -0.27). In contrast, low to moderate concentrations (25–50%) enhanced the germination process, with peak performance observed at 50% (CGI: 112.32 area units). Bioactive compound levels were also significantly influenced, with total phenolics (P=0.003) and FRAP (P≤0.001) varying throughout germination. A notable recovery in antioxidant power values between 72 and 96 hours, particularly at lower concentrations, suggests a stress-adaptive response potentially linked to the increased phenolic content observed after 72 hours. We conclude that microalgal extracts can effectively enhance the germination and bioactivity of ayocote beans, but their application requires careful dose calibration, with an optimal range at or below 50%, to maximise biostimulation and avoid phytotoxic effects.