The phototactic collection of microalgae and its implications for the comprehensive utilization of microalgal biomass

Microalgal biomass harvesting is a critical bottleneck in microalgae biotechnology, accounting for 20–30 % of production costs while often introducing chemical contaminants that compromise biomass quality. This study demonstrates a novel, chemical-free harvesting method utilizing unilateral illumination to induce phototactic aggregation of Chlamydomonas reinhardtii under varying nutrient conditions. Under optimized conditions (10 5 cells/mL, 4000 Lux), remarkable aggregation efficiencies of 89.11 %, 85.24 %, and 80.02 % were achieved in 0.5 × , 1.0 × , and 2.0 × standard N/P media, respectively. Critically, post-harvesting analyses revealed preserved phototactic activity and minimal impact on photosynthetic efficiency, with Fv/Fm values (0.484–0.497) remaining within functional ranges despite a slight reduction from control conditions. Laser confocal Raman imaging confirmed that intracellular distribution and content of proteins, lipids, and carotenoids remained unaltered after harvesting, while three-dimensional fluorescence spectroscopy demonstrated that extracellular polymeric substances were influenced solely by nutrient conditions rather than the harvesting method. This chemical-free approach overcomes major limitations of conventional harvesting techniques, maintaining cellular integrity and biochemical composition essential for comprehensive biomass utilization in high-value applications. • A chemical-free phototactic method for microalgae collection was proved. • The N and P concentrations affected the movement trajectory of the microalgae. • The harvested microalgae retain their phototactic capabilities. • Phototactic harvesting preserves cellular components, enabling biomass utilization.