Two-Dimensional Microfluidic System for the Simultaneous Quantitative Analysis of Phototactic/Chemotactic Responses of Microalgae

Microalgae have been spotlighted as a renewable energy source to produce biofuels from CO₂ by photosynthesis. However, their innate inefficiency of CO₂ conversion using light energy has been a challenge to the commercialization of algae-based biofuel production. Photosynthetic organisms have evolved behavioral responses, including phototaxis and chemotaxis, to find optimal conditions for capturing light energy and inorganic carbon (Ci) sources for photosynthesis. In this context, investigation of phototaxis and chemotaxis to HCO₃ ^-, the predominant form of Ci in neutral aqueous solutions, is necessary to understand the physiological role of tactic responses in photosynthesis. In this study, a two-dimensional microfluidic system enabled efficient analysis of phototactic and chemotactic responses by investigation of cell distribution in the outlet chambers. From statistical analysis (skewness and kurtosis) of tactic responses of different algal strains to external stimuli, the preferred concentrations of HCO₃ ^- for Chlamydomonas reinhardtii CC125 (wild type), PTS42 (random insertional mutant of C. reinhardtii, high photosynthetic activity), and CC2702 (cia5 mutant of C. reinhardtii, unable to acclimate to low CO₂ concentration) were determined to be 27.22, 43.23, and 36.95 mM, respectively. From the analysis of tactic responses of wild type and 14 mutant strains, it was found that the photosystem II (PSII) operating efficiency and CO₂ fixation rate were strongly correlated with the phototactic (R² = 0.931) and chemotactic response (R² = 0.857), respectively. Finally, this system can be applied to high-throughput screening strategies for the rapid isolation of high photosynthetically productive microalgal strains based on their tactic responses.