Behavior of Euglena gracilis under simultaneous competing optical and chemical stimuli

In this study, we developed a microfluidic system to elucidate the behavioral response of Euglena gracilis cells to simultaneous competing optical and chemical stimuli. The system illuminated a nonuniform blue light of 0.5-18 mW/cm2 on cells confined in a 2D microchamber to induce a photophobic response (step-up photo-shock). After cells accumulated in areas of weak blue light, a CO2 gradient (0%-100%) was generated in the microchamber to induce counter chemotaxis (aerotaxis). E. gracilis cells showed negative chemotaxis for areas of higher CO2 concentrations rather than strong blue light, suggesting that CO2 chemotaxis is dominant over blue light photophobicity. We also examined phototaxis instead of photo-shock responses, using in-plane uniform blue light illumination in a counter direction to the CO2 gradient. The cells exhibited both CO2 chemotaxis and blue light phototaxis, and swam back and forth randomly. In both cases, some sensitive cells accumulated in niche areas, primarily with CO2 concentration < 25% and secondarily with a weaker blue light intensity (further away from the blue light source). The results indicate that competing chemotactic and photophobic or phototactic stimuli induced superimposed responses, although E. gracilis was more strongly affected by CO2 chemotaxis than by blue light. We propose that the two independent chemotaxis and photoresponse signal-transduction pathways are merged into one flagellar control mechanism. Moreover, a small proportion of cells were resistant to CO2 or blue light, showing the diversity in cell metabolic status.