Phototaxis and photo-shock responses of Euglena gracilis under gravitaxis

Abstract

We quantitatively investigated Euglena gracilis photo-responses and photoadaptation under gravity by observing the distribution and movements of cells confined in a microchamber. All the movements in the microchamber were analyzed via video observation with longer/shorter time scales from seconds to hours. Normally the cells distributed at the upper area of the microchamber, exhibiting negative gravitaxis. Two types of negative gravitactic motions were suggested for non-explicit photo stimulation: circular swimming probably due to tail-heavy cell body and run and tumbling with a larger chance to turn upward. When the step-up or step-down photo-shock was induced by light stimulus, cells started sinking downward with on-site and continuous rotation. Positive phototaxis can be superimposed on negative gravitaxis, whereas negative phototaxis overcomes competing gravitaxis. The adaptation of cells to high-flux blue light (2780 μmol/m2 s, irradiated from the bottom) was clearly observed as cell distribution changed from the bottom (step-up photo-shock) to the top (negative phototaxis) of the microchamber. Faster adaptation was achieved by shortening the interval period of blue light stimulation. Our microchamber observation/analysis system is advantageous for quantitative investigation of photo-responses of microorganic cells under gravity, since the entire cellular movements in the microchamber can be tracked over time, from seconds to hours, with varying light illumination.