Temporal Evolution of the Gravitaxis of Euglena gracilis from a Single Cell

Gravitaxis is one of the most important issues in the growth of microalgae in the water column; it determines how easily cells receive sunlight with a comfortable intensity that is below the damaging threshold. We quantitatively investigated and analyzed the gravitaxis and cell multiplication of Euglena gracilis using vertically placed microchambers containing a single cell. A temporal change in gravitaxis and cell multiplication was observed after transferring the cells to fresh culture medium for 9 days. We performed 29 individual experiments with 2.5 mm × 2.5 mm × 0.1 mm square microchambers and found that the cells showed positive, negative, and moderate gravitaxis in 8, 7, and 14 cases, respectively, after transferring to fresh culture medium. A common trend was observed for the temporal change in gravitaxis for the eight initially positive gravitaxis cases. The cells with initially positive gravitaxis showed a higher rate of cell multiplication than those with initially negative gravitaxis. We also discussed the gravitaxis mechanism of E. gracilis from the observed trend of gravitaxis change and swimming traces. In addition, bioconvection in a larger and thicker chamber was investigated at a millimeter scale and visualized.