Abstract
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.
Highlights
Gravitaxis is a fundamental characteristic of both land and sea plants
We report the gravitactic behavior of a single E. gracilis cell and its temporal changes with cell multiplication
Whether those cells originally exhibited positive gravitaxis before transferring to fresh culture medium is not certain; it is probable that they exhibited negative gravitaxis before transferring, given that the majority of swimming cells after 4 days of culture showed negative gravitaxis (Figure 6d compared with Figure 6c)
Summary
Gravitaxis is a fundamental characteristic of both land and sea plants. Even in the absence of light, gravity indicates the direction in which plants should grow to obtain sunlight. A plant seed lying in soil can respond to gravity to determine the direction to extend its roots and shoots. Several motile microalgae in water exhibit negative gravitaxis and gather near the surface area of the water column to catch sunlight [1,2,3]. One of the simplest ways to swim toward or against gravity is to use buoyancy, i.e., rising to the surface of the water column and sinking to the bottom by buoyancy-affected heading mechanisms [4,5]
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