Abstract
The adaptation to a strong light is one of the essential characteristics of green algae, yet lacking relatively the information about the photophobic responses of Eukaryotic microalgae. We investigated the photophobic step-up responses of Euglena gracilis over a time course of several hours with alternated repetition of blue-light pulse illumination and spatially patterned blue-light illumination. Four distinctive photophobic motions in response to strong blue light were identified in a trace image analysis, namely on-site rotation, running and tumbling, continuous circular swimming, and unaffected straightforward swimming. The cells cultured in autotrophic conditions under weak light showed mainly the on-site rotation response at the beginning of blue-light illumination, but they acquired more blue-light tolerant responses of running and tumbling, circular swimming, or straightforward swimming. The efficiency of escaping from a blue-light illuminated area improved markedly with the development of these photophobic motions. Time constant of 3.0 h was deduced for the evolution of photophobic responses of E. gracilis. The nutrient-rich metabolic status of the cells resulting from photosynthesis during the experiments, i.e., the accumulation of photosynthesized nutrient products in balance between formation and consumption, was the main factor responsible for the development of photophobic responses. The reduction-oxidation status in and around E. gracilis cells did not affect their photophobic responses significantly, unlike the case of photophobic responses and phototaxis of Chlamydomonas reinhardtii cells. This study shows that the evolution of photophobic motion type of E. gracilis is dominated mainly by the nutrient metabolic status of the cells. The fact suggests that the nutrient-rich cells have a higher threshold for switching the flagellar motion from straightforward swimming to rotation under a strong light.
Highlights
Motile microalgae such as Euglena gracilis and Chlamydomonas reinhardtii are fascinating organisms, because they have a high photosynthetic capacity and are able to move using their flagella
This means that the photo-induced movements we observed in our system were not phototaxis, i.e., photo-directional motions, but photophobic responses, i.e., non-directional motions, with which they try to escape from the strong light by changing their swimming direction randomly
We deduced that photosynthesis during these experiments improved the metabolic status of the cells, and the cells with a nutrient-rich metabolic status did not rotate on-site, which is ineffective for escaping from harsh blue light
Summary
Motile microalgae such as Euglena gracilis and Chlamydomonas reinhardtii are fascinating organisms, because they have a high photosynthetic capacity and are able to move using their flagella. Higher oil production has been achieved by modifying genes in C. reinhardtii [6,7] and E. gracilis [8,9], and the development of appropriate media has allowed E. gracilis to be cultured on a pond-scale [10]. Another potential use of motile microalgae is in cell-based biosensors. It is faster and easier to measure the locomotive responses of motile microalgae cells than to measure changes in the viability of mammalian cells. That study demonstrated that computational performance could be improved by the various photophobic responses of the microalgae cells
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