High Light Exposure Leads to a Sign Change of Gravitaxis in the Flagellate Euglena gracilis

Abstract

The unicellular flagellate Euglena gracilis orients itself in the water column by means of pronounced phototaxis and gravitaxis. The antagonism of phototaxis and gravitaxis brings the cells in a position in the water column providing them with optimal light conditions for their photosynthetic apparatus (about 30 Wm^(-2)). Long exposure to solar or artificial radiation induces a loss of the negative gravitactic orientation in Euglena gracilis or very often a pronounced, persistent (＞4 h) sign change in gravitaxis. The effects on gravitaxis are exclusively due to UV and intensive blue light as experiments with different light qualities revealed. This phenomenon is not caused by the phototaxis photoreceptor or chloroplast processes, because also the colorless and blind (no photoreceptor) Euglena gracilis 1f mutant and Astasia longa reverse the sign of gravitaxis upon strong radiation. The sign change is oxygen-dependent, because gravitaxis is not affected in oxygen free medium (in wild type Euglena gracilis, as well as in the 1f mutant and Astasia longa). This indicates the involvement of oxygen radicals as a trigger of gravitaxis sign reversal. As the destruction of the photoreceptor molecules by light leads to a loss of phototaxis, the switch from negative to positive gravitaxis might be an adaptation mechanism of the cells to escape from deleterious radiation even after loss of the ability to perceive the light direction.