Possible involvement of the membrane potential in the gravitactic orientation of Euglena gracilis

Euglena gracilis, a unicellular photosynthetic flagellate, uses light and g ravity as environmental hints to reach and stay in regions optimal for grow th and reproduction. The current model of gravitaxis (the orientation with respect to the earth's gravitational field) is based on the specific densit y difference between cell body and medium. The resulting sedimentation of t he cell body applies a force to the lower membrane. This force activates me chano-sensitive ion channels. The resulting ion flux changes the membrane p otential, which in turn triggers reorientational movements or the trailing flagellum. One possibility for recording the predicted membrane potential c hanges during reorientation is the use of potential-sensitive dyes, such as Oxonol VI. The absorption changes of the dye indicating potential changes were recorded with a custom-made photometer, which allows a high precision measurement with a high temporal resolution. After a gravitactic stimulatio n, a short period of hyperpolarization was detected, followed by a massive depolarization of the cell. The membrane potential returned to initial valu es after a period or approximately 200s. Parallel measurements of the preci sion of orientation and the membrane potential showed a close relationship between both phenomena. The obtained results support the current model of g ravitaxis of Euglena gracilis.