A gravity-induced regulation of swimming speed in Euglena gracilis

We investigated the autotrophic flagellate Euglena gracilis for gravity-ind uced modulation of the speed of swimming as previously documented for large r protozoan cells. Methods of video-tracking of swimming and sedimenting ce lls under 1 g and hypergravity up to 2 g, and computer-assisted data proces sing were applied. The vertical and horizontal swimming speed, and sediment ation rates of immobilized cells, were found to be linear functions of acce leration. Accounting for sedimentation in the observed upward and downward movements of Euglena, the active component of speed (propulsion) rose in pr oportion to acceleration. No saturation of gravikinesis was seen within the g-range tested. Gravity-dependent augmentation of speed was maximal in upw ard swimmers and decreased continuously over horizontal to downward swimmer s. Linear extrapolations of the data to zero-g conditions suggest the absen ce of a threshold of gravikinesis in Euglena. Energetic considerations indi cate a high sensitivity of gravitransduction near the level of Brownian mol ecular motion.