Graviperception and graviorientation in flagellates

In the flagellate Euglena gracilis Klebs, gravitaxis is mediated by an active physiological receptor and is not the result of passive alignment of the cells in the water column. The threshold of this response was found at 0.08 < threshold <0.16 g during a recent space flight on the American shuttle Columbia, where the cells were subjected to different accelerations between 0 and 1.5 g; the response saturated at 0.32 < saturation less than or equal to 0.64 g. Over the whole duration of the mission no adaptation of the response to microgravity was observed. The whole body of the cell, rather than intracellular organelles, seems to act as statolith since suspending the cells in a density-adjusted medium (Ficoll) resulted in an inhibition of gravitaxis and even reversal of orientation at higher densities. Thus, the cytoplasm seems to exert a pressure on the respective lower membrane where it is hypothesized to activate stretch-sensitive specific ion channels, as indicated by inhibitor studies with gadolinium. One of the early steps in the sensory transduction chain seems to be a modulation of the membrane potential since ion-channel blockers, ionophores and ATPase inhibitors strongly inhibit gravitaxis in this flagellate without seriously affecting motility and phototaxis.