Calcium is involved in the gravitactic orientation in colorless flagellates

The colorless flagellate Astasia longa shows a pronounced negative gravitax is. The calcium fluorescence indicator Calcium Crimson was used to detect c hanges of the intracellular calcium concentration during gravitactical orie ntation. Astasia shows an increase of the fluorescence after a lag phase of about 10 s, a maximum after about 30 s and a decrease to the basic level w ithin 60 s during gravitactic reorientation. The observed change in fluores cence corresponds to an almost doubling of the initial free calcium concent ration. The influence of inhibitors, known to impair gravitaxis, on the cal cium concentration of Astasia longa was tested. Addition of caffeine, an in hibitor of phosphodiesterase, increases, while addition of gadolinium, an i nhibitor of mechanosensitive ion channels decreases the fluorescence signal . While gravitactic stimulation of caffeine-treated cells resulted in a kin etics of fluorescence intensity changes comparable to control cells the add ition of gadolinium inhibited any calcium concentration change. Dynamic flu orescence imaging was used during a sounding rocket experiment (MAXUS 3 cam paign). Different accelerations interrupted by mug intervals were applied t o Astasia cells. The cells show an increase in the calcium signal upon acce leration and a decrease during the mug state. The results strongly reemphas ize the working model of gravitaxis which is based on the activation of mec hano-sensitive ion channels as one of the primary events in signal percepti on.