Phototaxis of Euglena gracilis at low external calcium concentration

Abstract

Negative and positive phototaxis of the flagellated alga Euglena gracilis was studied at various external Ca 2+ concentrations. The swimming velocity and the time of phototactic orientation after an 180° change in the direction of incident light were measured with individual cells after different times of incubation in the Ca 2+ solutions. Both parameters are influenced by the external Ca 2+ concentration. Low Ca 2+ concentrations (⩽10 −6 mol l −1) slow down the reorientation during negative phototaxis, whereas during positive phototaxis reorientation is enhanced. But even at the extremely low Ca 2+ concentration of about 10 −9 mol l −1 neither negative nor positive phototaxis was suppressed. These results lead us to the conclusion that, although Ca 2+ is involved in the reactions to light, a Ca 2+ influx from the external medium is not essential for phototaxis.