Effect of external ionic environment on phototaxis of Volvox carteri

The sign of phototaxis in Volvox carteri is temperature-dependent;positive at room temperature and negative at low temperature.Modification of the tactic sign by external ions, pH and chemicalswas studied. The addition of 30 mM potassium ion to the mediumchanged the tactic sign from positive to negative, and a mediumwith a high pH elicited positive phototaxis. An increase inthe potassium or hydrogen ion concentration raised the reversaltemperature of the phototactic sign, and the addition of magnesiumor calcium ion also raised the reversal temperature of the signslightly. Valinomycin, a highly specific ionophore of potassium,raised the reversal temperature. CCCP, DCMU and DCCD, whichdepolarize biological membranes, also raised the reversal temperature.These results show that the sign of phototaxis is determinedby membrane polarization; on depolarization of the membranethe sign of phototaxis changes from positive to negative. Ethanol lowered the reversal temperature, and sodium azide inhibitedboth positive and negative phototaxis. The effects of ethanoland azide indicate that depolarization of the membrane is notthe only factor that induces change in the phototactic sign. (Received July 23, 1979; )