Effect of external ionic environment on phototaxis ofVolvox carteri

摘要

The sign of phototaxis inVolvox carteriis temperature-dependent; positive at room temperature and negative at low temperature. Modification of the tactic sign by external ions, pH and chemicals was studied. The addition of 30 mMpotassium ion to the medium changed the tactic sign from positive to negative, and a medium with a high pH elicited positive phototaxis. An increase in the potassium or hydrogen ion concentration raised the reversal temperature of the phototactic sign, and the addition of magnesium or calcium ion also raised the reversal temperature of the sign slightly. Valinomycin, a highly specific ionophore of potassium, raised the reversal temperature. CCCP, DCMU and DCCD, which depolarize biological membranes, also raised the reversal temperature. These results show that the sign of phototaxis is determined by membrane polarization; on depolarization of the membrane the sign of phototaxis changes from positive to negative.Ethanol lowered the reversal temperature, and sodium azide inhibited both positive and negative phototaxis. The effects of ethanol and azide indicate that depolarization of the membrane is not the only factor that induces change in the phototactic sign.