Illumination does not affect the membrane potential or the plasma membrane H+-ATPase activity in Micrasterias

The membrane potential (MP) of the unicellular green alga Micrasterias torreyi was found to be −46 to −47 mV (when cultured in Waris medium). In contrast to plant cells in general, light-dark changes neither affected the potential or the membrane resistance in Micrasterias . In comparison, the freshwater plant Elodea showed a light-induced hyperpolarization due to the activating effect of light on the plasma membrane adenosine triphosphatases (PM ATPases) through a signal from chloroplasts. In Micrasterias , the PM H+-ATPase inhibitors Na-orthovanadate and diethylstilbestrol depolarized the potential, but it remained at the same level in light and dark. On the other hand, fusicoccin, which activates the PM H+-ATPases, hyperpolarized the potential clearly (to −56 mV). 3-(3',4'-dichlorophenyl)-1,1-dimethylurea, which blocks the electron transport chain from photosystem (PS)II to PSI and thereby prevents the possible signal transmission from chloroplasts to the PM, depolarized the MP slightly, but did not affect the (lacking) light changes either. The results indicate the presence of a continuous (low) activity of PM H+-ATPases in Micrasterias , which is not stimulated by light. The lack of rapid light-induced changes in Micrasterias MP may be due to an unusual functioning of giant chloroplasts in the ion metabolism of the Micrasterias cell.