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

The membrane potential (MP) of the unicellular green alga Micrasterias torr eyi was found to be -46 to -47 mV (when cultured in Waris medium). In contr ast to plant cells in general, light-dark changes neither affected the pote ntial or the membrane resistance in Micrasterias. In comparison, the freshw ater plant Elodea showed a light-induced hyperpolarization due to the activ ating effect of light on the plasma membrane adenosine triphosphatases (PM ATPases) through a signal from chloroplasts, In Mia aster ins, the PM H+ AT Pase inhibitors Na-orthovanadate and diethylstilbestrol depolarized the pot ential, but it remained at the same level in light and dark. On the other h and, fusicoccin, which activates the PM H+-ATPases, hyperpolarized the pote ntial clearly (to -56 mV). 3-(3',4'-dichlorophenyl)-1,1-dimethylurea, which blocks the electron transport chain from photosystem (PS)Il to PSI and the reby prevents the possible signal transmission from chloroplasts to the PM, depolarized the MP slightly, but did not affect the (lacking) light change s either. The results indicate the presence of a continuous (low) activity of PM H+-ATPases in Micrasterias, which is not stimulated by light. The lac k of rapid light-induced changes in Micrasterias RIP may be due to an unusu al functioning of giant chloroplasts in the ion metabolism of the Micraster ias cell.