pH Changes in the Cytoplasm of the Blue-Green Alga <i>Anacystis nidulans</i> Caused by Light-dependent Proton Flux into the Thylakoid Space

The pH in the cytoplasmic and thylakoid spaces of the blue-green alga, Anacystis nidulans, has been determined in the light and in the dark by uptake of 5,5-dimethyloxazolidine-2,4-dione and methylamine into the sucrose-impermeable (3)H-H(2)O space, as measured by silicon layer filtering centrifugation.Illumination causes an alkalinization in the cytoplasm which is accompanied by an acidification in the thylakoid space, reflecting light-dependent proton transport across the thylakoid membrane. Under light conditions, a pH gradient of approximately 2.8 between the cytoplasmic and thylakoid spaces has been measured that can be abolished almost completely by addition of the uncoupler, 3-chlorocarbonyl cyanide phenylhydrazone. The pH in the cytoplasm is independent of the pH in the medium.