pH control of the chlorophyll a fluorescence in algae.

The pH of the suspension medium was found to have a remarkable influence on the "slow" (min) time course of Chlorophyll a fluorescence yield in the green alga Chlorella pyrenoidosa and in the blue-green alga Anacys t i s nidulans. In Chlorella, the decay of fluorescence yield, in the Ito 5-rain region, is strongly retarded at alkaline pH ; this decay rate shows an optimum at pH 67. In Anacystis, the rise of fluorescence yield, in the same time range, is decreased optimally at pH 6-7; poisoning with 3(3,4-dichlorophenyl)-i,i-dimethylurea reverses the direction of this pH effect. These observations suggest a correlation of the H + status (or the processes associated with it such as photophosphorylation and resulting conformational changes) of the chloroplast to the yield of chlorophyll a fluorescence in vivo.