Role of Photosynthetic Reactions in the Activity of Carbonic Anhydrase in Synechococcus sp. (UTEX 2380) in the Light

Abstract

The role of the photosystems in the exchange of 18O between species of inorganic carbon and water was studied in suspensions of the cyanobacterium Synechococcus sp. (UTEX 2380) using membrane-inlet mass spectrometry. This 18O exchange is caused by the hydration-dehydration cycle of CO2 and is catalyzed by carbonic anhydrase. We observed the complex 18O exchange kinetics including dark-light-dark transients in suspensions of whole cells and found these to be identical to the 18O exchange kinetics of physiologically fully active spheroplast preparations. There was no enhancement effect of inorganic nitrogen on inorganic carbon accumulation. Membrane preparations exhibited no uptake of inorganic carbon and very little carbonic anhydrase activity, although these membranes were photosynthetically fully competent. DCMU, the inhibitor of photosystem II, eliminated almost entirely the 18O exchange activity of whole cells in the light. But this effect of DCMU could be reversed by addition of the electron donor couple 3,6-diaminodurene/ascorbate, suggesting the involvement of photosystem I in the events leading to 18O exchange. Iodoacetamide, an inhibitor of CO2 fixation, enhanced the 18O exchange in whole cell suspensions and inhibited neither the uptake of inorganic carbon nor the dehydration of bicarbonate in the light. The proton carrier carbonylcyanide m-chlorophenylhydrazone and the inhibitors diethylstilbestrol and N,N''-dicyclohexyl carbodiimide affecting the membrane potential, totally abolished 18O exchange in the light. From 18O-labeled inorganic carbon experiments we conclude that one of the roles of photosystem I is to provide the active uptake of inorganic carbon into the cells, where carbonic anhydrase catalyzes the interconversion between CO2 and HCO3- resulting in the 18O exchange from inorganic carbon to water.