Dependence of H+ exchange and oxygen evolution on K+ in the marine cyanobacterium Synechococcus sp. strain UTEX 2380

Abstract

Light-induced net H+ efflux, photosynthetic oxygen evolution, and medium alkalization during the steady state of photosynthesis were specifically stimulated by K+ from pH 6 through 8.5 in the marine cyanobacterium Synechococcus sp. strain UTEX 2380. Net proton efflux in the light was completely abolished by the uncoupler carbonylcyanide m-chlorophenylhydrazone and the membrane ATPase inhibitor diethylstilbestrol or partially abolished by the membrane ATPase inhibitor N,N′-dicyclohexyl-carbodiimide and by orthovanadate. H+ extrusion in the light was accompanied by K+ uptake at rates of 30 μmol∙mg chlorophyll−1∙h−1. During the steady state of CO2 fixation, potassium was excreted simultaneously with medium alkalization. The K+ content of the cells was 432 mM for air-grown cells. K+ in the cells was displaced by diethanolamine, which was inhibited by m-chlorophenylhydrazone. Na+-loaded cells showed nearly complete inactivation of oxygen evolution and medium alkalization. Both activities were reactivated by the addition of K+ or Rb+. A fivefold increase in inorganic carbon uptake in the light was observed in the presence of K+. The pH of the cytoplasm in the light increased from 7.2 to 8.04 in the pH range 6.6–8.6. These results suggest that a light-dependent, proton-excreting ATPase is active in conjunction with a K+ uptake system in a marine Synechococcus species, while a K+–H+ antiporter may function as a regulator of cytoplasmic pH during photosynthesis.Key words: potassium transport, oxygen, cyanobacterium, Synechococcus, proton extrusion.