Evidence for HCO3− Transport by the Blue-Green Alga (Cyanobacterium) Coccochloris peniocystis

The possibility of HCO(3) (-) transport in the blue-green alga (cyanobacterium) Coccochloris peniocystis has been investigated. Coccochloris photosynthesized most rapidly in the pH range 8 to 10, where most of the inorganic C exists as HCO(3) (-). If photosynthesis used only CO(2) from the external solution the rate of photosynthesis would be limited by the rate of HCO(3) (-) dehydration to CO(2). Observed rates of photosynthesis at alkaline pH were as much as 48-fold higher than could be supported by spontaneous dehydration of HCO(3) (-) in the external solution. Assays for extracellular carbonic anhydrase were negative. The evidence strongly suggests that HCO(3) (-) was a direct C source for photosynthesis.Weakly buffered solutions became alkaline during photosynthesis with a one-to-one stoichiometry between OH(-) appearance in the medium and HCO(3) (-) initially added. Alkalization occurred only during photosynthesis and was blocked by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, diuron. It is suggested that HCO(3) (-) was transported into cells of Coccochloris in exchange for OH(-) produced as a result of HCO(3) (-) fixation in photosynthesis.The inorganic C concentration required to support a rate of photosynthesis of half the maximum rate (K(m)) was 6 micromolar at pH 8.0 or, in terms of available CO(2), a K(m) of 0.16 micromolar. This value is two orders of magnitude lower than reported K(m) values for the d-ribulose-1,5-bisphosphate carboxylase for blue-green algae. It is suggested that the putative HCO(3) (-) transport by Coccochloris serves to raise the CO(2) concentration around the carboxylase to levels high enough for effective fixation.