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

TLDR: The evidence strongly suggests that HCO(3) (-) was a direct C source for photosynthesis and serves to raise the CO(2) concentration around the carboxylase to levels high enough for effective fixation.

Abstract: 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.