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. 1980 Feb;65(2):397–402. doi: 10.1104/pp.65.2.397

Evidence for HCO3− Transport by the Blue-Green Alga (Cyanobacterium) Coccochloris peniocystis1

Anthony G Miller 1, Brian Colman 1
PMCID: PMC440336  PMID: 16661199

Abstract

The possibility of HCO3− 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 HCO3−. If photosynthesis used only CO2 from the external solution the rate of photosynthesis would be limited by the rate of HCO3− dehydration to CO2. Observed rates of photosynthesis at alkaline pH were as much as 48-fold higher than could be supported by spontaneous dehydration of HCO3− in the external solution. Assays for extracellular carbonic anhydrase were negative. The evidence strongly suggests that HCO3− 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 HCO3− initially added. Alkalization occurred only during photosynthesis and was blocked by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, diuron. It is suggested that HCO3− was transported into cells of Coccochloris in exchange for OH− produced as a result of HCO3− fixation in photosynthesis.

The inorganic C concentration required to support a rate of photosynthesis of half the maximum rate (Km) was 6 micromolar at pH 8.0 or, in terms of available CO2, a Km of 0.16 micromolar. This value is two orders of magnitude lower than reported Km values for the d-ribulose-1,5-bisphosphate carboxylase for blue-green algae. It is suggested that the putative HCO3− transport by Coccochloris serves to raise the CO2 concentration around the carboxylase to levels high enough for effective fixation.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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