Evidence for HCO3- [bicarbonate ion] transport by the blue-green alga (Cyanobacterium) Coccochloris peniocystis

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 t...

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Bibliographic Details
Published in:Plant physiology (Bethesda) 1980-02, Vol.65 (2), p.397-402
Main Authors: Miller, A.G, Colman, B
Format: Article
Language:English
Subjects:
Algae
bicarbonates
Carbon dioxide
Cell membranes
Chemical suspensions
Cyanobacteria
Cyanobacterium
Dehydration
Electrodes
Photosynthesis
Plants
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Summary: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.
ISSN:0032-0889
1532-2548
DOI:10.1104/pp.65.2.397