SUSTAINED NET CO2 EVOLUTION DURING PHOTOSYNTHESIS BY MARINE MICROORGANISMS

Background: Many aquatic photosynthetic microorganisms possess an inor ganic-carbon-concentrating mechanism that raises the CO2 concentration at the intracellular carboxylation sites, thus compensating for the r elatively low affinity of the carboxylating enzyme for its substrate. In cyanobacteria, the concentrating mechanism involves the energy-depe ndent influx of inorganic carbon, the accumulation of this carbon - la rgely in the form of HCO3- - in the cytoplasm, and the generation of C O2 at carbonic anhydrase sites in close proximity to the carboxylation sites. Results: During measurements of inorganic carbon fluxes associ ated with the inorganic-carbon-concentrating mechanism, we observed th e surprising fact that several marine photosynthetic microorganisms, i ncluding significant contributors to oceanic primary productivity, can serve as a source of CO2 rather than a sink during CO2 fixation. The phycoerythrin-possessing cyanobacterium Synechococcus sp. WH7803 evolv ed CO2 at a rate that increased with light intensity and attained a va lue approximately five-fold that for photosynthesis. The external CO2 concentration reached was significantly higher than that predicted for chemical equilibrium between HCO3- and CO2, as confirmed by the rapid decline in the CO2 concentration upon the addition of carbonic anhydr ase. Measurements of oxygen exchange between water and CO2, by means o f stable isotopes, demonstrated that the evolved CO2 originated from H CO3- taken up and converted intracellularly to CO2 in a light-dependen t process. Conclusions: We report net, sustained CO2 evolution during photosynthesis. The results have implications for energy balance and p H regulation of the cells, for carbon cycling between the cells and th e marine environment, and for the observed fractionation of stable car bon isotopes. (C) Current Biology Ltd ISSN 0960-9822.