PHOTOSYNTHESIZING MARINE MICROORGANISMS CAN CONSTITUTE A SOURCE OF CO2 RATHER THAN A SINK

The unexpected finding that certain major marine photosynthetic microo rganisms can serve as a source of CO2 rather than a sink emerged durin g measurements of inorganic carbon fluxes associated with the CO2-conc entrating mechanism. During steady-state photosynthesis, CO2 was evolv ed at sustained rates up to 5-fold that of photosynthesis; the steady- state external CO2 concentration reached was significantly higher than that at CO2-HCO3- equilibrium. The evolved CO2 originated from HCO3- taken up and intracellularly converted to CO2 in a light-dependent pro cess. Our results bear implications for carbon cycling in the marine e nvironment; the use of naturally-observed stable carbon isotope fracti onations as paleobarometer and productivity probe; and for intracellul ar energy balance and pH regulation.