Photosynthesizing marine microorganisms can constitute a source of CO₂ rather than a sink

The unexpected finding that certain major marine photosynthetic microorganisms can serve as a source of CO₂ rather than a sink emerged during measurements of inorganic carbon fluxes associated with the CO₂-concentrating mechanism. During steady-state photosynthesis, CO₂ was evolved 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 process. Our results bear implications for carbon cycling in the marine environment; the use of naturally-observed stable carbon isotope fractionations as paleobarometer and productivity probe; and for intracellular energy balance and pH regulation.