Massive light-dependent cycling of inorganic carbon between oxygenic photosynthetic microorganisms and their surroundings

Membrane inlet mass spectrometry indicated massive light-dependent cycling of inorganic carbon between the medium and the cells of various phytoplankton species representing the main groups of aquatic primary producers. These included diatoms, symbiotic and free living dinoflagellates, a coccolithophorid, a green alga and filamentous and single cell cyanobacteria. These organisms could maintain an ambient CO₂ concentration substantially above or below that expected at chemical equilibrium with HCO₃^-. The coccolithophorid Emiliania huxleyi shifted from net CO₂ uptake to net CO₂ efflux with rising light intensity. Differing responses of CO₂ uptake and CO₂ fixation to changing light intensity supported the notion that these two processes are not compulsorily linked. Simultaneous measurements of CO₂ and O₂ exchange and of the fluorescence parameters in Synechococcus sp. strain PCC 7942, showed that CO₂ uptake can serve as a sensitive probe of the energy status of the photosynthetic reaction centers. However, during transitions in light intensity, changes in CO₂ uptake did not accord with those expected from fluorescence change. Quantification of the net fluxes of CO₂, HCO₃^- and of photosynthesis at steady-state revealed that substantial HCO₃^- efflux accompanied CO₂ uptake and fixation in the case of 'CO₂ users'. On the other hand, 'HCO₃^- users' were characterized by a rate of net CO ₂ uptake below that of CO₂ fixation. The results support the notion that entities associated with the CCM function not only in raising the CO₂ concentration at the site of Rubisco; they may also serve as a means of diminishing photodynamic damage by dissipating excess light energy.