title: Extracellular carbonic anhydrase facilitates carbon dioxide availability for photosynthesis in the marine dinoflagellate prorocentrum micans
abstract: This study investigated inorganic carbon accumulation in relation to photosynthesis in the marine dinoflagellate Prorocentrum micans. Measurement of the internal inorganic carbon pool showed a 10-fold accumulation in relation to external dissolved inorganic carbon (DIC). Dextran-bound sulfonamide (DBS), which inhibited extracellular carbonic anhydrase, caused more than 95% inhibition of DIC accumulation and photosynthesis. We used real-time imaging of living cells with confocal laser scanning microscopy and a fluorescent pH indicator dye to measure transient pH changes in relation to inorganic carbon availability. When steady-state photosynthesizing cells were DIC limited, the chloroplast pH decreased from 8.3 to 6.9 and cytosolic pH decreased from 7.7 to 7.1. Re-addition of HCO3- led to a rapid re-establishment of the steady-state pH values abolished by DBS. The addition of DBS to photosynthesizing cells under steady-state conditions resulted in a transient increase in intracellular pH, with photosynthesis maintained for 6 s, the amount of time needed for depletion of the intracellular inorganic carbon pool. These results demonstrate the key role of extracellular carbonic anhydrase in facilitating the availability of CO2 at the exofacial surface of the plasma membrane necessary to maintain the photosynthetic rate. The need for a CO2-concentrating mechanism at ambient CO2 concentrations may reflect the difference in the specificity factor of ribulose-1,5 bisphosphate carboxylase/oxygenase in dinoflagellates compared with other algal phyla.