CALCIFICATION IN CHARA-CORALLINA - CO2 HYDROXYLATION GENERATES PROTONS FOR BICARBONATE ASSIMILATION

The alga Chara corallina (Charophyta, Charophyceae) apparently generates protons through the hydroxylation and precipitation of CO2, which leaks from the cell into highly alkaline pericellular regions. Under mildly alkaline conditions, the loss of CO2 to calcification is compensated by the production of twice as much CO2 from HCO3-, using the protons generated during calcification. Experiments capitalize on Chara's large cells with their alternating alkaline (calcareous) and acidic (noncalcareous) pericellular zones. C-14 uptake into carbonate and organic phases is fastest when the isotope is applied to the acidic zones. The ratio of calcification to photosynthesis is approximately 1 at pH 8 and increases with pH approximately as simulated by a model in which both calcification and photosynthesis use carbon assimilated in the acidic zones. PIPES buffer (pKa = 6.8) inhibits calcification and photosynthesis equally, suggesting competition with bicarbonate as a proton acceptor in the acidic zones. CAPS buffer (pKa = 10.4) has little effect, suggesting that exogenous bicarbonate is not very important for calcification. Divalent Sr and Mn added to the medium precipitate primarily between the cell and calcareous encrustations, but phosphate does not. This result is consistent with the use of cellular Ca and CO2 in extracellular calcification.