Involvement of Plasmalemmasomes and Carbonic Anhydrase in Photosynthetic Utilization of Bicarbonate in Chara corallina

There is a clear relationship between the density of plasmalemmasomes (PLSs) in the acid band regions of whorl cells of Chara corallina and the capacity of these cells to utilize HCO*¯/3 for photosynthesis. For cells grown in alkaline media, high PLS densities on the plasmalemma (50-85% coverage) were always correlated with high rates of HCO*¯/3 dependent O2 evolution (pH 9.3), reaching rates of 17-23 mol O2 mg Chl-¹ h-¹ at 5 mM inorganic carbon concentration. In alkaline culture, development of high PLS densities and high rates of HCO*¯/3 usage were related to provision of limiting levels of CO2 (< 12 M) in the media. High growth levels of CO2 (267 M) markedly reduced PLS densities and rates of HCO*¯/3 usage. Bicarbonate dependent O2 evolution (pH 9.3) was extremely sensitive to the carbonic anhydrase inhibitor ethoxyzolamide (20 M), whilst CO2-dependent O2 evolution (pH 5.5) was insensitive. Carbonic anhydrase (CA) activity was present in homogenates and in intact cells, suggesting that a periplasmic location is possible. It seems reasonable to suggest that, for Chara corallina, high PLS densities and CA activity are required for efficient utilization of HCO*¯/3 at alkaline pH. Plasmalemmasomes may be part of the HCO*¯/3-utilizing mechanism by acting as sites for localized pH generation, thus facilitating HCO*¯/3 utilization by either H+/HCO*¯/3 active cotransport or CA and pH-mediated external conversion of HCO*¯/3 to CO2 in the periplast.