Inorganic-carbon assimilation in the green seaweed Ulva rigida C.Ag. (Chlorophyta)

Mechanisms of carbon assimilation were investigated in thalli and protoplasts of Ulva rigida by measuring HCO 3 (-) -dependent O2 evolution at pH 6.5 and 8.6. In thalli, dextran-bound azetazolamide (DBAZ), a specific inhibitor of extracellular carbonic anhydrase (CA), inhibited the rate of O2 evolution at pH 8.6 when HCO 3 (-) was the only available form of inorganic carbon (Ci) in the medium. At pH 6.5 when CO2 is accessible, DBAZ did not affect photosynthetic O2 evolution. Inhibition of total CA activity (extracellular and intracellular) by ethoxyzolamide (EZ) inhibited photosynthesis at pH 6.5 and 8.6. During illumination of thalli the medium was alkalized at a rate which increased with increasing light. This alkalization decreased during inhibition of extracellular CA by DBAZ. Protoplasts at pH 6.5 exhibited a higher rate of O2 evolution than in pH 8.6. Addition of CA to protoplasts at pH 8.6 increased the rate of O2 evolution, whereas EZ was inhibitory at both pH 6.5 and 8.6, and DBAZ did not affect photosynthesis at either pH. We suggest that both extracellular and intracellular CA are present and that Ulva rigida assimilates HCO 3 (-) by an indirect mechanism. A theoretical scheme for carbon utilization is suggested.