The role of carbonic anhydrase (CA) in photosynthesis of the marine diatom Skeletonema costatum grown at ambient level of CO(2) was investigated. Extracellular CA activity was very low. In comparison, intracellular CA activity was great part of total CA activity. The inhibition of external CA by acetazolamide (AZ) caused little change in net photosynthetic rate (P(n)), but the inhibition of intracellular CA by ethoxyzolamide (EZ) resulted in the strong reduction of P(n). EZ reduced the light-saturated photosynthesis, the saturation radiance and the affinity of inorganic carbon for photosynthesis, raised inorganic carbon compensation point and enhanced the inhibition of photosynthesis by high O(2) and light. It is concluded that extracellular CA exerted a minor role in the photosynthesis, but intracellular CA enhanced the efficiency of photosynthetic carbon fixation and the capacity of acclimation to stress conditions (high light, O(2) and low CO(2)) by catalytically converting HCO(-)(3) to CO(2) and facilitating CO(2) supply to the cell.