Ethoxyzolamide Inhibition of CO2-Dependent Photosynthesis in the Cyanobacterium Synechococcus PCC7942

Cells of the cyanobacterium, Synechococcus PCC7942, grown under high inorganic carbon (C(i)) conditions (1% CO(2); pH 8) were found to be photosynthetically dependent on exogenous CO(2). This was judged by the fact that they had a similar photosynthetic affinity for CO(2) (K(0.5)[CO(2)] of 3.4-5.4 micromolar) over the pH range 7 to 9 and that the low photosynthetic affinity for C(i) measured in dense cell suspensions was improved by the addition of exogenous carbonic anhydrase (CA). The CA inhibitor, ethoxyzolamide (EZ), was shown to reduce photosynthetic affinity for CO(2) in high C(i) cells. The addition of 200 micromolar EZ to high C(i) cells increased K(0.5)(CO(2)) from 4.6 micromolar to more than 155 micromolar at pH 8.0, whereas low C(i) cells (grown at 30 microliters CO(2) per liter of air) were less sensitive to EZ. EZ inhibition in high and low C(i) cells was largely relieved by increasing exogenous C(i) up to 100 millimolar. Lipid soluble CA inhibitors such as EZ and chlorazolamide were shown to be the most effective inhibitors of CO(2) usage, whereas water soluble CA inhibitors such as methazolamide and acetazolamide had little or no effect. EZ was found to cause a small drop in photosystem II activity, but this level of inhibition was not sufficient to explain the large effect that EZ had on CO(2) usage. High C(i) cells of Anabaena variabilis M3 and Synechocystis PCC6803 were also found to be sensitive to 200 micromolar EZ. We discuss the possibility that the inhibitory effect of EZ on CO(2) usage in high C(i) cells of Synechococcus PCC7942 may be due to inhibition of a ;CA-like' function associated with the CO(2) utilizing C(i) pump or due to inhibition of an internal CA activity, thus affecting CO(2) supply to ribulose bisphosphate carboxylase-oxygenase.