Ethoxyzolamide Inhibition of CO2-Dependent Photosynthesis in the Cyanobacterium Synechococcus PCC7942 1

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Cells of the cyanobacterium, Synechococcus PCC7942, grown under high inorganic carbon (Ci) conditions (1% CO2; pH 8) were found to be photosynthetically dependent on exogenous CO2. This was judged by the fact that they had a similar photosynthetic affinity for CO2 (K0.5[CO2] of 3.4-5.4 micromolar) over the pH range 7 to 9 and that the low photosynthetic affinity for Ci 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 CO2 in high Ci cells. The addition of 200 micromolar EZ to high Ci cells increased K0.5(CO2) from 4.6 micromolar to more than 155 micromolar at pH 8.0, whereas low Ci cells (grown at 30 microliters CO2 per liter of air) were less sensitive to EZ. EZ inhibition in high and low Ci cells was largely relieved by increasing exogenous Ci up to 100 millimolar. Lipid soluble CA inhibitors such as EZ and chlorazolamide were shown to be the most effective inhibitors of CO2 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 CO2 usage. High Ci 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 CO2 usage in high Ci cells of Synechococcus PCC7942 may be due to inhibition of a `CA-like' function associated with the CO2 utilizing Ci pump or due to inhibition of an internal CA activity, thus affecting CO2 supply to ribulose bisphosphate carboxylase-oxygenase.

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