Selective and reversible inhibition of active CO2 transport by hydrogen sulfide in a cyanobacterium
1989
Espie, G.S. | Miller, A.G. | Canvin, D.T.
The active transport of C02 in the cyanobacterium Synechococcus UTEX 625 was inhibited by H2S. Treatment of the cells with up to 150 micromolar H2S + HS- at ph 8.0 had little effect on Na+-dependent HCO3-transport or photosynthetic 02 evolution, but C02 transport was inhibited by more than 90%. C02 transport was restored when H2S was removed by flushing with N2. At constant total H2S + HS-concentrations, inhibition Of C02 transport increased as the ratio of H2S tO HS-increased, suggesting a direct role for H2S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H2S and Na+-dependent HCO3-transport, the extracellular C02 concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H2S. The inhibition of C02 transport, therefore, revealed an ongoing leakage from the from the cells of C02 which was derived from the intracellular dehydration of HCO3- which itself had been recently transported into the cells. Normally, leaked C02 is efficiently transported back into the cell by the C02 transport system, thus maintaining the extracellular C02 concentration near zero. It is suggested that C02 transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering C02 lost from the cell. A schematic model describing the relationship between the C02 and HCO3-transport systems is presented.
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