PHOTOSYNTHESIS OF PROCHLOROTHRIX-HOLLANDICA UNDER SULFIDE-RICH ANOXICCONDITIONS

The photosynthetic activity and photosystem II fluorescence of Prochlo rothrix hollandica were studied under anoxic, sulfide-rich conditions. Oxygenic photosynthetic activity with water as the electron donor was highly resistant to inhibition by sulfide. Cells still retained 50% o f their oxygenic photosynthetic activity at > 1 mM sulfide. In the pre sence of DCMU [N-(3,4-dichlorophenyl)-N'-dimethylurea], an inhibitor o f photosystem II activity, P. hollandica cells exhibited a low but sig nificant anoxygenic photosynthetic activity when sulfide was present. This activity increased with higher sulfide concentrations and reached maximal rates at concentrations exceeding 1 mM sulfide. The effects o f hydroxylamine on both oxygen evolution and fluorescence induction ki netics were similar to those observed for sulfide. It was concluded th at the oxidizing site of photosystem II was the site of sulfide action leading to reduced or even fully inhibited electron donation to photo system II. These observations bear similarity to the situation in some cyanobacteria in which both hydroxylamine and sulfide inhibit electro n donation from H2O to P-680. The high resistance of photosystem II to sulfide is related to the hydrophobic nature of the manganese-stabili zing protein in P. hollandica (T. S. Mor, A. F. Post, and I. Ohad, Bio chim. Biophys. Acta 1141:206-212, 1993). The observed sulfide toleranc e of P. hollandica may confer a competitive advantage in its natural e nvironment, where it forms a dominant fraction of phytoplankton in wat ers in which sulfide presence is a recurring phenomenon.