Sulfide and pH effects on variable fluorescence of photosystem II in two strains of the cyanobacterium Oscillatoria amphigranulata

Abstract

Changes in fluorescence of photosystem II (PS II) chlorophyll were used to monitor the in vivo effects of sulfide and pH on photosynthesis by the cyanobacterium Oscillatoria amphigranulata. O. amphigranulata is capable of both oxygenic photosynthesis and sulfide dependent anoxygenic photosynthesis. A genetic variant of O. amphigranulata which photosynthesizes oxygenically at normal rates, but is incapable of anoxygenic photosynthesis and cannot tolerate sulfide, was also used to explore the mode of action of sulfide. In vivo fluorescence responses of PS II chlorophyll in the first few seconds of exposure to light (Kautsky transients) reflected the electrochemical states of PS II and associated electron donors and acceptors. Kautsky transients showed a distinct difference between PS II of the wild type and the variant, but sulfide lowered fluorescence in both. Kautsky transients with sulfide were similar to transients with addition of NH2OH, NH4 (+) or HCN, indicating sulfide interacts with a protein on the donor side of PS II. The fluorescence steady-state (after 2 min) was measured in the presence of sulfide, cyanide and ammonium with pH ranging from 7.2-8.7. Sulfide and cyanide had the most impact at pH 7.2, ammonium at pH 8.7. This suggests that the uncharged forms (HCN, NH3 and H2S) had the strongest effect on PS II, possibly because of increased membrane permeability.