Oxygen and sulfur isotope fractionation during sulfide oxidation by the purple sulfur bacterium Allocromation vinosum and the green sulfur bacterium Chlorobaculum tepidum

Abstract

Sulfide mediated anoxygenic photosynthesis (SMAP) carried out by anaerobic phototrophic bacteria may have played an important role in sulfur cycling and the formation of sulfate in the Earth's Archaean oceans. Determination of epsilon (super 34) S (sub SO4-HS) and epsilon (super 18) O (sub SO4-H2O) values for bacterial sulfide oxidation will permit more accurate interpretation of the delta (super 34) S and delta (super 18) O (sub SO4) values measured in the ancient rock record and modern anoxic environments. This is especially true in the Archaean where more accurate interpretation of the rock record could bring insight into the origin of sulfate in the ocean and perhaps also reveal early bacterial metabolisms operating at that time. Laboratory experiments with anaerobic purple and green sulfur phototrophs, Allochromatium vinosum and Chlorobaculum tepidum, respectively, were conducted to determine the sulfur and oxygen isotope fractionation during the oxidation of sulfide to sulfate. Replicate experiments were conducted at 25 degrees C and 45 degrees C for A vinosum and C. tepidum, respectively, and in duplicate at three different starting oxygen isotopic values of water to accurately determine sulfate-water oxygen isotope fractionations (epsilon (super 18) O (sub SO4-H2O) ). epsilon (super 18) O (sub SO4-H2O) values of 5.6+ or -0.2 ppm and 5.4+ or -0.1 ppm were obtained for A. vinosum and C. tepidum, respectively. Temperature had no apparent effect on the epsilon (super 18) O (sub SO4-H2O) values. By combining all data from both cultures, an average epsilon (super 18) O (sub SO4-H2O) value of 5.6+ or -0.3 ppm was obtained for SMAP. This fractionation is unique and falls between that previously reported for bacterial oxidation of sphalerite and elemental sulfur (7-10 ppm) and abiotic and biotic oxidation of pyrite and chalcopyrite (2-4 ppm). There was negligible sulfur isotope fractionation between sulfide and sulfate formed by A. vinosum (0.1+ or -0.2 ppm) during all experiments. Sulfur isotope fractionation between sulfide and sulfate formed by C tepidum was also small (-0.7+ or -0.3 ppm), although a larger fractionation of -2.3+ or -0.5 ppm was observed during earlier stages of oxidation when the sulfate concentration was below 0.5 mM.