Oxygen and sulfur isotope fractionation during sulfide oxidation by anoxygenic phototrophic bacteria

Abstract

Sulfide-mediated anoxygenic photosynthesis (SMAP) carried out by anaerobic phototrophic bacteria may have played an important role in sulfur cycling, formation of sulfate, and, perhaps, primary production in the Earth’s early oceans. Determination of ε34SSO4-Sulfide- and ε18OSO4-H2O values for bacterial sulfide oxidation will permit more refined interpretation of the δ34S and δ18OSO4 values measured in modern anoxic environments, such as meromictic lakes where sulfide commonly extends into the photic zone, and in the ancient rock record, particularly during periods of the Precambrian when anoxic and sulfidic (euxinic) conditions were believed to be more pervasive than today. 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°C for A. vinosum and 45°C for C. tepidum, and in duplicate at three different starting oxygen isotope values for water to determine sulfate-water oxygen isotope fractionations accurately (ε18OSO4-H2O). ε18OSO4-H2O values of 5.6±0.2‰ and 5.4±0.1‰ were obtained for A. vinosum and C. tepidum, respectively. Temperature had no apparent effect on the ε18OSO4-H2O values. By combining all data from both cultures, an average ε18OSO4-H2O value of 5.6±0.3‰ was obtained for SMAP. This value falls between those previously reported for bacterial oxidation of sphalerite and elemental sulfur (7–9‰) and abiotic and biotic oxidation of pyrite and chalcopyrite (2–4‰). Sulfur isotope fractionation between sulfide and sulfate formed by A.vinosum was negligible (0.1±0.2‰) during all experiments. For C. tepidum an apparent fractionation of −2.3±0.5‰ was observed during the earlier stages of oxidation based on bulk δ34S measurements of sulfate and sulfide and became smaller (−0.7±0.3‰) when sulfate concentrations rose above 0.5mM and sulfide concentrations had became negligible.