Stable sulfur isotope fractionation and discrimination between the sulfur atoms of thiosulfate during oxidation by Halothiobacillus neapolitanus

Abstract

Growing cultures and nongrowing suspensions of Halothiobacillus neapolitanus selectively fractionated 32S and 34S during the oxidation of the sulfane- and sulfonate-sulfur atoms of thiosulfate. Sulfate was enriched in 32S, with δ34S reaching -6.3‰ relative to the precursor sulfonate-sulfur of thiosulfate, which was progressively resynthesized from the thiosulfate-sulfane-sulfur during thiosulfate metabolism. Polythionates, principally trithionate, accumulated during thiosulfate oxidation and showed progressive increase in the relative 34S content of their sulfonate groups, with δ34S values up to +20‰, relative to the substrate sulfur. The origins of the sulfur in the sulfate and polythionate products of oxidation were tracked by the use thiosulfate labelled with 35S in each of its sulfur atoms, enabling determination of the flow of the sulfur atoms into the oxidation products. The results confirm that highly significant fractionation of stable sulfur isotopes can be catalyzed by thiobacilli oxidizing thiosulfate, but that differences in the 34S/32S ratios of the nonequivalent constituent sulfur atoms of the thiosulfate used as substrate mean that the oxidative fate of each atom needs separate determination. The data are very significant to the understanding of bacterial sulfur-compound oxidation and highly relevant to the origins of biogenic sulfate minerals. © 2008 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.