Effect of pH on the anaerobic microbial cycling of sulfur in mining-impacted freshwater lake sediments

Lakes caused by coal strip-mining processes are characterized by low pH, low nutrient status, and high concentrations of Fe(II) and sulfate due to the oxidation of pyrite. Most microbiological studies of sulfidic mine tailings concentrate on processes in the oxic zone, and on the ability of acidophilic bacteria to promote the production of toxic leachates. Few studies have studied microbial processes in anoxic sediment zones, where the reduction of Fe(III) often is the dominant electron-accepting process in acidic sediments, and the reduction of sulfate occurs in sediments of elevated pH. In this study, microcosm experiments were performed with sediments of a coal mining-impacted lake to study the microbial turnover of sulfur under anoxic conditions. Microcosms with sediment of the zone of maximum sulfate-reducing activity indicated that sulfidic minerals subsequently formed due to the reduction of sulfate were not stable under anoxic conditions. Supplemental Na2S stimulated the formation of Fe(II) and sulfate, whereas supplemental S0 had no effect under pH 5 conditions. Low numbers of S0-utilizing Fe(III)-reducing bacteria were enumerated in this sediment zone. In contrast, sediment microcosms of the upper acidic sediment zone demonstrated that supplemental elemental S0 stimulated the formation of Fe(II) and sulfate, whereas supplemental Na2S had no effect. Most probable number estimates demonstrated that S0-utilizing Fe(III)-reducing bacteria cultivated at pH 3 approximated to 1% of the total direct counts of bacteria in the upper acidic sediment zone. Sulfate reducers cultured at pH 5.2 were more abundant than sulfate reducers cultured at pH 6.4 in both sediment zones. A sulfate-reducing bacterium, Lau III, was isolated at the in situ pH of 5.2 from the highest growth positive dilution series from the zone of maximum sulfate-reducing activity. Analysis of the 16S rRNA gene sequence (1498 base pairs) of Lau III demonstrated that it was a member of the Clostridium-Bacillus subphylum of gram-positive bacteria related to Desulfosporosinus orientis Y11571 (96.4% 16S rRNA gene sequence similarity). Lau III was a spore-forming, lactate-utilizing sulfate reducer capable of growing at a pH range of 4.9-6.1 and a pH optimum of 5.5. These collective results indicate that, (i) acid-tolerant sulfate reducers are involved in the reduction of sulfate; (ii) sulfide or sulfidic minerals are reoxidized under anoxic conditions; and (iii) the anaerobic turnover of sulfur is affected by the pH gradient in this impacted heterogeneous lake sediment.