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, lo w nutrient status, and high concentrations of Fe(II) and sulfate due to the oxidation of pyrite. Most microbiological studies of sulfidic mine tailing s concentrate on processes in the oxic zone, and on the ability of acidophi lic 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 sedim ents, and the reduction of sulfate occurs in sediments of elevated pH. In t his study, microcosm experiments were performed with sediments of a coal mi ning-impacted lake to study the microbial turnover of sulfur under anoxic c onditions. Microcosms with sediment of the zone of maximum sulfate-reducing activity indicated that sulfidic minerals subsequently formed due to the r eduction of sulfate were not stable under anoxic conditions. Supplemental N a2S stimulated the formation of Fe(II) and sulfate, whereas supplemental S- 0 had no effect under pH 5 conditions. Low numbers of S-0-utilizing Fe(III) -reducing bacteria were enumerated in this sediment zone. In contrast, sedi ment microcosms of the upper acidic sediment zone demonstrated that supplem ental elemental S-0 stimulated the formation of Fe(II) and sulfate, whereas supplemental Na2S had no effect. Most probable number estimates demonstrat ed that S-0-utilizing Fe(III)-reducing bacteria cultivated at pH 3 approxim ated to 1% of the total direct counts of bacteria in the upper acidic sedim ent zone. Sulfate reducers cultured at pH 5.2 were more abundant than sulfa te reducers cultured at pH 6.4 in both sediment zones. A sulfate-reducing b acterium, Lau III, was isolated at the in situ pH of 5.2 from the highest g rowth positive dilution series from the zone of maximum sulfate-reducing ac tivity. 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-u tilizing 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-toleran t sulfate reducers are involved in the reduction of sulfate; (ii) sulfide o r sulfidic minerals are reoxidized under anoxic conditions; and (iii) the a naerobic turnover of sulfur is affected by the pH gradient in this impacted heterogeneous lake sediment. (C) 2001 Elsevier Science B.V. All rights res erved.