Microbial cycling of iron and sulfur in sediments of acidic and pH-neutral mining lakes in Lusatia (Brandenburg, Germany)

Abstract

A vast number of lakes developed in the abandoned opencast lignite mines of Lusatia (East Germany) contain acidic waters (<pH 3) with high concentrations of dissolved iron and sulfate. The elimination of iron and sulfate by microbially mediated reduction processes produces alkalinity, whereas the reoxidation of reduced sulfur compounds consumes alkalinity. In order to determine the influence of environmental factors with the prospect of biological remediation strategies, these processes were investigated in sediments of two acidic and one pH-neutral mining lake. Slightly acidic sediment underlying permanently anoxic waters showed a higher accumulation rate of reduced inorganic sulfur than the pH-neutral sediment (60 v.s. 45 mmol S m−2 a−). Potential Fe(III) reduction measured by the accumulation of Fe(II) during anoxic incubation yielded similar rates in both types of sediments, however, the responses towards the supplementation of Fe(III) and organic carbon were different. Sulfate reduction rates estimated with 35S-radiotracer were much lower in the slightly acidic sediment than in the pH-neutral sediment (156 v.s. 738 mmol SO4 2− m−2 a−1). However, sulfate reduction rates were increased by the addition of organic carbon. Severe limitation of sulfate-reducing bacteria under acidic conditions was also reflected by low most probable numbers (MPN). High MPN of acidophilic iron- and sulfur-oxidizing bacteria in acidic sediments indicated a high reoxidation potential. The results show that potentials for reductive processes are present in acidic sediments and that these are determined mainly by the availability of oxidants and organic matter.