A sediment exchange experiment to assess the limiting factors of microbial sulfate reduction in acidic mine pit lakes

Purpose

Microbial sulfate reduction is an alkalinity-producing process and potentially supports the neutralization of acidic mine pit lakes. In many acidic lakes the process does not occur. Sulfate-reducing bacteria are known to be pH sensitive. There are, however, several reports of sulfate reduction occurring in the sediment of acidic lakes. To find out why sulfate reduction occurs in some acidic lakes but not in others, we conducted a field experiment.

Materials and methods

Surface sediment from lake ML111 (pH?2.6, no sulfate reduction), in the Koyne-Plessa lignite mining district of Lusatia in Germany, was incubated in the less-acidic lake ML117 (pH?3.4, sulfate reduction) and vice versa. After 19?weeks of incubation, the sediments were sampled and analyzed for microbial sulfate reduction rates, bacterial numbers, and geochemical composition.

Results and discussion

Incubation of ML117 sediment in ML111 resulted in a partial inhibition of sulfate reduction while incubation of ML111 sediment in ML117 did not initiate sulfate reduction. We observed a linear relationship between sediment pH and sulfate reduction, while there was no relation with sedimentary iron content. Sulfate reduction was not only affected by the water quality but also by the experimental treatment. Homogenization of the sediment prior to incubation stimulated microbial sulfate and iron reduction. Due to the low pH, incubation in ML111 resulted in the dissolution of reduced inorganic sulfur.

Conclusions

We conclude that the water pH is the major regulator of sulfate reduction in the surface sediment of acidic lakes. The rate of sulfate reduction in the sediment of acidic lakes depends on a fragile equilibrium between proton flux between water and sediment, and buffering reactions in the sediment.