Iron Sulfide and Pyrite as Potential Electron Donors for Microbial Nitrate Reduction in Freshwater Wetlands

The potential of iron sulfide minerals to function as electron donors for microbial nitrate reduction has important consequences for freshwater wetland ecosystems. Nitrate influxes in iron sulfide-rich soils, could ultimately lead to adverse processes, such as internal eutrophication or metal toxicity. In the present study, the potential of two iron sulfide minerals (pyrite and iron sulfide) to function as electron donors for microbial nitrate reduction was investigated in a bioreactor under anoxic, freshwater conditions. Crystalline pyrite did not function as an electron donor. In contrast, an active denitrifying microbial community developed upon addition of iron sulfide. The dominant process in the iron sulfide amended bioreactor was identified, based upon stoichiometric calculations, as the oxidation of sulfide to sulfate, at the expense of nitrate or nitrite. A clone library (16S rRNA gene) generated from the reactors biomass contained sequences of Thiobacillus-like, Geothrix-like, OP10-like, and Acidovorax-like bacteria, which covered 90% of all clones. Together these results suggest that the type of iron sulfide mineral is an important determinant for the occurrence of anoxic iron sulfide mineral oxidation, and Thiobacillus-species may be key players in nitrate-dependent iron sulfide dissolution in freshwater ecosystems.