Effect of supplemental electron donors on the microbial reduction of Fe(III), sulfate, and CO2 in coal mining-impacted freshwater lake sediments

In acidic mining-impacted lake sediments, the microbial reduction of Fe(III ) is the dominant electron-accepting process, whereas the reduction of sulf ate seems to be restricted to a narrow sediment zone of elevated pH and low er amounts of total and reactive iron. To evaluate the microbial heterogene ity and the commensal interactions of the microbial community, the flow of supplemental carbon and reductant was evaluated in four different zones of the sediment in anoxic microcosms at the in situ temperature of 12 degreesC . Substrate consumption, product formation, and the potential to reduce Fe( III) and sulfate were similar with both upper and lower sediment zones. In the upper acidic iron-rich sediment zone, the rate of Fe(II) formation 204 nmol ml(-1) d(-1) was enhanced to 833 nmol ml(-1) d(-1) and 462 nmol ml(-1) d(-1) by supplemental glucose and H-2, respectively. Supplemental lactate and acetate were not consumed under acidic conditions and decreased the rat e of Fe(II) formation to 130 nmol ml(-1) d(-1) and 52 nmol ml(-1) d(-1), re spectively. When the pH of the upper sediment increased above pH 5, acetate -dependent reduction of sulfate was initiated even though the pool of Fe(II I) was not depleted. In deeper sediment zones with elevated pH, the rapid c onsumption of acetate was always coincident to a decrease in the concentrat ion of sulfate and soluble Fe(II), indicating the formation of Fe(II) sulfi des. Although the reduction of Fe(III) was still an ongoing process in deep er sediment zones, the formation of Fe(II) was only slightly enhanced by th e consumption of glucose or cellobiose, but not by H-2 or acetate. H-2-util izing acetogens seemed to be involved in the consumption of H-2. These coll ective results indicated (i) that the reduction of Fe(III) predominated ove r the reduction of sulfate as long as the sediment remained acidic and carb on-limited, and (ii) that the sulfate-reducing microbiota in this heterogen eous sediment were better adapted to the geochemical gradients present than were other neutrophilic dissimilatory Fe(III) reducers.