DISTRIBUTION OF FE(II) OXIDIZING MICROBIAL ACTIVITIES IN ACID MINE DRAINAGE-DERIVED IRON MOUNDS

Acid mine drainage (AMD) results when water fills abandoned coal mines, leading to the oxidative dissolution of subsurface iron sulfides, creating iron rich acidic fluids. When AMD emerges on the terrestrial surface, dissolved Fe(II) and atmospheric oxygen provide substrates for acidophilic Fe(II) oxidizing bacteria capable of mediating the precipitation of the Fe(III) (hydr)oxides. This microbial activity facilitates the formation of Fe(III) rich deposits called “iron mounds.” Our previous work indicated that Fe(III) and O₂ reduction occur simultaneously in the uppermost sediments where atmospheric oxygen is abundant, and Fe(II) oxidation occurs in sediments depleted in O₂. To further characterize the potential for Fe(II) oxidation in O₂ depleted sediments, we collected core samples from an iron mound at the Mushroom Farm in North Lima, Ohio, and returned them to our laboratory for analysis. We established vertical geochemical and electrochemical profiles in the iron mound cores by combining pH and dissolved oxygen micro-profiling, electrical potential measurements and quantification of sediment-associated Fe(II) by depth. Our results indicated that Fe(II) oxidation occurs in the iron mound sediments despite limited O₂ availability, as evidenced by decreasing Fe(II) concentrations and decreasing pH with increasing depth. These signatures correspond with electrical potential measurements that may be indicative of microbial extracellular electron transfer processes. Further characterization is currently underway in laboratory incubations to evaluate geochemical signatures associated with a Fe(III) reducing/Fe(II) oxidizing bacterial cultures isolated from iron mound sediments. The organism will be incubated in agarose plugs constructed using synthetic acid mine drainage (SAMD) and a similar approach to that employed above will be used to evaluate the influences of O₂ diffusion on the depth-dependent activities of this organism.