Depth-dependent geochemical and microbiological gradients in Fe(III) deposits resulting from coal mine-derived acid mine drainage.

Justin S. Brantner,Christopher M. Menge,Shane T. Hotchkiss,John M. Senko,Zachary J. Haake,John E. Burwick

doi:10.3389/fmicb.2014.00215

Abstract

We evaluated the depth-dependent geochemistry and microbiology of sediments that have developed via the microbially-mediated oxidation of Fe(II) dissolved in acid mine drainage (AMD), giving rise to a 8–10 cm deep “iron mound” that is composed primarily of Fe(III) (hydr)oxide phases. Chemical analyses of iron mound sediments indicated a zone of maximal Fe(III) reducing bacterial activity at a depth of approximately 2.5 cm despite the availability of dissolved O2 at this depth. Subsequently, Fe(II) was depleted at depths within the iron mound sediments that did not contain abundant O2. Evaluations of microbial communities at 1 cm depth intervals within the iron mound sediments using “next generation” nucleic acid sequencing approaches revealed an abundance of phylotypes attributable to acidophilic Fe(II) oxidizing Betaproteobacteria and the chloroplasts of photosynthetic microeukaryotic organisms in the upper 4 cm of the iron mound sediments. While we observed a depth-dependent transition in microbial community structure within the iron mound sediments, phylotypes attributable to Gammaproteobacterial lineages capable of both Fe(II) oxidation and Fe(III) reduction were abundant in sequence libraries (comprising ≥20% of sequences) from all depths. Similarly, abundances of total cells and culturable Fe(II) oxidizing bacteria were uniform throughout the iron mound sediments. Our results indicate that O2 and Fe(III) reduction co-occur in AMD-induced iron mound sediments, but that Fe(II)-oxidizing activity may be sustained in regions of the sediments that are depleted in O2.

Highlights

In the Appalachian coal mining regions of the northeastern United States, acid mine drainage (AMD) arises when coal seamassociated iron sulfide phases are exposed to O2-rich fluids during or upon completion of mining activities (Baker and Banfield, 2003)
We evaluated the depth-dependent geochemistry and microbiology of sediments that have developed via the microbially-mediated oxidation of Fe(II) dissolved in acid mine drainage (AMD), giving rise to a 8–10 cm deep “iron mound” that is composed primarily of Fe(III)oxide phases
AMD that emerges from the basement window well of the vacant house flows as a sheet over the terrestrial surface, which has given rise to an iron mound covering an area of approximately 45 m2 (Gouin et al, 2013) that is composed of 7 mmol Fe/g (75% Fe(III) phases by mass, if Fe(OH)3 is assumed; Bertel and Senko, unpublished)

Summary

Introduction

In the Appalachian coal mining regions of the northeastern United States, acid mine drainage (AMD) arises when coal seamassociated iron sulfide phases are exposed to O2-rich fluids during or upon completion of mining activities (Baker and Banfield, 2003). Geochemical evaluations of the MF iron mound indicated that aerobic microbial activities were occurring most extensively in the upper 2.5 cm of the sediments, but Fe(III) reduction was occurring in this region, despite the presence of dissolved O2 to a depth of approximately 3 cm (Figures 1A,B).

Results

Conclusion