Geomicrobiology of Ferromanganese Deposits in Caves of the upper Tennessee River Basin

Ferromanganese deposits in epigenic cave systems located within the Knox Dolomite bedrock formation of the upper Tennessee River Basin were analyzed to determine the role of microbes in the cycling of manganese and the transformation of cave mineralogy. Results from geochemical analyses, molecular surveys, and culture-dependent surveys indicate that Mn(II)-oxidizing bacteria are abundant and environmentally relevant members of ferromanganese deposits in the study area, and that these cave systems harbor unique microbial communities. Over the duration of the study period (July 2009-2011), a decline in the intensity of ferromanganese deposits in a cave biofilm was noted. Because a sewage discharge may have occurred at this site prior to July 2009, it was hypothesized that the decline may have been associated with the gradual abatement of an acute, point source of pollution. The change in biofilm appearance combined with water chemistry data and molecular-based and culture-dependent results suggest an association between the bloom of Mn(II)-oxidizing bacteria and a nutrient loading event. More work is needed to determine if blooms of Mn(II)-oxidizing bacteria are widespread where contamination is prevalent. However, if this hypothesis is validated, monitoring of Mn-oxidizing microbial communities would provide a method of assessing anthropogenic impact in shallow cave systems.