Sustained Anthropogenic Impact in Carter Saltpeter Cave, Carter County, Tennessee and the Potential Effects on Manganese Cycling

Abstract

Anthropogenic impact is a pervasive problem in heavily trafficked cave systems and fecal contamination is equally problematic in many cave and karst waters worldwide. Carter Saltpeter Cave in Carter County, Tennessee exhibits Mn(III/IV) oxide coatings associated with groundwater seeps, as well as manganese oxide growth on litter. Culturing results revealed that Mn(III/IV) oxide production on litter was associated with Mn(II)-oxidizing fungi. Immediately prior to this study, a massive Mn(II)-oxidizing biofilm bloomed at a cave seep. During the course of this study from 2009-2011, the seep exhibited a dramatic visual reduction in Mn(III/IV) oxide production, which was hypothesized to correlate with a decrease in fecal nutrient input. Molecular methods (16S rRNA gene sequencing) confirmed the presence of Bacteroides-Prevotella human fecal indicators in this seep, and most probable number assays and ion chromatography of the associated seep water confirmed nutrient loading at the site. Further, phylogenetic analysis from clone sequences suggested a strong initial human-specific fecal signature (50% of the sequences clustering with human feces sequences) in July 2009, and a weaker human signature (20% clustering) by June 2011. Most Probable Number (MPN) analyses of heterotrophic bacteria at this site suggested that Mn(II) oxidation was correlated with heterotrophic activity, due to point source exogenous nutrient loading.