Mn(II)-oxidizing Bacteria are Abundant and Environmentally Relevant Members of Ferromanganese Deposits in Caves of the Upper Tennessee River Basin

Abstract

The upper Tennessee River Basin contains the highest density of our nation's caves; yet, little is known regarding speleogenesis or Fe and Mn biomineralization in these predominantly epigenic systems. Mn:Fe ratios of Mn and Fe oxide-rich biofilms, coatings, and mineral crusts that were abundant in several different caves ranged from ca. 0.1 to 1.0 as measured using ICP-OES. At sites where the Mn:Fe ratio approached 1.0 this represented an order of magnitude increase above the bulk bedrock ratio, suggesting that biomineralization processes play an important role in the formation of these cave ferromanganese deposits. Estimates of total bacterial SSU rRNA genes in ferromanganese biofilms, coatings, and crusts measured approximately 7×107–9×109 cells/g wet weight sample. A SSU-rRNA based molecular survey of biofilm material revealed that 21% of the 34 recovered dominant (non-singleton) OTUs were closely related to known metal-oxidizing bacteria or clones isolated from oxidized metal deposits. Several different isolates that promote the oxidation of Mn(II) compounds were obtained in this study, some from high dilutions (10–8–10–10) of deposit material. In contrast to studies of caves in other regions, SSU rRNA sequences of Mn-oxidizing bacterial isolates in this study most closely matched those of Pseudomonas, Leptothrix, Flavobacterium, and Janthinobacterium. Combined data from geochemical analyses, molecular surveys, and culture-based experiments suggest that a unique consortia of Mn(II)-oxidizing bacteria are abundant and promoting biomineralization processes within the caves of the upper Tennessee River Basin.