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.
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The authors would like to thank Dr. Clara Chan for assistance with electron microscopy and sample preparation, and Dr. Guichuan Hou and Oliver Burns for assistance with light, confocal, and electron microscopy. We thank Dr. Yongli Gao, Taylor Burnham, Seth Hewitt, John Rossi, Milton Starnes, Robbie Winters, and John Matthews for providing field expertise and scouting locations of cave ferromanganese deposits, and to the landowners of the caves for site access. We are appreciative of assistance from Dr. Carol Babyak, Dr. Shea Tuberty, Daniel Jackson, and Yosuke Sakamachi in preparing samples and in conducting ICP-OES analyses. We are deeply grateful for the assistance of Dr. Trevor Craig, Zach Anderson, Jared Butler, Kornelia Galior, Ashley Hawkins, Daniel Parker, Leigh Anne Roble, Marlie Shelton, and Bryan Zorn in culture maintenance, field assistance, and procedural development. Partial support was provided through a National Science Foundation grant 0935270 to S. Bräuer, two North Carolina Space Grant New Investigators Program Awards to S. Carmichael and to S. Bräuer, and through a North Carolina Space Grant Graduate Research Fellowship awarded to M.J. Carmichael. Support was also provided by Appalachian State University.
- Tennessee karst
- cave geomicrobiology
- ferromanganese deposits
- manganese oxidation