GEOMICROBIOLOGY OF CAVE MN OXIDE DEPOSITS IN EASTERN TENNESSEE

Manganese oxide coatings and deposits are very common in caves and karsts in the southern Appalachians. They play an important role in cave ecology, as they are associated with unusual microbial communities that use the available Mn for chemosynthesis. In cave systems that contain abundant manganese oxides, it is not yet known if these deposits form via biomineralization processes or if the microbes are metabolizing the existing mineral deposits.

Carter Salt Peter Cave and Worley’s Cave, both located in the Knox Dolomite in east Tennessee, contain a variety of Mn oxide deposits. Deposit morphologies include smooth black surface coatings over flowstones, rimstones, and muds, and a black, bubbly “popcorn” texture.

Leucobeurbelin Blue (LBB) field tests for bacterial Mn(II)-oxidation yielded strong positive reactions for all of the biofilm samples collected, suggesting high concentrations of Mn oxides. Preliminary ICP analyses of available Mn and Fe oxides, defined as oxalic acid-reducible Fe and Mn, were conducted using biofilm material from a muddy substrate in Carter Salt Peter Cave, called the Mn slide. The ICP data supported the LBB test observations by revealing unusually high Mn to Fe ratios (ca. 4.0) for the biofilm material compared to that of wetland or deep sea biofilms studied previously (ca 0.003-0.1). These data suggest that Mn oxidation dominates over Fe oxidation in these samples. DNA sequencing coupled with ARB, PHYLIP, and DOTUR analyses of two samples from that same biofilm (Mn slide) indicated that the genetic biodiversity within the biofilm community was significantly greater than that of bacterial biofilms found in typical Fe-dominated wetland environments.

SEM imaging and X-ray mapping of these Mn oxide coatings and “popcorn” samples from Carter Salt Peter Cave showed that the Mn oxides were distinct from Fe oxides, i.e. no solid solutions between the two were found. Mn oxides formed smooth, thin (<20 µm) coatings over existing pebbles, and raised structures that resembled rounded snowflakes.

Powder X-ray diffraction (XRD) analysis of the HCl-insoluble residues for Carter Salt Peter Cave samples indicated the presence of quartz, kaolinite, illite, and birnessite (MnO₂). Future TEM imaging and analyses of these samples will allow more detailed chemical and morphological characterizations.