FUNGI AND BACTERIA AS IMPORTANT FACILITATORS FOR MANGANESE OXIDE MINERAL FORMATIONS IN MADISON CAVE, VIRGINIA

Ferromanganese cave deposits largely arise from redox gradients in which anoxic groundwater interacts with oxygenated cave atmospheres. Microbial-induced oxidative precipitation is increasingly recognized as a potential mechanism for manganese (Mn) oxide formation. The extent of microbial involvement in cave systems remains unclear. To better understand the role of microbes in the formation of Mn oxide cave deposits, Madison Cave, Virginia, USA, was studied due to its historical significance, ecological importance, and abundant ferromanganese deposits. The aqueous geochemistry of the two cave lakes were analyzed, revealing low aqueous Mn concentrations (<1.5 pbb). Sterile swabs and solids from various deposits (e.g., vermiculations, vugs, and wall coatings) that tested positive for oxidized Mn via the leucoberlin blue (LBB) test were collected for microbial and geochemical analyses. Mn oxidizing microbes were then isolated from sterile swabs and sediment dilutions using a series of AY and K media containing plates with and without HEPES buffer. Post isolation, LBB staining confirmed Mn oxidation activity. A total of 10 Mn-oxidizing microbes were successfully isolated, the majority originating from vermiculations, with polymerase chain reaction (PCR) identifying seven as fungi (ITS-1 and ITS-4 primers) and one as bacterial (16S primer). The rates of microbial growth and Mn oxidation was assessed in the absence and presence of HEPES, finding several microbes grow faster with HEPES. Scanning electron microscopy paired with energy-dispersive X-ray spectroscopy of the cave sediments reveals Mn oxides from Madison Cave contain appreciable concentrations of aluminum, calcium, magnesium, and nickel. Additionally, X-ray diffraction confirms the presence of quartz, gypsum, calcite, and aragonite within the cave sediments, as well as ranciéite. The successful isolation of Mn-oxidizing microbes, coupled with their demonstrated Mn oxidation rates, indicates microbes likely play an active role in the formation of Mn oxide cave deposits in Madison Cave. The elemental and mineralogical analyses further highlight the complexity of the cave’s geochemical environment, where both abiotic and biotic processes interact to produce the observed deposits.