THE ORGANIZATION OF MICROBIAL MN-OXIDE PRECIPITATES INTO MINERAL PHASES: A COMPARISON STUDY OF CULTURED AND NATURAL MATERIALS

Microbial oxidation of Mn is common in the terrestrial environment and can be found in deep marine, lake sediment, and hot springs environments. We are studying Lechuguilla and Spider Caves in Carlsbad Caverns National Park, New Mexico where Mn-oxides are abundant on cave surfaces and present even on some speleothems. Mn-minerals (mainly todorokite and feitknechite) are often associated with Fe- and Al-minerals including goethite, hematite and nordstrandite. Amorphous Mn and Fe oxyhydroxides are also present. The most Mn-rich samples contain over 20 wt% MnO, and are enriched relative to the underlying bedrock by as much as 4 orders of magnitude. Prior work has shown that these deposits are microbially influenced. Evidence includes growth of microorganisms on Mn and Fe enrichment medium, fluorescence microscopy revealing both active and inactive microorganisms, and DNA analysis that yields a community of microorganisms, including Mn-oxidizers. In culture, biotically precipitated Mn-oxides are initially amorphous or poorly crystalline oxides. The exact mechanism of precipitation and metabolic pathway is poorly understood, as is the path by which the amorphous material organizes into crystalline minerals.

We are isolating and growing organisms from cave deposits on Mn-enriched media. The resultant mineral products in the cultures are identified by x-ray diffraction, SEM imaging, TEM imaging and electron diffraction, and EELS characterization of oxidation state. Cultures only a few weeks old show visible growth of dark Mn-rich material, while control samples show no visible growth. Both live and killed cultures and controls are examined at intervals of weeks to months to determine whether microbial activity has an influence on the degree of crystallinity. Initial results indicate that young cultures produce mainly amorphous phases with only weak crystalline peaks visible in XRD. Crystallinity is increased as the cultures age. This seems to parallel the natural samples where a range of both crystallinity and oxidation state are present.