Surface and Subsurface Manganese Biofilms: A Comparison of Biofilms from Desert Varnish and Caves

Manganese oxides are a common product of biofilms in metal-oxidizing environments. The morphologies of the oxide products are diverse but the forms of manganese oxides are not always characteristic of the environment in which they occur. For example, desert varnish is thinly laminated and rarely more than 200 micrometers thick, but displays contortions and variability in thickness within the layers, whereas deep marine nodules and fluvial manganese deposits exhibit smooth and simple laminations.

Snowy River, a recently opened extension to Ft. Stanton Cave, NM consists of snow-white calcite lining the floor of "borehole" passage and walls and ceilings lined with jet black manganese oxide. With a cursory examination, this dark coating resembles surface desert varnish, except that the coating and underlying clay are soft and will deform under light finger pressure. In the scanning electron microscope (SEM), the manganese oxides appear as grape-like clusters of 2 micrometer spheres of oxide; thus, the very dark coating is actually discontinuous in the microscale. In cross-section the clusters are laminated on a sub-micrometer scale, and are seen to originate below the surface and extend upward in continuously laminated "pillars," not unlike stromatolites. These patterns are similar to those in desert varnish from Socorro, NM and Hanksville, UT, where cyanobacteria communities form laminated varnish, with botryoidal forms resembling microstromatolites. Preliminary DNA sequence results from the manganese crusts in Snowy River indicate that closest relatives of the organisms are from other caves (ferromanganese deposits in Lechuguilla Cave, NM; Frassasi Cave, Italy; and Weebubbie Cave on the Nullabor Plain, Australia), iron oxidizers and soil and rhizosphere residents. Although Lechuguilla Cave hosts similar microbial communities to both desert varnish, the processes are different (corrosion of carbonate bedrock), and thus the characteristic of the manganese oxides in Lechuguilla is distinctly different from those environments.