SURFACE AND SUBSURFACE MANGANESE MICROBIAL ENVIRONMENTS

Manganese and iron oxidizing microorganisms are abundant in surface rock varnishes and in unique deposits within some caves. Within these caves, manganese (and possibly iron) oxidation may provide an energy source for the microbial communities. In both arid land caves and surface desert environments, microbial communities interact with iron and manganese, yielding deposits of intimately associated Fe and Mn oxides. The geological context and bedrock composition of caves and overlying surficial outcrops may be similar but the environmental conditions are vastly different. Nevertheless, there appear to be similarities in the microbial community composition and the product minerals.

There is evidence that manganese-depositing organisms may be less susceptible to ionizing radiation (Daly et al. 2004). Furthermore, Horsburgh et al (2002) describe evidence that the accumulation of manganese may form the basis for catalytic detoxification of harmful reactive oxygen species. Thus manganese-rich environments such as rock varnish may serve as a niche for radiation-resistant life-forms.

Rock varnish communities that we have analyzed exhibit colonial habits and in cross-section resemble microscopic stromatolites protected by micro-pits in the rock surface. In samples from Utah, green cyanobacterial layers are present within white sandstone and orthoquartzites, underneath a dark surface manganese layer, which may act as a sunshade, protecting the microbial layer below. Such an arrangement resembles the organization of endolithic communities in Antarctic Dry Valley sandstones. Analysis by denaturing gradient gel electrophoresis (DGGE) reveals similarities amongst cave communities and between cave communities and surface rock varnish, while rDNA analysis indicates that cyanobacteria and actinomycetes are predominant in the varnish.

References: Daly MJ, et al. Science, 306:1025-1028 (2004). Horsburgh M, et al. Trends in Microbiology (http://tim.trends.com) (2002).