Abstract :
[en] Bacteria are ubiquitous microorganisms that are able to colonize extreme environments such as cave which are nutrient-limited and devoided of light. Microorganisms so have to use other energy sources because the photosynthesis is not possible, they use the rock itself. The specific microorganisms can use iron or/and manganese oxidation as an energy source through a process called chemolitotrophy.
The interaction between bacteria and environment in karst systems is still poorly known. However, microbial influence on the corrosion of limestone has been demonstrated in caves in New Mexico (Northup et al., 2000). Microorganisms in these caves cause the dissolution of the wall-rock through acidic metabolic by-products (the so-called 'punk-rock') and ferromanganese oxides accumulate on its surface.
In Azé Cave, south Burgundy, Saône-et-Loire (France), ferromanganese deposits form dark coatings of a few millimeters to several centimeters in thickness. Biological analyses confirmed the presence of bacteria in these deposits. However, DGGE and biomass estimation based on DAPI staining showed a poor biodiversity, suggesting a highly-specialized microbiota. The carbonate bedrock beneath the ferromanganese coating is systematically weathered into a soft, porous and Fe-stained material (the so-called 'ghost rock'). No attempt has been made yet to investigate the presence of metabolically-active bacteria in the ghost rock. However, iron-oxidizing bacteria are ubiquitous in dysaerobic environments, where low oxygen concentration allows then to compete with inorganic oxidation. Ghost rock and Mn-coatings are probably parts of the same biochemical system in which microorganisms uptake and oxidize metals from the host carbonate. Oxidized metals then migrate over a certain distance and precipitate according to their specific chemical properties. This would account for the observed decoupling of the Fe and Mn accumulation zones (Baele et al., this volume).
The next step in our investigation is to identify the microorganisms by DNA-sequencing using a metagenomic technique. This will allow us to better understand the biotic processes that are taking place in the cave. Identifying known iron- and manganese-oxidizers would provide the best evidence for supporting the hypothesis of bacterially-influenced dissolution of the wall rock and, hence formation of ghost rock.
Reference
Northup D.E., Dahm C.N., Melim L.A., Spilde M.N., Crossey L.J., Lavoie K.H., Mallory L.M., Boston P.J., Cunningham K. I. and Barns S.M., 2000. Evidence for geomicrobiological interactions in Guadalupe caves. Journal of Cave and Karst Studies, 62(2) : 80 -90
