Bacterial diversity and seasonal changes in iron microbial mats formed in neutrophilic, fresh-water environments.

Microorganisms play an important and sometimes underestimated role in the cycling of elements in Earth's subsurface environments. Iron-bacteria are particular microorganisms which use ferrous (Fe2+) to ferric (Fe3+) iron oxidation as an energy source through chemolithotrophic process. These bacteria produce cm- to dm-thick ochre deposits called ferruginous or iron bacterial mats. Such mats are commonly observed in natural or anthropogenic environments such as quarries where iron-rich groundwater flows out at spring mouths. Similar deposits also form in mine drainage systems and may cause clogging problems in drinking water wells. The importance of iron bacterial mats in geology and industry is demonstrated by the large number of studies devoted to them. The retention capacity of heavy metals in these mats is actively investigated for its importance in environmental problems. The sorption properties of iron oxi-hydroxides such as ferrihydrite are often put forward against biotic processes but few studies have investigated the role of bacteria so far. In this study, we focus on iron bacterial mats which develop in neutrophilic freshwater environments. Two sites in natural environment (Trô Maret Valley near Malmédy and the Helle Valley near Eupen) and one site in anthropogenic environment (Hautrage quarry near Mons) are currently being analyzed on the basis of a two years sampling survey. The samples were first analyzed using chemical (FTIR, ICP-MS), mineralogical (XRD) and biological techniques such as 4'6'-diamidino-2-phenylindole (DAPI) and denaturing gradient gel electrophoresis (DGGE). Filamentous structures (morphotypes) were studied and counted under the optical microscope. Their relative abundance is clearly varying over time and could relate to seasonal change in bacterial activity. DAPI stains show a consistent trend. Variations of environmental parameters such as pH, temperature and dissolved species are recorded but not fully understood at the moment. Biodiversity as reflected by DGGE shows that iron bacterial mats are favorable for bacterial growth since up to 30 different bacterial species were detected. Morphologically-distinct iron-oxidizing bacteria such as Gallionella and Leptothrix were observed under the optical microscope but they seemingly represent only a portion of the bacterial community in the mat. The identification of all the bacteria in the community is necessary to first investigate the biotic processes that are taking place in the system and secondly to assess their sensitivity to environmental factors. The identification of the different bacteria is currently in progress using cloning and sequencing of the 16S rRNA.