Prokaryotic Abundance and Community Composition in a Freshwater Iron-Rich Microbial Mat at Circumneutral pH

The abundance, diversity and composition of bacterial and archaeal communities in a freshwater iron-rich microbial mat were investigated using culture-dependent and culture-independent methods. The sampling site is a mixing zone where ferrous-iron-rich fluids encounter oxygen-rich environments. Quantitative PCR analysis shows that Bacteria dominated the mat community (>99% of the total cell numbers). Phylotypes related to iron-oxidizers in Gallionellaceae, methano/methylotrophs in Methylophilaceae and Methylococcaceae, sulfide-oxidizers in Sulfuricurvum and an uncultured clone group, called Terrestrial group I or the 1068 group, in the Epsilonproteobacteria were detected in the clone library from the original sample and/or the enrichment cultures. This result suggests that these members may play a role in Fe, S and C cycling in the mixing zone. Although Archaea were minor constituents numerically, phylogenetic analysis indicates that unique and diverse yet-uncultivated Archaea are present in the iron-rich mat. The phylotypes of these yet-uncultivated Archaea belong to environmental clone groups that have been recovered from other mixing zones in terrestrial and marine environments, and some of our phylotypes have significantly low similarity (80% or lower) with the archaeal clones reported previously. Our results provide further insights into the bacterial and archaeal communities in a microaerobic iron-rich freshwater environment in mixing zones.