Phylogenetic and physiological diversity of dissimilatory ferric iron reducers in sediments of the polluted Scheldt estuary, Northwest Europe

The potential for dissimilatory ferric iron [Fe(Ill)] reduction in intertidal sediments of the polluted Scheldt estuary, Northwest Europe, was assessed by combining field-based geochemical measurements with laboratory experiments on the associated microbiology. Microbial communities at a freshwater and brackish location were characterized by culture-independent 16S rRNA gene analysis, as well as enrichments, strain isolation and physiological screening. Dilution-to-extinction batch enrichments using a variety of Fe(III) sources were performed. The dilution factor of the inoculum in the enrichments had a more determining effect on the Fe(III)-reducing microbial community structure than the Fe(III) source. Well-known Fe(III) reducers, including members of the family Geobacteraceae and the genus Shewanella, constituted only a small fraction (<= 1%) of the in situ microbial community. Instead, facultative anaerobic Ralstonia and strictly anaerobic, spore-forming Clostridium species dominated Fe(III) reduction. These species were able to utilize a variety of electron acceptors. This flexibility may help the organisms to survive in the dynamic estuarine environment. The high diversity and abundance of culturable Fe(III) reducers (4.6 x 10(5) and 2.4 x 10(4) cells g(-1) sediment at the freshwater and brackish site respectively), plus the high concentrations of chemically reducible solid-phase Fe(III) at the sites, implied a high potential for dissimilatory Fe(III) reduction in the estuarine sediments. Pore water chemical data further supported in situ dissimilatory Fe(III) reduction.