Interaction of Fe(III)-reducing bacteria with heavy metals in contaminated soils
Iron oxides are important metal-sorbents and Fe(III)-reducing bacteria (FeRB) can directly or indirectly alter the availability of metals in soils. The sediment subjacent to a former leaching heap and the bank soil of the creek Gessenbach in a former U-mining district (Ronneburg, Germany) showed high concentrations of metals and radionuclides both in the porewater and the solid phase. In this thesis i) the impact of FeRB on metal retention and ii) the tolerance of indigenous FeRB towards metal stress were investigated. In the sediment subjacent to the former leaching heap metal retention was likely not influenced by microbial activity due to low microbial abundance and activity and carbon-limitation. In the creek soil metals were enriched in an iron-rich horizon (Btlc) and in a sulfate-rich horizon. Geochemical data indicated that Fe(III)-reduction might be an important process in Btlc but reduction rates were low suggesting an inhibition of Fe(III)-reduction in situ. In biostimulated soil slurries, concentrations of soluble metals, including U, increased during Fe(III)-reduction, likely by desorption during reductive dissolution of Fe(III)-oxides. The active Fe(III)-reducing population as determined by stable isotope probing was dominated by δ-Proteobacteria (Geobacter) in 13C-ethanol amended microcosms. A more diverse community was present in 13C-lactate amended microcosms including especially Acidobacteria and Firmicutes. Incubation of Fe(III)-reducing enrichment cultures in the presence of metals suggested, that Fe(III)-reduction might be inhibited by soil concentrations of Cu and Ni. Sequencing of DGGE bands demonstrated a dominance of Firmicutes in Zn-tolerant cultures, suggesting an importance of Firmicutes for Fe(III)-reduction in contaminated environments. Altogether, FeRB in Btlc might be able to facilitate the release of metals and U. However, their activity seems to be inhibited in situ and might depend on a metal detoxification mechanism.