Identifizierung eisenreduzierender Mikroorganismen in anoxischem Reisfeldboden mit Hilfe stabiler Isotope

The importance of microbial iron reduction and the microorganisms involved in this process increased within the last decades. Dissimilatory iron reduction is the second most important process of organic matter mineralization after methanogenesis in flooded rice field soil. However, very little is known about the diversity of microbial populations that conserve energy to support growth from ferric iron reduction. In this project, the microbial reduction of iron was analyzed for the first time in flooded rice field soil in the presence of distinct iron(III) oxides (ferrihydrite, lepidocrocite, goethite and hematite) and different carbon sources (13C2-acetate and 13C3-lactate). Stable isotope probing of RNA is a direct way of linking function and phylogenetic affiliation to metabolic active microorganisms and allows us to identify known and as yet uncultured iron reducers as functional guild in rice field soil. The addition of acetate as central intermediate in the anaerobic degradation of organic matter resulted in the detection of Geobacter and Anaeromyxobacter spp. (delta-Proteobacteria) under iron-reducing conditions. In the presence of goethite a coexistence of these species was observed, indicating the development of distinct ecological niches within the system. Novel bacterial clusters were assigned to the Clostridiaceae and Paenibacillaceae (Firmicutes) but have so far not been directly linked to dissimilatory iron reduction. Also in the control (without additional iron oxide), a specific incorporation of label (13C) was observed and the analysis of the 16S rRNA showed a strong activation of Rhodocyclaceae-related populations (beta-Proteobacteria). The presence of this population in the microcosms with goethite and hematite confirm our hypothesis, that these as yet uncultured Rhodocyclaceae can reduce iron(III) mineral phases with low bioavailability. Besides identifying dissimilatory iron reducers under distinct physiological conditions, we analyzed the influence of different properties (crystallinity, surface area and particle size) of the iron oxide mineral phases on the microbial reduction. The amendment of different phases of iron oxides selected for distinct bacterial populations of iron reducers in flooded rice field soil. Apart from the development of iron oxide specific and phylogenetically diverse populations, species of the genus Geobacter showed a selective reduction of certain iron oxides with different redox potentials. This observation indicates a high functional diversity within this phylogenetic, on the basis of 16S rRNA very similar, group of microorganisms. In summary, our study shows that iron reducing microorganisms are much more diverse in nature as assumed so far. Not only the available electron donor but also the iron mineral phase has a substantial influence on the metabolically active microbial community. Furthermore, the identification of the iron reducing microorganisms as functional guild contributes significantly to the knowledge of iron reduction and associated microbial processes in the model system rice field soil.