Microbial redox cycling of iron in Lake Grosse Fuchskuhle
Peatlands constitute >3% of the Earth’s terrestrial area but store approximately one third of global soil organic carbon. Although peatlands act as sinks for atmospheric carbon, they are net emitters of greenhouse gasses, like CH4 and N2O, into the atmosphere. Hence, most of the studies conducted...
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|Summary:||Peatlands constitute >3% of the Earth’s terrestrial area but store approximately one third of global soil organic carbon. Although peatlands act as sinks for atmospheric carbon, they are net emitters of greenhouse gasses, like CH4 and N2O, into the atmosphere. Hence, most of the studies conducted on peatlands focused on methanogenesis and the role of environmental factors influencing this process and very few studies focused on other electron-accepting processes. Recent studies have shown indications that Fe(III) reduction could be playing an important role in the mineralization of organic carbon in mildly acidic peat bogs. However, this process in peatlands has not been well investigated. In the first part of the work the role of Fe(III) reduction and methanogenesis as electron-accepting processes was investigated. Unlike the earlier hypothesis of sequential reduction of electron acceptors according to their redox potentials in sediments, a simultaneous reduction of Fe(III) and methanogenesis was observed in the sediment of Lake Grosse Fuchskuhle. Quantitative comparison of these processes showed that Fe(III) reduction is the dominant organic matter mineralization process compared to methanogenesis during the course of the incubations. After an initial Fe(III) reduction a fluctuating Fe(II) concentration was observed during the course of our incubation indicating a continuous anaerobic Fe(II) oxidation and reduction in this sediment. Following the above results, the second part of the work focused on identifying, enriching and characterizing microorganisms involved in anaerobic nitrate-dependent Fe(II) oxidation. These investigations indicated the chemolithotrophic nitrate-dependent Fe(II)-oxidizing nature of TM3 Actinobacteria and that these organisms could be involved in mediating anaerobic oxidation of Fe(II) in the sediment. Previous culture-independent studies had shown a widespread distribution of these Actinobacteria in natural environments and were hypothesized to be contributing to ecologically important processes; however, the physiological capabilities of these microorganisms remained unknown. To the best of our knowledge this is the first study to show the autotrophic nitrate-dependent Fe(II)-oxidizing nature of TM3 group of uncultured Actinobacteria. The third part of the thesis deals with the role of humic substances in abiotic and microbial Fe(II) oxidation. Despite the fact that Fe(II) is predominantly present in natural environments as chelated to humic substances, the role of humic substances in mediating Fe(II) oxidation has not been elucidated. Our findings indicate that the presence of humic substances could be beneficial for microorganisms oxidizing Fe(II) due to reduced abiotic Fe(II) oxidation and also possibly due to an increased energy yield caused by a lowering of the redox potential of chelated Fe(II) compared to free Fe(II). Estimations of nitrate-dependent Fe(II)-oxidizing microorganisms from Lake Grosse Fuchskuhle sediment using a cultivation-based approach showed a two-order of magnitude higher number of chemolithotrophic nitrate-dependent Fe(II)-oxidizing microorganisms when including humic substances in the growth medium. The incubations of sediment under chemolithotrophic nitrate-dependent Fe(II)-oxidizing conditions showed the enrichment of microorganisms belonging to the genus Thiomonas. Further characterization of these enrichments provided preliminary evidence of a chemolithotrophic nitrate-dependent Fe(II)-oxidizing capability of these Thiomonas strains. Lastly, Thiomonas arsenivorans strain 3As was tested for chemolithoautotrophic nitrate-dependent Fe(II) oxidation since the presence of all the genes required for mediating this physiological process were identified in the genome. These assays were performed both in the presence and absence of humic substances. A stoichiometric consumption of Fe(II) and nitrate consistent with nitrate-dependent Fe(II) oxidation was observed in the presence of humic substances under autotrophic growth conditions. In contrast, no Fe(II) oxidation either under autotrophic or heterotrophic conditions was observed in the absence of humic substances, indicating the importance of humic substances in mediating nitrate-dependent Fe(II) oxidation. To the best of our knowledge this is the first study to show a chemolithotrophic nitrate-dependent Fe(II)-oxidizing physiology in a bacterial pure culture. Furthermore, the findings of the study indicate that humic substances are beneficial for microbial Fe(II) oxidation.|