Abstract
Wetland ecosystems are important as sites of rapid biogeochemical cycling of bioactive elements, among which iron features prominently. The redox cycling of iron exerts a strong influence on soil chemistry and the metabolism of plants and microorganisms. Studies have shown that bacteria play an important role in the process of
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iron oxidation in wetlands. This study explores the diversity and distribution of iron-oxidizing bacteria (FeOB) in soils and sediments of freshwater and brackish wetlands of circum-neutral pH, and analyzes the potential environmental factors influencing them. Gradient tubes were used to enrich FeOB from soil and sediment samples. From these enrichments, a clone library was established on the basis of the almost complete 16S rRNA gene. Specific probes and primers were developed using Gallionella-related sequences from this library. The newly designed Gallionella-specific 16S rRNA gene primer set was applied to community DNA obtained from three contrasting wetland environments, followed by Denaturing Gradient Gel Electrophoresis (DGGE) analysis and cloning. The retrieved 16S rRNA gene sequences yielded novel iron oxidizers most closely related to Gallionella spp. A novel quantitative PCR (qPCR) assay was developed. In combination with the nested PCR-DGGE approach, it was used to delineate the spatial and temporal distributions of FeOB at a number of locations characterized by different plant species in a tidal freshwater marsh (Appels, Belgium). The presence of Gallionella-related FeOB was confirmed in vegetated and non-vegetated sediments of the tidal marsh. A high temporal variability of the composition of the iron-oxidizing community was observed. Iron-oxidizing bacteria flourished especially in early spring. The simultaneous accumulation of iron (hydr)oxides further indicated increased iron oxidizing activity. Overall, the results implied a highly dynamic FeOB community structure. The spatial distribution of FeOB communities was also analyzed along a flooding gradient in an irregularly flooded riparian wetland (Ewijk, Netherlands). In addition, the co-occurrence of methane-oxidizing bacteria (MOB) was investigated, as MOB represent a biological component that may affect the distribution of FeOB under conditions of oxygen limitation. A clear trend of increasing abundance of FeOB was observed with increasing elevation and, hence, decreasing flooding intensity and soil moisture content. The abundance of FeOB exceeded that of MOB. With the modified gradient tube method, we successfully isolated a novel FeOB strain, representing a new genus. This neutrophilic iron-oxidizing bacterium, Ferrocurvibacter nieuwersluisensis gen. nov., sp. nov., was isolated from an iron-rich grassland. This isolate belongs to the same group as detected by the specific primers. In conclusion, this thesis describes novel findings about the occurrence of neutrophilic iron-oxidizing bacteria in a number of wetland ecosystems in the delta area of the Netherlands and Belgium, by characterizing the distribution of FeOB in various wetlands and by enriching and isolating these organisms. This work opens up new opportunities for using molecular tools to study microbial iron oxidation at circum-neutral pH and contributes to the knowledge of iron cycling in redox-stratified environments. Future work should focus on the spatial and temporal distributions of FeOB in additional environment settings, the mechanism of bacterial iron oxidation, and its interactions with other key biotic and abiotic processes, in particular microbial iron reduction
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