Iron oxidizing bacteria at the groundwater/surface water interface: Presence, diversity, activity and role in natural iron deposition

The groundwater surface water interface (GSI) is a transition zone with biogeochemical properties determined by both the adjacent groundwater and surface water bodies. We examined the presence of iron oxidizing bacteria (IOB) at a GSI impacted by reduced groundwater originating as leachate from an unlined landfill. The vertical IOB distribution was quantified and mirrored the distribution of iron reducing bacteria (IRB). Bench-scale experiments demonstrated that the enriched IOB could accelerate iron oxidation rates under low DO conditions (0.5-0.6mg/L) compared to chemical or the metabolically inactivated IOB controls. Analyses of 16S rRNA gene clone libraries from IOB enrichments indicated that the IOB were very diverse with clones mainly belonging to α-, β-, and γ-Proteobacteria. As part of this analysis, an improved cell collection method was developed for IOB enrichments. This method only involves agarase digestion for agarose removal and was demonstrated to be simpler and faster than the published heating plus oxalic acid treatment method. The obtained cell pellets were applicable for further FISH performance and resulted in higher reliable DNA yields for further 16S rRNA gene recovery and IOB ecological community investigation. Active microbial communities in the GSI were captured by a series of glass-bead (GB)/activated carbon (AC) columns deployed in the sediment over several sequential short periods. The qPCR based molecular analysis of column samples revealed that although microbes might accelerate iron oxidation during the flooding season when more reducing environments developed in the study field, bacterial activities measured in columns did not vary significantly throughout the whole year. The spatial effects on bacterial activities retrieved from columns were detected mainly during the flooding seasons. Higher activities were measured in the column-trapped microbial communities than in the ambient sediments. Bacterial densities exhibited clear stratification trends in the sediment samples (but not in the columns) with similar depth-resolved bacteria distribution profiles obtained from these freeze core sediment samples as those previously retrieved from another two sediment samples via direct counting or dilution-to-extinction cultivation method. A 16S rRNA gene clone library was developed from the GB columns and demonstrated that the in-situ active microbial communities from different locations were phylogenetically diverse but similar. Several retrieved clones, which clustered with the known IOB enrichments derived clones or known IOB species, further indicated that IOB are metabolically active community members in the redox transition zone ecosystem. This is the first report of the depth-resolved IOB diversity distributions and of the seasonal and spatial effects of the associated metabolically active microbial community activity distributions in the GSI. The findings of high abundance of readily enriched IOB from the study site which accelerated iron oxidation rates over abiotic reactions under site pH and low DO conditions or even under anoxic conditions shed new light on the significance of microbial iron oxidization in retention of pollutants at the GSI.