Geomicrobiology of Iron Layers in the Sediment of Lake Superior

Previous studies of deeply oxygenated (8–12 cm) sediments in Lake Superior revealed prominent iron (Fe)-rich layers, up to 2 cm thick. Whereas metal enrichments are common near the oxic–anoxic boundary in aquatic sediments, several geomicrobiological aspects of their formation remain unknown. In this study, we analyzed geochemical and microbiological signatures associated with the Fe-rich layer in Lake Superior at two locations (230 and 310-m water depth) in the Eastern Basin of the lake. We measured oxygen and metal contents in the sediment solid phase and imaged the Fe-rich layer with transmission and scanning electron microscope (TEM and SEM). Bacterial communities were characterized using tag-encoded FLX small subunit ribosomal gene amplicon pyrosequencing. The SEM images reveal filamentous structures encrusted with spheres of ca. 20 nm in diameter. TEM and energy dispersive X-ray spectroscopy observations of thin sections indicate that bacteria cells served as nucleation surfaces for Fe-oxide formation. The Fe-rich layers, in contrast to the layers directly above them, were dominated by members of the NC10 phylum, known for anaerobic methane oxidation, and also harbored a high proportion of Nitrospira species that were most similar to members of a Fe-rich seep. Taken together the results suggests that microorganisms, whose cell surfaces are serving as templates for iron oxide precipitates that might also be mediating the iron oxidation, support formation of the Fe-rich layers.