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The role that neutrophilic iron-oxidizing bacteria play in the Arctic tundra is unknown. This study surveyed chemosynthetic iron-oxidizing microbial mat communities on the North Slope of Alaska, near Toolik Field Station (TFS) at Toolik Lake (68.63N-149.60W). Microbial iron mats were common in submerged habitats with stationary or slowly flowing water, and had their greatest areal extent coating plant stems and sediments in wet sedge meadows. The known mor-photypes of Fe-oxidizing bacteria (FeOB) dominated all the mats. Cool water temperatures (9–11°C), reduced pH (pH 5.0 – 6.6), and the shallow water table constrained by underlaying per-mafrost likely contributed to an active iron cycle. A microbial survey of five sites based on 16S rRNA genes found a predominance of Proteobacteria, with Betaproteobacteria and members of the Comamondaceae being the most prevalent operational taxonomic units (OTUs). Clades of known lithotrophic FeOB composed 5-10% of the communities. Somewhat surprisingly OTUs related to cyanobacteria and chloroplasts accounted 3–25% of the communities. High resolution oxygen profiles showed evidence for oxygenic photosynthesis in the surface layer of some mats, indicating co-existence of photosynthetic and FeOB populations. Iron mats incubated anaerobi-cally with 10 mM acetate rapidly initiated Fe-reduction, and OTUs related to the Geobacterace-ae, were relatively abundant at all sites, indicating that iron mats are zones of active iron cycling. The prevalence of iron mats in tundra habitats could impact the carbon cycle through lithoauto-trophic chemosynthesis, anaerobic respiration of organic carbon coupled to iron-reduction, and potentially influence phosphorus dynamics through adsorption of phosphorus to iron oxides. |