Estimating iron oxidizing bacteria distribution from remote sensing in Alaskan tundra

Abstract:

Permafrost contains an abundance of organic carbon (OC) that is poised for degradation upon thaw. The fate of thawed OC is a balance between the microbial community, rates, and degradation pathways, which primarily produce either carbon dioxide or methane in shallow freshwater habitats. Microbial iron reduction is a potentially prominent terminal oxidation pathway for OC in Arctic tundra freshwater sediments and competitively inhibits methanogenesis when favorable Fe(III)-oxides are present as an electron acceptor. The activity of microbial iron reduction promotes iron oxidizing bacteria (FeOB) at the oxic-anoxic interface, which produce visible FeOB mats as a byproduct of their respiration. The Alaskan north slope lends itself to remote sensing of these FeOB mats as the landscape contains few trees or other tall vegetation, which typically preclude this analysis; furthermore, foot surveys have revealed extensive presence of iron mats associated with waterways on the tundra. The central aims of this research are to determine, first, if iron mats are detectable by remote sensing, and second, if the activity of Fe-cycling communities can be inferred from these observations. We used aerial imagery to estimate the distribution of FeOB mats on the sub-catchment scale near Toolik Field Station. The imagery was used to create a redness index (RI) to compare against in situ dissolved iron and extracted iron data from three sites, which included pond and wet sedge meadow habitats representing the presence or absence of FeOB mats, and identify a range of RI values at which FeOB mats occurred. Potential iron reduction rates were quantified using anoxic incubations with native water and sediment from the three sites. We found that potential iron reduction rates are substantially higher in sites with FeOB mats. RI analysis shows FeOB mats concentrated to waterlogged soils, watertracks, and shallow ponds. Given the differences in potential rates of iron reduction driven by the presence of FeOB, mats provide favorable Fe(III)-oxides for iron reduction, suggesting a significant role of iron reduction in OC degradation in the Alaskan tundra. Further, remote sensing is a promising method for estimating the distribution of FeOB and an indicator of their activity.