Microbial Metabolisms Connecting Iron and Carbon in Terrestrial Wetlands: A Metagenomic and Metatranscriptomic Study of the Savannah River Site
Authors
Gracee K. Tothero1* (gtothero@udel.edu), Daniel I. Kaplan2, Pamela Weisenhorn3, Clara S. Chan1 (cschan@udel.edu)
Institutions
1University of Delaware, Newark, DE; 2University of Georgia, Athens, GA; 3Argonne National Laboratory, Lemont, IL
Abstract
At the Savannah River Site (SRS) in South Carolina, extensive iron-oxidizing microbial mats form and appear to be a major sink of uranium. To understand the significance of microbial iron oxidation and its connection to carbon (C) cycling and to incorporate these metabolisms into hydro-biogeochemical models, researchers need to know the rates and mechanisms of biotic and abiotic oxidation. To evaluate this, the team conducted two field campaigns to the SRS to sample iron mats in the Tim’s Branch stream and wetlands. Researchers performed 16S rRNA gene sequencing to identify the major iron-oxidizing bacteria (FeOB), the flanking community, and metagenomic sequencing to identify the major biogeochemical transformations catalyzed by FeOB metabolisms.
Additionally, researchers performed iron oxidation kinetics experiments to quantify biotic iron oxidation rates, connected to metatranscriptomics sequencing to identify the expression of metabolic pathways in response to Fe(II) stimulus. The iron mats were dominated by known FeOB, notably a diverse set of Gallionella and Leptothrix taxa. Scanning electron microscopy shows the major morphologies in the mats are FeOB biominerals, including twisted stalks (characteristic of Gallionella) and sheaths (characteristic of Leptothrix). Researchers compare biotic oxidation rates with abiotic azide-killed controls and show that mat iron oxidation is dominated by biotic oxidation while oxidation by abiotic mechanisms is much slower. The team presented the results of metagenomic and metatranscriptomic analyses of the Fe mat communities used in the kinetics experiments, including the major physiological mechanisms of the dominant FeOB. The team described the biogeochemically relevant activities of all major iron mat community members, detailing the metabolic links between iron oxidation and C metabolisms, and outlined the major interactions between taxa. These results set the stage for microbial modeling work towards longer-term goals to link FeOB metabolic models and kinetics to biogeochemical models in order to predict Fe, C, nutrient, and contaminant metal cycling.