PT - JOURNAL ARTICLE AU - Melissa J. Warren AU - Xueju Lin AU - John C. Gaby AU - Cecilia B. Kretz AU - Peter L. Morton AU - Jennifer Pett-Ridge AU - David J. Weston AU - Christopher W. Schadt AU - Joel E. Kostka AU - Jennifer B. Glass TI - Alphaproteobacteria fix nitrogen in a <em>Sphagnum-</em>dominated peat bog using molybdenum-dependent nitrogenase AID - 10.1101/114918 DP - 2017 Jan 01 TA - bioRxiv PG - 114918 4099 - http://biorxiv.org/content/early/2017/03/07/114918.short 4100 - http://biorxiv.org/content/early/2017/03/07/114918.full AB - Microbial N2 fixation (diazotrophy) represents an important nitrogen source to oligotrophic peatland ecosystems, which are important sinks for atmospheric CO2 and susceptible to changing climate. The objective of this study was to investigate the pathways and controls of diazotrophy, as well as the active microbial groups that mediate the process in Sphagnum-dominated peat bogs. In an ombrotrophic peat bog (S1) in the Marcell Experimental Forest (Minnesota, USA), low (μM) levels of inorganic nitrogen were observed, suggesting that diazotrophy could have a significant influence on ecosystem properties. Despite higher dissolved vanadium (V) (11 nM) than molybdenum (Mo) (3 nM) in surface peat, a combination of metagenomic, amplicon sequencing and activity measurements indicated that Mo-containing nitrogenases dominate over the V-containing form. Diazotrophy was only detected in surface peat exposed to light, with the highest rates observed in peat collected from hollows with the highest water content. Rates were suppressed by O2, and unaffected by CH4 and CO2 amendments. Acetylene fully inhibited CH4 consumption under oxic conditions, but only partially inhibited 15N2 incorporation in degassed incubations, and had a minimal effect on oxic 15N2 incorporation. Through a close coupling of process rate measurements with molecular analysis of the metabolically active microbial communities, our findings suggest that diazotrophy in surface layers of the S1 bog is mediated by Alphaproteobacteria (Bradyrhizobiaceae and Beijerinckiaceae) supported by photosynthate, rather than methane, for carbon and/or energy.Importance Previous studies indicate that diazotrophy provides an important nitrogen source and is linked to methanotrophy in Sphagnum-dominated peatlands. However, the environmental controls and enzymatic pathways of diazotrophy, as well as the metabolically active microbial populations that catalyze this process in peatlands, remain in question. Our findings indicate that oxygen levels and photosynthetic activity override low nutrient availability in limiting diazotrophy, and that primarily non-methanotrophic members of the Alphaproteobacteria (Bradyrhizobiaceae and Beijerinckiaceae) catalyze this process primarily at the bog surface.