Alphaproteobacteria fix nitrogen in a Sphagnum -dominated peat bog using molybdenum-dependent nitrogenase

Microbial N 2 fixation (diazotrophy) represents an important nitrogen source to oligotrophic peatland ecosystems, which are important sinks for atmospheric CO 2 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 O 2 , and unaffected by CH 4 and CO 2 amendments. Acetylene fully inhibited CH 4 consumption under oxic conditions, but only partially inhibited 15 N 2 incorporation in degassed incubations, and had a minimal effect on oxic 15 N 2 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.

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