Molybdenum-based diazotrophy in aSphagnumpeatland in northern Minnesota

Abstract

AbstractMicrobial N2fixation (diazotrophy) represents an important nitrogen source to oligotrophic peatland ecosystems, which are important sinks for atmospheric CO2and susceptible to changing climate. The objectives of this study were: (i) to determine the active microbial group and type of nitrogenase mediating diazotrophy in a ombrotrophicSphagnum-dominated peat bog (the S1 peat bog, Marcell Experimental Forest, Minnesota, USA); and (ii) to determine the effect of environmental parameters (light, O2, CO2, CH4) on potential rates of diazotrophy measured by acetylene (C2H2) reduction and15N2incorporation. Molecular analysis of metabolically active microbial communities suggested that diazotrophy in surface peat was primarily mediated byAlphaproteobacteria(BradyrhizobiaceaeandBeijerinckiaceae). 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. Acetylene reduction was only detected in surface peat exposed to light, with the highest rates observed in peat collected from hollows with the highest water content. Incorporation of15N2was suppressed 90% by O2and 55% by C2H2, and was unaffected by CH4and CO2amendments. These results suggest that peatland diazotrophy is mediated by a combination of C2H2- sensitive and C2H2- insensitive microbes that are more active at low O2and show similar activity at high and low CH4.ImportancePrevious studies indicate that diazotrophy provides an important nitrogen source and is linked to methanotrophy inSphagnum-dominated peatlands. However, the environmental controls and enzymatic pathways of peatland diazotrophy, as well as the metabolically active microbial populations that catalyze this process remain in question. Our findings indicate that oxygen levels and photosynthetic activity override low nutrient availability in limiting diazotrophy, and that members of theAlphaproteobacteria(Rhizobiales) catalyze this process at the bog surface using the molybdenum - based form of the nitrogenase enzyme.