Peat mosses of the genus <em>Sphagnum</em> are ecosystem engineers that frequently predominate over photosynthetic production in boreal peatlands. <em>Sphagnum</em> spp. host diverse microbial communities capable of nitrogen-fixation (diazotrophy) and methane oxidation (methanotrophy), thereby potentially supporting plant growth under severely nutrient-limited conditions. Moreover, diazotrophic-methanotrophs represent a possible "missing link" between the carbon and nitrogen cycles, but the functional contributions of the <em>Sphagnum</em>-associated microbiome remain in question<em>.</em> A combination of metagenomics, metatranscriptomics, and dual-isotope incorporation assays was applied to investigate <em>Sphagnum</em> microbiome community composition across the North American continent and provide empirical evidence for diazotrophic-methanotrophy in <em>Sphagnum</em>-dominated ecosystems. Remarkably consistent prokaryotic communities were detected in over 250 <em>Sphagnum</em> SSU rRNA libraries from peatlands across the US (5 states, 17 bog/fen sites, 18 <em>Sphagnum</em> species), with twelve genera of the core microbiome comprising 60% of the relative microbial abundance. Additionally, nitrogenase (<em>nifH</em>) and SSU rRNA gene amplicon analysis revealed that nitrogen-fixing populations made up nearly 15% of the prokaryotic communities, predominated by <em>Nostocales</em> cyanobacteria and <em>Rhizobiales</em> methanotrophs. While cyanobacteria comprised the vast majority (>95%) of diazotrophs detected in amplicon and metagenome analyses, obligate methanotrophs of the genus <em>Methyloferula</em> (order <em>Rhizobiales</em>) accounted for one-quarter of transcribed <em>nifH</em> genes. Furthermore, in dual-isotope tracer experiments, members of the <em>Rhizobiales</em> showed substantial incorporation of <sup>13</sup>C-CH<sub>4</sub> and <sup>15</sup>N-N<sub>2</sub> isotopes into their rRNA. Our study characterizes the core <em>Sphagnum</em> microbiome across large spatial scales and indicates that diazotrophic methanotrophs, here defined as obligate methanotrophs of the rare biosphere (<em>Methyloferula</em> spp. of the <em>Rhizobiales</em>) that also carry out diazotrophy, play a keystone role in coupling of the carbon and nitrogen cycles in nutrient-poor peatlands.
|Date made available||2021|