Peat mosses of the genus Sphagnum play a major role in global carbon storage and predominate over photosynthetic production in many northern peatland ecosystems, which are currently being subjected to some of the most rapid climate change on Earth. The objectives of this research are 1.) to investigate the role of the Sphagnum phytobiome (plant host + constituent microbiome + environment) in regulating the carbon and nutrient cycles of peatland ecosystems, and 2.) to provide a mechanistic understanding of the role of the phytobiome in the resilience of peatlands to climate change drivers (warming and elevated CO2). Sphagnum phytobiomes were sampled from peatlands across North America, including field sites sponsored by the U.S. Dept. of Energy to investigate the ecosystem level effects of climate change, at the Marcell Experimental Forest (MEF) of northern Minnesota, where the Oak Ridge National Lab (ORNL) has established an experimental warming site known as Spruce and Peatland Response Under Climatic and Environmental Change (SPRUCE). Sphagnum plants contain a diverse microbiome within dead, water-filled hyaline cells that is distinct from the surrounding decomposing peat. In comparison to bulk peat samples, phytobiomes contained a lower relative abundance of Acidobacteria and a higher relative abundance of Proteobacteria, Cyanobacteria, and Actinobacteria. We hypothesize that this reflects the influence of commensal taxa and the elevated pH (pH 5.5) of the Sphagnum endosphere relative to the bulk peat (pH 4 or less). Phytobiomes from two peat moss species, S. fallax and S. magellanicum, revealed that both microhabitat and plant species act as selective forces in shaping microbiome composition. Metatranscriptome analysis indicates that cyanobacteria and fungi are among the most metabolically active microbiome groups, and nitrogen-fixing microbiome members are dominated by cyanobacteria of the Nostocales and methanotrophs within the Rhizobiales. Multiple lines of evidence indicate that members of the Rhizobiales play a key role in coupling nitrogen fixation to methanotrophy. These results corroborate biogeochemical field data which show a coupling of N fixation and CH4 consumption in Sphagnum-dominated peatlands.