Methanomassiliicoccales in fens
The Methanomassiliicoccales  are a recently identified order of hydrogen-dependent methylotrophic methanogens, that consist of two broad family-level clades, namely the gastrointestinal tract clade (GIT) and environmental clade. While the GIT clade is comparably well studied, the environmental clade has been rather poorly studied to date. In this 16S rRNA gene amplicon sequencing based study we investigated the temporal and spatial dynamics of the Methanomassiliicoccales in three common temperate fen types: percolation fen, coastal fen and alder swamp. Fen microbiome composition was continuously monitored during 2.5 years in three peat depths and revealed a dynamic methanogen and Methanomassiliicoccales composition across space and time. Four Methanomassiliicoccales phylotypes were found that were differentially distributed in the fen types. The wetland-ASV cluster was omnipresent in all sites along all depths and also dominant in abundance. The Methanomassiliicoccales cluster was highly abundant in topsoil while the AB364942 cluster was exclusively found in deeper regions of a rewetted fen. The cluster affiliated with Ca. Methanogranum gryphiswaldense was only detected in one fen. We discuss the distribution of the four clusters with implications to their ecophysiology, where oxygen tolerance as well as substrate spectrum might play major roles. In conclusion, the Methanomassiliicoccales are widespread in the temperate fens studied here and do account for a significant proportion of methanogens, which might suggest their importance for methane formation.