Drained peatlands are significant sources of the greenhouse gas (GHG) carbon dioxide. Rewetting is a proven strategy to protect carbon stocks; however, it can lead to increased emissions of the potent GHG methane. The response to rewetting of soil microbiomes as drivers of these processes is poorly understood, as are biotic and abiotic factors that control community composition. We analyzed the pro- and eukaryotic microbiomes of three contrasting pairs of minerotrophic fens subject to decade-long drainage and subsequent rewetting. Also, abiotic soil properties including moisture, dissolved organic matter, methane fluxes and ecosystem respiration rates. The composition of the microbiomes was fen-type-specific, but all rewetted sites showed higher abundance of anaerobic taxa compared to drained sites. Based on multi-variate statistics and network analyses we identified soil moisture as major driver of community composition. Furthermore, salinity drove the separation between coastal and freshwater fen communities. Methanogens were more than tenfold more abundant in rewetted than in drained sites, while their abundance was lowest in the coastal fen, likely due to competition with sulfate reducers. The microbiome compositions were reflected in methane fluxes from the sites. Our results shed light on the factors that structure fen microbiomes via environmental filtering.

An aerobic methanotroph, strain Kb3^T, was isolated from a rhizospheric soil sample collected from a tropical Indian rice field. The cells were motile, Gram-negative bacilli, formed pink colonies and pink turbid/pellicles in the liquid medium. Biochemical characteristics showed that strain Kb3^T utilised only methane and methanol as its sole carbon and energy sources. The isolate's 16S rRNA gene sequence expressed 99.52% similarity to the recently described valid species Methylomonas fluvii EbB (Mmf), with 92% query cover. But examining the genome similarity between Kb3 and EbB, a DDH value of 44.20% [41.6 - 46.7%] and an Ortho-ANI value of 91.48 was observed that were below the current cut-off values for species differentiation. Also, the 16S rRNA gene phylogeny and the phylogenomic analysis branched the two species separately. The major fatty acid in Methylomonas sp. Kb3 was C_14:0, followed by C_16:1 ω5c. The genome sequence revealed the size of strain Kb3 is 5.1 Mb, with the G + C content of 51.8%. Strain Kb3^T shared the closest relatedness with Methylomonas sp. LW13 reveals a 99.66% 16S rRNA gene similarity, an Ortho-ANI value of 97.9%, a DDH value of 87.3%, and a close branching in the phylogenomic tree Kb3 and LW13 together form a new species. The genomic and phylogenetic distinction between species Mmf and strain Kb3 supports Kb3^T to be described as a novel species within the genus Methylomonas, with the proposed name, Methylomonas aquatica sp. nov. (Mma) and the type strain being Kb3^T (=MCC 4012, =JCM 33634, =KCTC 72521).