Abstract <p>The environmental 16S rRNA gene sequences representing the phylogenetic clade OM190 were retrieved for the first time three decades ago from coastal marine waters. These sequences formed a deep evolutionary branch related to the planctomycetes-defined line of descent. Further studies revealed that members of the clade OM190 commonly occur in phytodetrital marine aggregates and epiphytic microbial communities on marine micro- and macroalgae, as well as in bacterioplancton assemblages in oxygenated hypolimnion in deep freshwater lakes. Recent molecular analyses of prokaryote communities in different types of wetlands identified OM190 as one of the specific bacterial groups in neutral eutrophic fens, which constituted up to 2% of total bacterial diversity. This study examined the patterns of OM190 diversity in six boreal fens of European Russia. Canonical correspondence analysis identified pH value and phosphorus content as the most important environmental variables defining distribution of OM190 planctomycetes in peatlands. The pool of 12148 OM190-affiliated 16S rRNA gene sequences was extracted from the Silva database v.138.2 and analyzed with regard to their environmental sources. The analysis revealed that OM190-affiliated bacteria inhabit various aquatic and terrestrial habitats, which range with regard to salinities but are uniform with regard to neutral or nearly-neutral pH reaction. A clear preference for the particle-associated lifestyle, with marine and freshwater sediments and activated sludge as the major habitats, was also suggested for members of the OM190 clade.</p>