Planctomycetes in boreal and subarctic wetlands: diversity patterns and potential ecological functions

Members of the phylum Planctomycetes are common inhabitants of boreal Sphagnum peat bogs and lichen-dominated tundra wetlands. These bacteria colonize both oxic and anoxic peat layers and reach the population size of 107 cells per gram of wet peat. The 16S rRNA gene sequences from planctomycetes comprise 5%-22% of total 16S rRNA gene reads retrieved from peat samples. Most abundant peat-inhabiting planctomycetes affiliate with the families Isosphaeraceae and Gemmataceae, and with as-yet-uncultured Phycisphaera-related group WD2101. The use of metatranscriptomics to assess the functional role of planctomycetes in peatlands suggested the presence of versatile hydrolytic capabilities in these bacteria. This evidence was further confirmed by the analysis of genome-encoded capabilities of isolates from wetlands. Large (up to 12 Mbp) genomes of planctomycetes encode wide repertoires of carbohydrate-active enzymes including many unclassified putative glycoside hydrolases, which suggests the presence of extremely high glycolytic potential in these bacteria. Experimental tests confirmed their ability to grow on xylan, pectin, starch, lichenan, cellulose, chitin and polysaccharides of microbial origin. These results provide an insight into the ecological roles of peat-inhabiting planctomycetes and suggest their participation in degradation of plant-derived polymers, exoskeletons of peat-inhabiting arthropods as well as exopolysaccharides produced by other bacteria.