Wetlands are ecosystems in which the water table is permanently or periodically close to the soil surface. These water-saturated terrestrial environments, such as boreal Sphagnum-dominated peat bogs and lichen-dominated tundra peatlands, are abundantly colonized by members of the phylum Planctomycetes. The 16S rRNA gene fragments from these bacteria comprise one of the major groups of reads retrieved from oxic and anoxic peat in cultivation-independent studies. Most 16S rRNA gene reads from planctomycetes in Sphagnum- derived peat commonly affiliate with the family Isosphaeraceae. By contrast, the assemblages of planctomycetes in lichen-dominated peatlands of tundra have a high proportion of representatives affiliated with the Phycisphaera-related group WD2101 (order Tepidisphaerales). Members of the family Gemmataceae are also common in boreal and tundra wetlands.
Despite their wide distribution, the functional role of planctomycetes in peatlands remains poorly understood. We used metatranscriptomics to assess the activity response of peat-inhabiting planctomycetes to biopolymers abundantly present in these habitats, i.e. cellulose, xylan, pectin and chitin. Each of the four polymers induced specific planctomycete responses, suggesting the presence of hydrolytic capabilities in these bacteria. The strongest substrate-induced response was detected on chitin for Gemmata- and Phycisphaera-like planctomycetes. Given that most conventional tests used for assessing hydrolytic capabilities are not fully suitable for slow-growing planctomycetes, the comparative genomic approach was applied to unveil the hidden potential of these bacteria. The genomes of two recently described peat-inhabiting planctomycetes, Paludisphaera borealis PX4T (family Isosphaeraceae) and Fimbriiglobus ruber SP5T (family Gemmataceae), were sequenced and analyzed. The genomes encode wide repertoires of carbohydrate-active enzymes including many unclassified putative glycoside hydrolases, which suggests the presence of extremely high but partly hidden glycolytic potential in planctomycetes. We were also able to prove the presence of chitinolytic capability in Fimbriiglobus ruber SP5T, which showed the specific response to chitin availability in our metatranscriptomic study. This is the first member of the order Planctomycetales with confirmed chitinolytic capability. In summary, these results suggest participation of peat-inhabiting planctomycetes in degradation of plant-derived polymers, exoskeletons of peat-inhabiting arthropods as well as exopolysaccharides produced by other bacteria.