High Diversity of Planctomycetes in Soils of Two Lichen-Dominated Sub-Arctic Ecosystems of Northwestern Siberia

Abstract

A wide variety of terrestrial ecosystems in tundra have a ground vegetation cover composed of reindeer lichens (genera <i>Cladonia</i> and <i>Cetraria</i>). The microbial communities of two lichen-dominated ecosystems typical of the sub-arctic zone of northwestern Siberia, that is a forested tundra soil and a shallow acidic peatland, were examined in our study. As revealed by molecular analyses, soil and peat layers just beneath the lichen cover were abundantly colonized by bacteria from the phylum <i>Planctomycetes</i>. Highest abundance of planctomycetes detected by fluorescence <i>in situ</i> hybridization was in the range 2.2-2.7 × 10⁷ cells per gram of wet weight. 16S rRNA gene fragments from the <i>Planctomycetes</i> comprised 8-13% of total 16S rRNA gene reads retrieved using Illumina pair-end sequencing from the soil and peat samples. Lichen-associated assemblages of planctomycetes displayed unexpectedly high diversity, with a total of 89,662 reads representing 1723 operational taxonomic units determined at 97% sequence identity. The soil of forested tundra was dominated by uncultivated members of the family Planctomycetaceae (53-71% of total <i>Planctomycetes</i>-like reads), while sequences affiliated with the <i>Phycisphaera</i>-related group WD2101 (recently assigned to the order Tepidisphaerales) were most abundant in peat (28-51% of total reads). Representatives of the <i>Isosphaera</i>-<i>Singulisphaera</i> group (14-28% of total reads) and the lineages defined by the genera <i>Gemmata</i> (1-4%) and <i>Planctopirus-Rubinisphaera</i> (1-3%) were present in both habitats. Two strains of <i>Singulisphaera</i>-like bacteria were isolated from studied soil and peat samples. These planctomycetes displayed good tolerance of low temperatures (4-15°C) and were capable of growth on a number of polysaccharides, including lichenan, a characteristic component of lichen-derived phytomass.