Abstract
Arctic soils store vast amounts of carbon and are subject to intense climate change. While effects of thaw on the composition and activities of Arctic tundra microorganisms has been examined extensively, little is known about the consequences of temperature fluctuations within the subzero range in seasonally frozen or permafrost soils. This study identified tundra soil bacteria active at subzero temperatures using stable isotope probing (SIP). Soils from Kilpisjärvi, Finland were amended with 13C-cellobiose and incubated at 0, -4, and -16°C for up to 40 weeks. 16S rRNA gene sequence analysis of 13C-labelled DNA revealed distinct subzero-active bacterial taxa. The SIP experiments demonstrated that diverse bacteria, including members of Candidatus Saccharibacteria, Melioribacteraceae, Verrucomicrobiaceae, Burkholderiaceae, Acetobacteraceae, Armatimonadaceae, and Planctomycetaceae were capable of synthesizing 13C-DNA at subzero temperatures. Differences in subzero temperature optima were observed, for example with members of Oxalobacteraceae and Rhizobiaceae found to be more active at 0°C than at -4°C or -16°C, whereas Melioribacteriaceae were active at all subzero temperatures tested. Phylogeny of 13C-labelled 16S rRNA genes from the Melioribacteriaceae, Verrucomicrobiaceae, and Candidatus Saccharibacteria suggested that these taxa formed subzero-active clusters closely related to members from other cryo-environments. This study demonstrates that subzero temperatures impact active bacterial community composition and activity which may influence biogeochemical cycles.