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Thawing permafrost can stimulate microbial activity, leading to faster decomposition of formerly preserved organic matter and CO2 release. Detailed knowledge about the vertical distribution of the responsible microbial community that is changing with increasing soil depth is limited. In this study, we determined the microbial community composition from cores sampled in a high Arctic heath at Svalbard, Norway; spanning from the active layer (AL) into the permafrost layer (PL). A special aim has been on identifying a layer of recently thawed soil, the transition zone (TZ), which might provide new insights into the fate of thawing permafrost. A unique sampling strategy allowed us to observe a diverse and gradually shifting microbial community in the AL, a Bacteroidetes dominated community in the TZ and throughout the PL, a community strongly dominated by a single Actinobacteria family (Intrasporangiaceae). The contrasting abundances of these two taxa caused a community difference of about 60%, just within 3 cm from TZ to PL. We incubated subsamples at about 5°C and measured highest CO2 production rates under aerobic incubations, yet contrasting for five different layers and correlating to the microbial community composition. This high resolution strategy provides new insights on how microbial communities are structured in permafrost and a better understanding of how they respond to thaw.

Direct microscopic counts of virus-like particles (VLPs) from different soil types revealed approximately 10 SP 8 sp -10 SP 10 sp VLP per gram dry weight of soil (Williamson et al., [16]), with higher numbers in forest soils when compared to agricultural soils (Williamson et al., [14]). Over the past couple decades, there has been intense exploration of the soil microbiome using modern sequencing and bioinformatics approaches. SOIL VIRUSES HAVE AUXILIARY METABOLIC GENES Studies of soil viral sequences in soil metagenomes have shown that some viral genomes contain auxiliary metabolic genes (AMGs) that are not required for normal viral replication and reproduction. SOIL VIRUSES PLAY A PREVIOUSLY UNRECOGNIZED ROLE IN SOIL ECOLOGY Several recent studies have shown that soil viruses are influenced by changes in their environment. [Extracted from the article]