S130: Comparative Metagenomics of Alaskan Permafrost Soils

The fate of large organic carbon reserves in permafrost is uncertain yet important for addressing terrestrial feedbacks to climate change. With warming there is an increased probability of thermokarst formation, and CO₂ and CH₄ flux to the atmosphere. However, we understand little of the microbial controls of nitrogen and carbon cycling in permafrost. Here we applied shotgun metagenomics and 16S rRNA gene sequencing to study microbial communities in permafrost and seasonally thawed active layer soils collected from Alaska, plus a thermokarst bog. In total, over 170,000 pyrotags were sequenced and >40 Gb of metagenome data was obtained. The bacterial communities in the permafrost and bog samples were more similar to each other than to the active layer. Bog samples had a different archaeal community than the other samples, with a high predominance of methanogens, consistent with the bog being a site of high CH₄ production. Soil moisture and pH were the strongest predictors of community composition.  An uncultured representative of the Chloroflexi phylum was found to be cosmopolitan and highly abundant in all samples. We sorted cells by FACS and performed single-cell sequencing to obtain a draft genome of this novel Chloroflexi. Metagenome assembly and annotation revealed distinct differences between the sites. In addition, draft genomes were assembled through contig binning. Overall we found that permafrost soils harbor unique microbial communities that do not contain the diversity of archaea present in thermokarst bogs. Therefore important shifts in community composition must occur following thaw and before the initiation of methane production.