Thermophilic spore-forming anaerobes are present as dormant spores in cold marine sediments. Our work shows that in the Arctic, these thermospores are derived from warm marine subsurface ecosystems, such as hydrothermal fluid flow at mid-ocean ridges or deeply buried petroleum reservoirs, and then dispersed by ocean currents to marine sediments. Because these organisms are present in the Arctic as inactive spores that are buried in sediments over time, analysis of thermospore diversity along sediment depth cores provides a unique window into their evolutionary history and the ecological history of their deep biosphere sources. Therefore, our objective is to characterise the diversity and community composition of thermospores along two Arctic sediment depth cores, representing approximately 500 years, by using thermospore enrichment incubations followed by 16S rRNA amplicon sequencing. Changes in diversity with depth will be analysed by traditional OTU-based approaches combined with analysis of microdiversity within OTUs using Minimum Entropy Decomposition. We hypothesise that both OTU-level composition and fine-scale diversity within OTUs will vary with depth. Broader changes at the OTU-level will provide evidence of ecological or physical change within the subsurface habitats, while changes in microdiversity will reflect the evolutionary history of individual populations. This project is a novel step towards reconstructing both the evolutionary and ecological history of subsurface microbes. PI: China Hanson, School of Biological and Chemical Sciences, Queen Mary University of London.