The ocean covers 71% of the planet and deep-sea marine sediments harbor a remarkable microbial biomass. Yet the richness, ecosystem interactions, and functional diversity of subseafloor microbial communities remain poorly understood. Here, we propose to characterize the subseafloor microbial communities and virus-host interactions in well-dated sediment records spanning the last 3000 years from an anoxic coastal sinkhole in the Bahamas as an analog for deep-sea anoxic basins. The hypotheses this project will test are: (i) sedimentary organic carbon content at time of deposition is a persistent control on subseafloor variations in microbial communities, and (ii) viruses play a central role in their ecosystem through the viral shunt and horizontal gene transfer. Using metagenomics, incubation experiments and chemical analysis of sediments, we will confirm the effect of the type and concentration of organic carbon present at deposition on the resident microbial communities, and the effect of viruses on their hosts survival. For the first time, viral and prokaryotic metagenomics data will be generated from a same sediment sample, allowing for direct characterization of their interactions. This project will support two early-career assistant professors (one from an underrepresented group in STEM programs) and provide training to two students (one graduate, one undergraduate).