The deep-sea floor hosts a distinct microbial biome covering 67% of the Earth’s surface, characterized by cold temperatures, permanent darkness, high pressure and food limitation. The surface sediments are dominated by bacteria, with on average a billion cells per ml. Benthic bacteria are highly relevant to the Earth’s element cycles as they remineralize most of the organic matter sinking from the productive surface ocean, and return nutrients, thereby promoting ocean primary production. What passes the bacterial filter is a relevant sink for carbon on geological time scales, influencing global oxygen and carbon budgets, and fueling the deep subsurface biosphere. Despite the relevance of deep-sea sediment bacteria to climate, geochemical cycles and ecology of the seafloor, their genetic and functional diversity, niche differentiation and biological interactions remain unknown. Our preliminary work in a global survey of deep-sea sediments enables us now to target specific genes for the quantification of abyssal bacteria. We can trace isotope-labeled elements into communities and single cells, and analyze the molecular alteration of organic matter during microbial degradation, all in context with environmental dynamics recorded at the only long-term deep-sea ecosystem observatory in the Arctic that we maintain. I propose to bridge biogeochemistry, ecology, microbiology and marine biology to develop a systematic understanding of abyssal sediment bacterial community distribution, diversity, function and interactions, by combining in situ flux studies and different visualization techniques with a wide range of molecular tools. Substantial progress is expected in understanding I) identity and function of the dominant types of indigenous benthic bacteria, II) dynamics in bacterial activity and diversity caused by variations in particle flux, III) interactions with different types and ages of organic matter, and other biological factors.