The sedimentary deep subseafloor biosphere

Deep subseafloor sediments represent an intriguing marine habitat, populated by genetically diverse -yet poorly characterized- microbial communities. Microbially mediated processes in subseafloor sediments constitute an important component of a network of (bio)geochemical redox reactions that leave a strong imprint on pore water and solid phase compositions, alter inorganic sedimentary paleoceanographic records, and ultimately affect global biogeochemical element cycles. Here we give a brief overview of the abundance and diversity of microbes in deep subseafloor environments. Furthermore, we discuss the (bio)geochemical aspects of the sedimentary deep subseafloor biosphere with excursions into the carbon, sulfur, and silica cycles, and look at the causes and impacts of non-steady state conditions on the marine sedimentary system.