Marine deep biosphere

Marine sediments cover about 70% of the Earth’s surface and contain the largest reservoir of organic matter including significant amounts of fossil fuels. They have recently been estimated to be a major microbial habitat to depths greater than 2.5 km below sea floor, containing between 1 and 30% of global biomass. Prokaryotic cells, which are ubiquitous in these sediments, utilize organic matter for energy and generally decrease with depth, as remaining organic matter is increasingly recalcitrant. Remarkably, even ancient organic matter is apparently still used, but on geological timescales (thousands to millions of years) and enabling only extremely slow growth rates. In addition, there is marked subsurface stimulation of microbial populations at depths where there are additional energy sources, such as sulfate–methane interfaces, high organic matter layers, fluid flow within rock basement, and upward diffusion of compounds from deep, hot thermogenic reactions. However, there is also overlap between deep biosphere and thermogenic processes and prokaryotic reactions may play a wider role in deep geochemical processes than previously considered, including deep gas formation. The deep biosphere is dominated by a range of specific phylogenetic bacterial and archaeal groups that are adapted to life under severe energy limitation.