Deep Subsurface Microbiomes of the Marine Realm

The oceans cover ~71% of the Earth's surface, so their sediments constitute one of the largest biomes of the planet. Moreover, the marine deep subsurface is not only a colossal microbial habitat but also one of the most enigmatic and difficult to explore microbiomes on Earth. Albeit microbial cell density in the subsurface is manifold lower than in surficial ecosystems, the sheer vastness of the deep and dark extremities of the biosphere makes them the largest reservoir of organic carbon. The progressively organic carbon impoverished and energy-limited marine deep subsurface poses a unique set of physicochemical challenges (including high temperature and mechanical pressure) to metabolism, notwithstanding which the habitat is replete with microbial life and activity, often up to several hundreds of meters below the seafloor. The last few decades have witnessed quantum leaps in our understanding of the microbiology of the deep subsurface of the marine realm, and in tune with that aspects pertaining to physiology, adaptation, evolution, and astrobiology have been reviewed extensively. Here, we compile all the information available for in situ microbial processes and their geochemical manifestations (including typical geological features such as gas hydrate deposits, mud volcanoes and carbonate mounds) and aim to visualize a comprehensive biogeochemical picture of ecosystem functioning in the marine deep subsurface.