The deep-sea realm (depth > 200 m) is characterized by low temperature (0–4°C), absence of sunlight, and high hydrostatic pressure, which increases by 1 atm every 10 m depth. It is a key site for degrading organic matter back to carbon dioxide and into other inorganic compounds. This is a crucial process for driving the carbon cycle in the deep-sea mediated largely by microbes, especially prokaryotes. Recently, global carbon budget estimates indicated an unresolved mismatch between organic carbon supply and prokaryotic carbon demand in the deep-sea. Hence measured prokaryotic activity in the deep-sea exceeds the input of sedimenting organic carbon from the sunlit euphotic layer. Without realistic measurements on prokaryotic activity, we cannot advance our understanding on the ocean carbon cycle. Thus, one of the most important issues discussed for some decades remains still unresolved as is the role of hydrostatic pressure on deep-sea prokaryotic activity. Because prokaryotic activity is traditionally measured on board of research vessels under atmospheric pressure conditions, the results obtained hitherto on deep sea heterotrophic microbial activity might be biased if hydrostatic pressure does influence heterotrophic microbial activity. While it has been shown already in the 1940s, that there are some piezophilic (‘pressure-loving’) bacteria present in the deep-sea preferentially growing under high-pressure conditions, their ecological role is still not clear. Overall, the main objectives of HYADES project are to understand deep-sea prokaryotic activity and metabolism under in situ hydrostatic pressure and reveal the contribution of piezophilic prokaryotes to the total prokaryotic community in the dark ocean.
