Several areas in the deep sea can be considered as extreme environments on earth, extreme by the conditions prevailing there, and extreme from the point of view of adaptations of life forms. Areas of hot vents on the mid-ocean ridges and cold seeps on continental margins are areas where several parameters combine to form original environments whose understanding implies geological, chemical, biological and microbiological studies.
The very high pressure, ranging from 100 to 500 bars, plays an essential role on the physical and chemical properties of seawater. It controls the chemical reactions taking place within the rocks and produces hydrothermal fluids. Fluid emission are known from areas a few hundred meter deep to over 4000 meters.
The second parameter is the temperature of the fluids that can be over 400˚C on underwater volcanoes. The temperature of the fluids influences the geochemical and biological processes in the areas where the emissions occur..
In these deep environments, because of the lack of light, life does not rely on photosynthesis but rather by chemosynthesis. The nutrients necessary for the animal and microbiological development are gases such as H2, H2S and CH4, and/or mineral nutrients found in the fluids.
The combined action of pressure and temperature, in rocks of different types, produces fluids with highly variable chemical composition (acidity, metal contents, salinity, oxygen concentration, …). These fluids affect the geologic environment, control the type of sulfide deposits and the biological and microbiological environment.
These extreme environments are sensitive areas that can be seriously affected by deep and rapid changes due to the tectonic and/or magmatic activity. The brutal volcanic events and episodes induce modifications of the fluid composition, and strongly affect and transform the biological and microbiological world. In contrast, periods of stability are favorable to the biological and microbiological development.
Numerous scientific questions arise:
- How are these fluids produced in the depths of Earth?
- What is the importance of tectonics for fluid transfer?
- What is the influence of the type of rocks that are washed out on the composition of the fluids?
- What is the importance of chemical exchanges and transfers generated by these fluids?
- How do they bacteria get energy out of the chemical compounds contained in the fluids?, What adaptive mechanisms do the animals develop to settle in these acidic environments, laden with metals, overheated and poor in oxygen?
- What is the depth limit in rock at which bacterial life can be found?
- How do these fluids transport metals to concentrate them on the sea-floor?
- Do mantle extreme environments, poor in oxygen, reflect conditions that prevailed on Earth when life appeared, about 4 billion years ago?
- What can the extreme environments bring to our understanding of the formation of energetic and mineral resources, and of the biodiversity of the deep-sea?