Hidden from the eyes of the society by a layer of seawater several kilometers thick, the deep Ocean has long been considered as a vast desert of darkness, sometimes populated by sea monsters. Yet, this environment is the largest biome on our planet. Covering more than 70% of the Earth's surface, the deep (> 1000 m) aphotic Ocean encompasses more than 80% of the volume of seawater.

    With the rise of new technologies, exhaustion of terrestrial mineral resources shades the light on the deep Ocean, in particular on the massifs of polymetallic sulphides produced by hydrothermal activity along the ridges. Hence, developed and emerging countries are now racing to explore and potentially exploit the mineral resources found on the seafloor. However, our knowledge of the ecosystems they harbor remains deeply limited. It is therefore urgent to explore and establish, using a holistic approach, integrated and solid knowledge baselines of the geological diversity, deep habitats, and the taxonomic and functional biodiversity they contain. These baselines of knowledge are required to identify the ecological services associated with these ecosystems and to answer the following questions: What are the criteria for establishing the (good) functioning of a deep-sea ecosystem? How does the Deep Ocean system work? And, from there, how can deep Ocean react and adapt to changes, be they natural or anthropogenic? It is now essential to identify markers for monitoring and comparing these habitats using multidisciplinary and integrated approaches, as will be developed in BICOSE3 cruise.

    Below a water depth of 3500 meters on the Mid-Atlantic Ridge, the TAG hydrothermal field, an old and structured field of massifs ranging from a current active site up to about -125,000 years ago, and the young and volcanic Snake Pit hydrothermal field, are at the core of our studies of deep ecosystems. At about 300 km apart and separated by a major transforming fault (Kane) that plunges down to 6000 m depth shifting the axis of the oceanic ridge by 100 km, these sites harbor similarities in terms of biological diversity, raising the question of their biological connectivity and resilience. In the past 7 years, the observations we have carried out on two major oceanographic cruises identified the different habitats on these two active fields and explored the peripheral areas. However, the functional and life history traits of key biological species have only been partially described. Recent exploration of over 600 km of this Zone also revealed geochemical evidence for new active hydrothermal sites located between these TAG and Snake Pit site and in the south of Snake Pit. These intermediate sites constitute potential relays for the dispersal of species, a key point in the overall understanding of ecosystems and their relationships along ridge segments. The BICOSE3 campaign seeks to establish more precisely:

1. The functioning of the geological system that has been underling this hydrothermal activity at TAG for more than 125,000 years.
2. To precise maps of the distribution of communities at the scale of active fields using a combination of high-resolution optical surveys and inventories of species identified by integrative taxonomic approaches.
3. The connectivity of key species taking into account all active sites - including those newly identified - on these segments.
4. The in situ functioning of holobionts during their life cycle in contrasting habitats (including the key stage of symbiont acquisition) using dedicated tools.
5. The settlement processes for new recruits and the influence of environmental conditions on this settlement, using in situ short term colonization experiments deployed at active and inactive sites of different ages.
6. The better understanding the sensory and acclimatization capabilities of holobionts using in vivo approaches.
7. The diversity of microbial communities inhabiting inactive sulfur deposits, their function and their implication in the carbon cycle, and,
8. Finally, lead to a redefinition of the notions of active and inactive sites, taking into account the gradient between the two extremes and the distribution of biotopes and biodiversity (micro to macro) within the TAG district.

    Ultimately, in order to describe the functioning of these environments, a precise map of all active and inactive hydrothermal sites present on this 600 km ridge segment will be established. We intend to identify key ecological markers along activity gradients, in order to establish global-scale inter-comparison protocols. At a broader level, these studies will also have economical and societal implications towards management and preservation of these ecosystems that could soon face major natural and anthropogenic changes.