Hydrothermal vents on the sea floor are true oases. Photosynthesis is impossible without light but bacteria manage to produce organic matter through chemosynthesis, i.e., by using the energy of chemical components such as H2S. In these environments, animals, especially shrimps and worms, host colonies of chemosynthetic bacteria on part of their tissues; a real 'larder' in the sense that these bacteria are the main source of the organic molecules these animals need to survive and reproduce. A study carried out by Julie Ponsard and her colleagues, published in the International Society for Microbial Ecology (ISME) journal, shows that in the Rimicaris exoculata shrimp, the transfer of nutrients between the bacteria and the shrimp take place through the exoskeleton rather than through the digestive system. This calls the paradigm into question considering that the cuticle of crustaceans is highly impermeable to dissolved organic substances.

There are corners on earth that are still more mysterious than space. The abyssal plain is one of these ‘terra incognita’. Several decades ago, scientists still believed that primary production was impossible deep down, where the sun never shines and where photosynthesis is impossible. And then hydrothermal vents were discovered, sort of underwater volcanoes that generate heat and a geological and chemical activity favourable to the development of certain life forms. “They are the oases of the ocean floor”, explains Julie Ponsard, researcher at the University of Liège’s ultrastructural morphology unit (Functional and Evolutionary Morphology Laboratory). “Here, life is abundant and diversified”. According to some highly controversial theories, these hydrothermal vents may well have seen the first forms of life develop on earth four billion years ago.

It is in this environment that a strange shrimp known as Rimicaris exoculata lives. It can be found not far from the Azores, on the Mid-Atlantic Ridge, a range of volcanic mountains several kilometres long in the middle of the ocean. This spectacular relief of the sea floor is the point where the North American Plate and Eurasian plate meet. The ridge is riddled with vents through which gasses from the molten magma escape. Difficult to imagine any kind of life at the opening of these chimneys where temperatures can reach several hundred degrees. But the water is cooler (between 10 and 15 degrees) on the chimney walls. And here, there is an explosion of life! Microbial life first and foremost: chemosynthetic bacteria that get their energy from chemical reactions, rather than the sun, using elements from their environment (sulphur, methane, hydrogen, iron, etc.) to fix carbon and synthesise organic matter. Other organisms gravitate around these bacteria, animals that find a source of organic molecules in the bacteria, either by directly consuming the bacterial mats that have developed close to the hydrothermal vents, or by forming symbiotic associations with these bacteria.