Chemosynthesis and symbiosis

There are a lot of different types of bacteria and archaea in hot and warm vents. Many of them can produce food by a process called chemosynthesis. They use, for example, the toxic hydrogen sulfide – the gas of rotten eggs - of the vent fluid and the oxygen of the surrounding seawater to oxidize the hydrogen sulfide. At the same time the oxygen gets reduced during this reaction. A lot of energy yields from this process. This energy is used to turn inorganic carbon dioxide into organic carbon, which is their food. The principle is the same one as with the plants’ photosynthesis, but bacteria utilize the energy of chemical compounds rather than the Sun’s energy.

Several large animals such as the giant tubeworm, the pompeii worm, mussels and even small worm like molluscs live in symbiosis with sulfur bacteria. Either these bacteria are inside the animals like in the tubeworm or the mussel or they live on the skin of the animals, like in the pompeii worm and the mollusc worm. All these animals are called the host, the sulfur bacteria are called the symbionts.

The symbionts receive all chemicals from the host to fix inorganic carbon and produce food. This food the bacteria use to grow themselves, but they share it with the host. The host is fed by the symbionts. Some hosts additionally feed on free-living bacteria using their mouth for uptake and their gut to digest the food. This is the case in mussels, pompeii worms and mollusc worms. Only the tubeworms to not have a mouth and a gut anymore. The live exclusively on the food they get from their symbionts.

From left to right (Photographs Monika Bright)

Worm-like mollusc of the phylum Mollusca and the family Solenogastres with the scientific name Helicoradomenia. These about 3 mm long worms do not have a shell. Instead, they have tiny little bristles made out of the same material as the shells of mussels and snails. The symbionts live between these bristles but some of them were also found in the skin.

The Pomeii Worm belongs to the phylum Annelida of the family Alvinellidae (scientific name Alvinella pompejana). These worms live on black smokers. Where there is their limit of temperature is still controversial. They build tubes and temperatures up to 120° Celsius were measured. Because they worm can move in their tube and change therefore the temperature themselves, around 50° Celsius seems proven. No other animal can endure such heat in the ocean.

The so-called Giant Tubeworm Riftia pachyptila of the phylum Annelida, familily Siboglinidae has been studied for more than 20 years. This is the fastest growing invertebrate we know of. It can grow 80 cm in its first year of life. It does so without a mouth and a gut. Instead, it has bacteria inside the body in an organ, called the trophosome.

The mussel Bathymodiolus thermophilus belongs to the phylum Mollusca, family Bivalves. The sulfur bacteria are located in the gills of the animal. These mussels can move and therefore and therefore also can choose the right conditions for living. They like it relatively cool, only a few degrees above the surrounding cold seawater.