Which deep-sea vent ecosystems run on which energy process?

Chemosynthesis - bacteria oxidising hydrogen sulphide from the vent fluid to build sugars, with no sunlight involved anywhere in the chain. Before 1977 it was assumed that essentially all life on Earth ultimately ran on photosynthesis.

The discovery was accidental. A geological expedition to a ridge near the Galapagos went looking for hot water and found clams the size of dinner plates, crabs, and metre-long tube worms in dense colonies around the vents. The team had brought no biologist and no preservative, and reportedly resorted to the ship's vodka to keep specimens.

The giant tube worm is the strange part. It has no mouth, no gut and no anus; as a larva it acquires bacteria and then digests its own digestive system, spending the rest of its life as a sac of symbionts fed by a haemoglobin that carries both oxygen and sulphide at once - two molecules that ordinarily react with each other. Vents are also a leading candidate for where life started, and they mean the search for life elsewhere is no longer restricted to places that get sunlight: Enceladus and Europa both plausibly have hot rock under an ocean.