Here is an article from NOAA:

http://www.pmel.noaa.gov/vents/nemo/...synthesis.html

Chemosynthesis

Most life on Earth is dependent upon photosynthesis, the process by
which plants make energy from sunlight. However, at hydrothermal vents
in the deep ocean a unique ecosystem has evolved in the absense of
sunlight, and its source of energy is completely different:
chemosynthesis. Chemosynthesis is the process by which certain microbes
create energy by mediating chemical reactions. So the animals that live
around hydrothermal vents make their living from the chemicals coming
out of the seafloor in the vent fluids! Because they are a local food
source, hydrothermal vents typically have high biomass, in stark
contrast to the very sparse distribution of animals outside of vent
areas where animals are dependent on food dropping down from above.

Chemosynthetic microbes provide the foundation for biological
colonization of vents. Chemosynthetic microbes live on or below the
seafloor, and even within the bodies of other vent animals as symbionts.
Where microbial mat covers the seafloor around vents, grazers such as
snails, limpets, and scaleworms eat the mat, and predators come to eat
the grazers. Tubeworms flourish in small clumps, waving in the warm
fluids. A typical picture of an active hydrothermal vent is therefore
one with shimmering warm hydrothermal fluids, tubeworms and many other
vent species, all densely clustered around the vent, with white
microbial mat material covering the surrounding area.