How can deep-sea vent animals live on a chemical that poisons most life?

By handing the chemistry to microbes. Sunlight never reaches deep-sea hydrothermal vents, so nothing there can photosynthesize. Instead, the base of the food chain is made of chemosynthetic bacteria and archaea: they fix carbon using energy from chemicals such as sulfide, turning hydrogen sulfide, carbon dioxide and oxygen into organic molecules. Many vent animals carry these microbes inside their bodies, in their gills or special tissues, and that symbiosis is the reason multicellular life can survive there at all, because sulfide is extremely toxic to most life on Earth.

Giant tube worms push the partnership furthest. They have no mouth and no gut. Instead they have an organ called a trophosome, where their bacterial partners live and where nutrition is handled, and a bright red plume that takes up oxygen, hydrogen sulfide and carbon dioxide to feed those bacteria. The red comes from their hemoglobin, which can carry oxygen without being disrupted by sulfide, even though the two are normally very reactive. In 2005 researchers found out how: zinc ions bind the hydrogen sulfide in the worm's hemoglobin, keeping it from reacting with the oxygen, sparing the worm's tissues, and delivering it to the bacteria.

The partnership turns vents into oases. Hydrothermal vents are fissures in the seabed that discharge geothermally heated water, often where tectonic plates move apart, at temperatures of up to 464°C against about 2°C for the surrounding deep water. Compared with most of the deep sea, the areas around them are biologically more productive, with giant tube worms, clams, limpets and shrimp. Scientists were astounded when they first found vents teeming with life in 1977; by 1981 they understood that tube worms are fed by their bacteria, and the same kind of partnership turned up in clams in 1983 and in vent mussels in 1984.

Some scientists think vents like these may have helped life get started on Earth, and active vents are thought to exist today on Jupiter's moon Europa and Saturn's moon Enceladus.