Deep-ocean hydrothermal vents occur where there is intense volcanic activity. Seawater permeates rock, heats up and becomes enriched with substances from the rock, like metals, sulfide, dihydrogen, and methane. Mineral-rich chimneys, around which hydrothermal-vent animals live, then form when these heated fluids exit the seafloor. During the 1980s, scientists realized that these habitats supported an unusual type of primary production, fueled not by sunlight and photosynthesis, but by energy from reactions between chemicals found in the hydrothermal fluid, like sulfide, and the oxygen present in seawater. Amazingly, some basic, single-celled microorganisms can use this energy to build the parts of their one cell. Hydrothermal vents provided the first evidence that this process, called chemosynthesis, could sustain so much life in otherwise desert-like surroundings.
But what about the larger animals that live in these environments? How do they get the energy they need to survive? Well, many of these animals acquire their energy by maintaining close relationships with chemosynthetic bacteria. This type of relationship, where two different organisms live together closely is called symbiosis. In chemosynthetic symbioses, both organisms involved are believed to benefit from the relationship.

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