All life requires energy. Humans and other animals get energy from what they eat. Plants don't eat; they make their own food. The same it true for the microbes that live around hydrothermal vents. Plants use energy from the sun to make sugars, through a process called photosynthesis. But, for hydrothermal vent communities at the bottom of the ocean, this is not an option. No sunlight reaches that depth. Instead microbes  get their energy from different chemical in hydrothermal fluid, through a process called chemosynthesis.

Hydrothermal fluid that comes out of vents contains hydrogen sulfide. Microbes that live around the vents take up hydrogen sulfide, oxygen, and carbon dioxide from the water. Then, the microbes get their energy when the break down the hydrogen sulfide. They use this energy and oxygen to convert carbon dioxide into sugars. At the end of this process, the microbes release sulfur into the water.