Chemosynthesis utilizes energy from inorganic chemical reactions to create sugars, while photosynthesis uses sunlight for the same purpose. Chemosynthesis is common in organisms in deep-ocean habitats where sunlight is not present.
All ecosystems require producers that turn inorganic precursors into usable nutrients. Plants and some bacteria accomplish this easily in most habitats through photosynthesis. Photosynthetic organisms use energy from the sun, water and carbon dioxide to create glucose. Photosynthesis is also responsible for using up excess carbon dioxide and replenishing atmospheric oxygen.
Some environments are not conducive to photosynthesis. Hot springs and the darkness of the deep-ocean floor require organisms to obtain energy from sources other than sunlight. Organisms in these habitats use the energy from chemical reactions happening nearby to produce usable sugars. Whereas there is only one basic reaction for photosynthesis, there are several for chemosynthesis, depending on the environment.
One common chemosynthetic reaction occurs around deep-ocean vents. Hydrothermal vents give off hydrogen sulfide, which deep-sea bacteria combine with carbon dioxide and oxygen to create sulfur, water and sugar. Other bacteria use methane to yield sugar. Colonies of chemosynthesizing bacteria also form around whale falls – when a whale carcass sinks to the deep ocean floor. Because scavenging does not occur as quickly so deep in the ocean, whale falls harbor small, local ecosystems and provide nourishment to the deep-sea habitat for decades.