Photosynthesis is the process by which plants use the sun’s energy to make sugar (glucose) for food. Plants absorb energy from sunlight, take in carbon dioxide from the air through their leaves, take up water through their roots, and produce glucose and oxygen. Photosynthesis takes place on land and in shallow water where sunlight can reach seaweeds.

Chemosynthesis is the process by which food (glucose) is made by bacteria using chemicals as the energy source, rather than sunlight. Chemosynthesis occurs around hydrothermal vents and methane seeps in the deep sea where sunlight is absent. During chemosynthesis, bacteria living on the sea floor or within animals use energy stored in the chemical bonds of hydrogen sulfide and methane to make glucose from water and carbon dioxide (dissolved in sea water). Pure sulfur and sulfur compounds are produced as by-products.

In the diagram mussels and tubeworms are using the hydrogen sulfide released from a hydrothermal vent. The chemical equation given here for chemosynthesis is just one of a number of possibilities.