At the bottom of an energy pyramid in the ocean, you will find the primary producers. These are the organisms that convert light energy into chemical energy, which forms the basis of the food chain. In the ocean, primary producers are mainly phytoplankton, such as diatoms, dinoflagellates and coccolithophores. These microscopic algae use the sun's energy to photosynthesize, converting carbon dioxide and water into organic matter. Phytoplankton are the foundation of the marine food chain, and they provide food for zooplankton, small fish, and other organisms.