Phytoplankton form the basis of life in the ocean. They use photosynthesis to convert energy from sunlight into chemical energy (food). Some of this food passes directly along the food chain when zooplankton eat the phytoplankton and in turn are consumed by larger animals such as fish, whales, squid, shellfish and birds. Food produced by phytoplankton can also enter another pathway dominated by bacteria (and possibly viruses). This is the microbial loop, so named because the organisms are microscopic – about 100 times smaller than phytoplankton. These bacteria absorb chemicals released by decomposing phytoplankton, and enter the main food chain when they are eaten by zooplankton.