According to the Oceanic Research Group, phytoplankton are plants and do not eat anything. Rather, they use energy from sunlight to convert nutrients, such as silicon and phosphorus, into organic tissue through the process of photosynthesis.
There are two types of plankton, according to the Oceanic Research Group: phytoplankton and zooplankton. Phytoplankton are plants, and zooplankton are the animals that eat the phytoplankton. These two kinds of plankton act as the base of the entire marine food chain, as small fish and crustaceans eat the zooplankton, which are then eaten by the larger fish and marine mammals. Phytoplankton are also responsible for creating more than half of the oxygen that people breathe.