Phytoplankton, animation. Phytoplankton are the photosynthetic organisms that drift around the world磗 oceans, seas and almost all other bodies of water. These microscopic organisms use sunlight and carbon dioxide to produce oxygen and organic molecules. This makes them the base layer of the food chain, and the source of around half of the oxygen in the atmosphere. Not all of the phytoplankton are plants: the group contains a number of protozoa, bacteria and cyanobacteria blue_green algae.