Microscopic organisms that live in both saline and freshwater environments. Some are protists (simple eukaryotic organisms), while most are single-celled plants. The most common varieties are cyanobacteria, diatoms, dinoflagellates, green algae, and coccolithophores. Phytoplanktons use photosynthesis to transform light captured by chlorophyll to chemical energy. They are also the main source of organic matter used in the creation of petroleum.