Phytoplankton are, by definition, photosynthetic, and include cyanobacteria as well as algae. Most of the phytoplankton in inland waters obtain all of their energy from light and their chemical requirements from dissolved inorganic solutes, i.e., they are photolithotrophs. Growth rate limitation by resource supply in photolithotrophic phytoplankton can involve light, phosphate, combined nitrogen (except in cyanobacterial nitrogen-fixers), inorganic carbon, and iron. Some phytoplankton are mixotrophs, i.e., can obtain energy and carbon from organic compounds in the environment as well as from light and inorganic carbon. This category includes sapromixotrophs (using dissolved organic carbon) and phagomixotrophs (using particulate organic matter). Net uptake of dissolved organic matter by sapromixotrophy in phytoplankton is much rarer in the natural environment than is the loss of dissolved organic matter by photolithotrophs. Phagomixotrophy also occurs in the natural environment, and can relate to the acquisition of phosphorus, iron, and nitrogen as well as of organic carbon. Even in photolithotrophic phytoplankton cells phosphorus and combined nitrogen can be obtained from dissolved organic phosphate and organic nitrogen.