Potassium is an essential cation for biochemical functions in algal cells. It is used as an activator for enzymes involved in photosynthesis and respiration, involved in the synthesis of a number of proteins and carbohydrates and the regulation of osmotic potential. In natural seawater, potassium typically occurs at ~10 mM. At this concentration, sufficient potassium is present for microalgal needs in most marine media. For freshwater and artificial seawater media potassium needs to be added, typically as either phosphate or nitrate salts.