Mn-SOD is the type of SOD present in eukaryotic mitochondria, and also in most bacteria (this fact is in keeping with the bacterial-origin theory of mitochondria). The Mn-SOD enzyme is probably one of the most ancient, for nearly all organisms living in the presence of oxygen use it to deal with the toxic effects of superoxide (), formed from the 1-electron reduction of dioxygen. The exceptions, which are all bacteria, include Lactobacillus plantarum and related lactobacilli, which use a different nonenzymatic mechanism with manganese (Mn) ions complexed with polyphosphate, suggesting a path of evolution for this function in aerobic life.