Green algae
Many bacteria, including "Escherichia coli" found in the large intestine, can synthesize vitamin K2 (MK-7 up to MK-11), but not vitamin K1. Green algae and some species of cyanobacteria (sometimes referred to as blue-green algae) are able to synthesize vitamin K1. In the vitamin K2 synthesizing bacteria, menaquinone transfers two electrons between two different small molecules, during oxygen-independent metabolic energy production processes (anaerobic respiration). For example, a small molecule with an excess of electrons (also called an electron donor) such as lactate, formate, or NADH, with the help of an enzyme, passes two electrons to menaquinone. The menaquinone, with the help of another enzyme, then transfers these two electrons to a suitable oxidant, such as fumarate or nitrate (also called an electron acceptor). Adding two electrons to fumarate or nitrate converts the molecule to succinate or nitrite plus water, respectively. Some of these reactions generate a cellular energy source, ATP, in a manner similar to eukaryotic cell aerobic respiration, except the final electron acceptor is not molecular oxygen, but fumarate or nitrate. In aerobic respiration, the final oxidant is molecular oxygen, which accepts four electrons from an electron donor such as NADH to be converted to water. "E. coli", as facultative anaerobes, can carry out both aerobic respiration and menaquinone-mediated anaerobic respiration.