Micronutrients
Micronutrients are required for life. However, they are only required in small amounts. There are many of these.
Iron is used by cytochromes. Cytochromes are involved in energy (ATP) synthesis. Iron is transported into siderophores. Siderphores bind iron and transport it into cells.
Some bacteria can grow without iron. In this case, manganese is used in place of iron.