Heterotrophic bacteria derive energy from the breakdown of organic molecules. They may be facultative anaerobes , obligate anaerobes or aerobes . Some use light as a source of energy. The great majority of bacteria and other life forms are heterotrophs. Nitrogen-fixing bacteria are important components of ecosystems, as they convert atmospheric N2 to a form of nitrogen which can be assimilated by plants.
Bacteria play additional important roles in most biogeochemical cycles .
The Protista is a Kingdom of about 60,000 spp. They exhibit mitosis and meiosis. Protists probably include the ancestors of the kingdoms Fungi, Animalia, and Plantae. The kingdom is diverse, including both simple and fairly complex forms. Both uni- and multicellular forms are found within the group.