<unk>  exhibit an extremely wide variety of metabolic types. The distribution of metabolic traits within a group of<unk>  has traditionally been used to define their taxonomy, but these traits often do not correspond with modern genetic classifications.<unk> l metabolism is classified into nutritional groups on the basis of three major criteria: the source of energy, the electron donors used, and the source of carbon used for growth.<unk>  either derive energy from light using photosynthesis (called phototrophy), or by breaking down chemical compounds using oxidation (called chemotrophy). Chemotrophs use chemical compounds as a source of energy by transferring electrons from a given electron donor to a terminal electron acceptor in a redox reaction. This reaction releases energy that can be used to drive metabolism. Chemotrophs are further divided by the types of compounds they use to transfer electrons.<unk>  that use inorganic compounds such as hydrogen, carbon monoxide, or ammonia as sources of electrons are called lithotrophs, while those that use organic compounds are called organotrophs. The compounds used to receive electrons are also used to classify<unk> : aerobic organisms use oxygen as the terminal electron acceptor, while anaerobic organisms use other compounds such as nitrate, sulfate, or carbon dioxide. Many<unk>  get their carbon from other organic carbon, called heterotrophy. Others such as cyanobacteria and some purple<unk>  are autotrophic, meaning that they obtain cellular carbon by fixing carbon dioxide. In unusual circumstances, the gas methane can be used by methanotrophic<unk>  as both a source of electrons and a substrate for carbon anabolism. + Nutritional types in<unk> l metabolism Nutritional type Source of energy Source of carbon Examples Phototrophs Sunlight Organic compounds (photoheterotrophs) or carbon fixation (photoautotrophs) Cyanobacteria, Green sulfur<unk> , Chloroflexota, or Purple<unk>  Lithotrophs Inorganic compounds Organic compounds (lithoheterotrophs) or carbon fixation (lithoautotrophs) Thermodesulfobacteriota, Hydrogenophilaceae, or Nitrospirae Organotrophs Organic compounds Organic compounds (chemoheterotrophs) or carbon fixation (chemoautotrophs) Bacillus, Clostridium, or Enterobacteriaceae In many ways,<unk> l metabolism provides traits that are useful for ecological stability and for human society. One example is that some<unk>  called diazotrophs have the ability to fix nitrogen gas using the enzyme nitrogenase. This environmentally important trait can be found in<unk>  of most metabolic types listed above. This leads to the ecologically important processes of denitrification, sulfate reduction, and acetogenesis, respectively.<unk> l metabolic processes are also important in biological responses to pollution; for example, sulfate-reducing<unk>  are largely responsible for the production of the highly toxic forms of mercury (methyl- and dimethylmercury) in the environment. Non-respiratory anaerobes use fermentation to generate energy and reducing power, secreting metabolic by-products (such as ethanol in brewing) as waste. Facultative anaerobes can switch between fermentation and different terminal electron acceptors depending on the environmental conditions in which they find themselves.
Bacteria
Yeast
Virus
Enteric bacteria 
Chyme
Microgenia
Microorganism
Mold