8.13A: Energy Conservation and Autotrophy in Archaea
- Discuss archaea energy sources
Archaea exhibit a variety of chemical reactions in their metabolism and use many sources of energy. These reactions are classified into nutritional groups, depending on energy and carbon sources. Some archaea, called lithotrophs, obtain energy from inorganic compounds such as sulfur or ammonia. Other examples include nitrifiers, methanogens, and anaerobic methane oxidizers. In these reactions one compound passes electrons to another in a redox reaction, releasing energy to fuel the cell’s activities. One compound acts as an electron donor and one as an electron acceptor. The energy released generates adenosine triphosphate ( ATP ) through chemiosmosis in the same basic process that happens in the mitochondrion of eukaryotic cells.
Many basic metabolic pathways are shared between all forms of life. For example, archaea use a modified form of glycolysis (the Entner–Doudoroff pathway) and either a complete or partial citric acid cycle. These similarities to other organisms probably reflect both early origins in the history of life and their high level of efficiency.
Some Euryarchaeota are methanogens living in anaerobic environments such as swamps. This form of metabolism evolved early, and it is possible that the first free-living organism was a methanogen. A common reaction in methanogens involves the use of carbon dioxide as an electron acceptor to oxidize hydrogen. Methanogenesis uses a range of coenzymes that are unique to these archaea, such as coenzyme M and methanofuran. Other organic compounds such as alcohols, acetic acid, or formic acid are used as alternative electron acceptors by methanogens. These reactions are common in gut-dwelling archaea. Acetotrophic archaea also break down acetic acid into methane and carbon dioxide directly. These acetotrophs are archaea in the order Methanosarcinales, and are a major part of the communities of microorganisms that produce biogas.
Other archaea, called autotrophs, use CO 2 in the atmosphere as a source of carbon, in a process called carbon fixation. This process involves either a highly modified form of the Calvin cycle or a recently discovered metabolic pathway called the 3-hydroxypropionate/4-hydroxybutyrate cycle. In addition, the Crenarchaeota use the reverse Krebs cycle while the Euryarchaeota use the reductive acetyl-CoA pathway. Carbon–fixation is powered by inorganic energy sources.
Phototrophic archaea use sunlight as a source of energy; however, oxygen–generating photosynthesis does not occur in any archaea. Instead, in archaea such as the Halobacteria, light-activated ion pumps generate ion gradients by pumping ions out of the cell across the plasma membrane. The energy stored in these electrochemical gradients is then converted into ATP by ATP synthase. This process is a form of photophosphorylation. The ability of these light-driven pumps to move ions across membranes depends on light-driven changes in the structure of a retinol cofactor buried in the center of the protein.
Besides these, archaeal energy sources are extremely diverse, and range from the oxidation of ammonia by the Nitrosopumilales to the oxidation of hydrogen sulfide or elemental sulfur by species of Sulfolobus, using either oxygen or metal ions as electron acceptors.
Key Points
- Lithotrophic archaea use non- organic sources to live.
- Phototrophic archaea use light in a non-photosynthetic fashion to drive ion pumps needed to survive.
- Archaeal energy sources are extremely diverse, including light, metallic ions, and even acidic (pH)-dependent sources.
Key Terms
- autotroph : Any organism that can synthesize its food from inorganic substances, using heat or light as a source of energy.
- calvin cycle : A series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms.