8.14A: Habitats and Energy Metabolism of Crenarchaeota

Crenarchaeota exist in a wide range of habitats and exhibit a great variety of chemical reactions in their metabolism.

Learning Objectives

• Outline the various types of energy metabolism used by Crenarchaeota

Key Points

• The first-discovered archaeans were extremophiles.
• Extremophile archaea are members of four main physiological groups: halophiles, thermophiles, alkaliphiles, and acidophiles.
• Some archaea obtain energy from inorganic compounds such as sulfur or ammonia (they are lithotrophs).
• Other groups of archaea use sunlight as a source of energy (phototrophs) or CO2 in the atmosphere as a source of carbon (autotrophs).

Key Terms

• extremophile: An organism that lives under extreme conditions of temperature, salinity, and so on. They are commercially important as a source of enzymes that operate under similar conditions.
• phototroph: An organism that carries out photon capture to acquire energy. They use the energy from light to carry out various cellular metabolic processes.
• autotroph: Any organism that can synthesize its food from inorganic substances, using heat or light as a source of energy.

The Crenarchaeota are Archaea that have been classified as either a phylum of the Archaea kingdom, or in a kingdom of its own. Archaea exist in a broad range of habitats, and as a major part of global ecosystems, they may contribute up to 20% of earth’s biomass.

The first-discovered archaeans were extremophiles. Indeed, some archaea survive high temperatures, often above 100 °C (212 °F), as found in geysers, black smokers, and oil wells. Other common habitats include very cold habitats and highly saline, acidic, or alkaline water. However, archaea also include mesophiles that grow in mild conditions, in marshland, sewage, the oceans, and soils.

Figure: Microbial Mats Around the Grand Prismatic Spring: Thermophiles produce some of the bright colors of Grand Prismatic Spring, Yellowstone National Park

Extremophile archaea are members of four main physiological groups. These are the:

• halophiles
• thermophiles
• alkaliphiles
• acidophiles

These groups are not comprehensive or phylum-specific, nor are they mutually exclusive, since some archaea belong to several groups. Nonetheless, they are a useful starting point for classification Halophiles live in extremely saline environments such as salt lakes. Thermophiles grow best at temperatures above 45 °C (113 °F), in places such as hot springs; hyperthermophilic archaea grow optimally at temperatures greater than 80 °C (176 °F). Other archaea exist in very acidic or alkaline conditions.

Recently, several studies have shown that archae exist not only in mesophilic and thermophilic environments but are also present, sometimes in high numbers, at low temperatures as well, as found in cold oceanic environments.

Chemical reactions and energy sources

Archaea exhibit a great 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 obtain energy from inorganic compounds such as sulfur or ammonia (they are lithotrophs). These include nitrifiers, methanogens and anaerobic methane oxidisers. 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.

Other groups of archaea use sunlight as a source of energy (phototrophs). However, oxygen–generating photosynthesis does not occur in any of these organisms. 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 even possible that the first free-living organism was a methanogen. A common reaction involves the use of carbon dioxide as an electron acceptor to oxidize hydrogen. Methanogenesis involves a range of coenzymes that are unique to these archaea, such as coenzyme M and methanofuran. These reactions are common in gut-dwelling archaea. Acetic acid is also broken down into methane and carbon dioxide directly, by acetotrophic archaea. These acetotrophs are archaea in the order Methanosarcinales, and are a major part of the communities of microorganisms that produce biogas.

Other archaea use CO2 in the atmosphere as a source of carbon, in a process called carbon fixation (they are autotrophs). 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. The Crenarchaeota also use the reverse Krebs cycle while the Euryarchaeota also use the reductive acetyl-CoA pathway. Carbon–fixation is powered by inorganic energy sources. No known archaea carry out photosynthesis.

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.

Phototrophic archaea use light to produce chemical energy in the form of ATP. In the Halobacteria, light-activated ion pumps like bacteriorhodopsin and halorhodopsin 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 (photophosphorylation).

Some marine Crenarchaeota are capable of nitrification, suggesting these organisms may affect the oceanic nitrogen cycle, although these oceanic Crenarchaeota may also use other sources of energy. Vast numbers of archaea are also found in the sediments that cover the sea floor, with these organisms making up the majority of living cells at depths over 1 meter below the ocean bottom.