4. Prokaryotic Diversity
4.6 Archaea
Learning Objectives
- Describe the unique features of each category of Archaea
- Explain why archaea might not be associated with human microbiomes or pathology
- Give common examples of archaea commonly associated with unique environmental habitats
Like organisms in the domain Bacteria, organisms of the domain Archaea are all unicellular organisms. However, archaea differ structurally from bacteria in several significant ways, as discussed in Unique Characteristics of Prokaryotic Cells. To summarize:
- The archaeal cell membrane is composed of ether linkages with branched isoprene chains (as opposed to the bacterial cell membrane, which has ester linkages with unbranched fatty acids).
- Archaeal cell walls lack peptidoglycan, but some contain a structurally similar substance called pseudopeptidoglycan or pseudomurein.
- The genomes of Archaea are larger and more complex than those of bacteria.
Domain Archaea is as diverse as domain Bacteria, and its representatives can be found in any habitat. Some archaea are mesophiles, and many are extremophiles, preferring extreme hot or cold, extreme salinity, or other conditions that are hostile to most other forms of life on earth. Their metabolism is adapted to the harsh environments, and they can perform methanogenesis, for example, which bacteria and eukaryotes cannot.
Phylogeny
Between 1990 and 2002, archaea were classified into one of two phyla: Euryarchaeota and Crenarchaeota. Today, with advances in technology and analytics, the domain includes at least 4 major supergroups: Euryarchaeota, TACK, Asgard and DPANN (Figure 4.47).[1]
As far as we know, archaea are not associated with infectious diseases in humans, animals, plants, or microorganisms. However, many play important roles in the environment and may thus have an indirect impact on human health.
Metabolism and Physiology
Archaea exhibit a variety of chemical reactions in their metabolism and use many sources of energy. As a result of this and their widespread distribution, archaea play critical roles in cycling of elements in the biosphere. Some archaea are lithotrophs, obtaining energy from inorganic compounds such as elemental sulphur, H2S or ammonia. Lithotrophs are also autotrophic, using CO2 in the atmosphere as a source of carbon, “fixing” the carbon into biomass.
Metabolism
Some archaea are methanogens. These organisms are unique in that they can reduce carbon dioxide in the presence of hydrogen, producing methane. They can live in the most extreme environments and can reproduce at temperatures varying from below freezing to boiling. Methanogens have been found in hot springs as well as deep under ice in Greenland. Some scientists have even hypothesized that methanogens may inhabit the planet Mars because the mixture of gases produced by methanogens resembles the makeup of the Martian atmosphere.[2]
Methanogens are thought to contribute to the formation of anoxic sediments by producing hydrogen sulph