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.

The size and complexity of the archaeal genome makes it difficult to classify. Most taxonomists agree that within the Archaea, there are currently five major phyla: Crenarchaeota , Euryarchaeota , Korarchaeota , Nanoarchaeota , and Thaumarchaeota . There are likely many other archaeal groups that have not yet been systematically studied and classified.

With few exceptions, archaea are not present in the human microbiota, and none are currently known to be 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.

Crenarchaeota

Crenarchaeota is a class of Archaea that is extremely diverse, containing genera and species that differ vastly in their morphology and requirements for growth. All Crenarchaeota are aquatic organisms, and they are thought to be the most abundant microorganisms in the oceans. Most, but not all, Crenarchaeota are hyperthermophiles; some of them (notably, the genus Pyrolobus ) are able to grow at temperatures up to 113 °C. E. Blochl et al.“ Pyrolobus fumani , gen. and sp. nov., represents a novel group of Archaea, extending the upper temperature limit for life to 113 ^° C.” Extremophiles 1 (1997):14–21.

Archaea of the genus Sulfolobus ( [link] ) are thermophiles that prefer temperatures around 70–80°C and acidophiles that prefer a pH of 2–3. T.D. Brock et al. “ Sulfolobus : A New Genus of Sulfur-Oxidizing Bacteria Living at Low pH and High Temperature.” Archiv für Mikrobiologie 84 no. 1 (1972):54–68. Sulfolobus can live in aerobic or anaerobic environments. In the presence of oxygen, Sulfolobus spp. use metabolic processes similar to those of heterotrophs. In anaerobic environments, they oxidize sulfur to produce sulfuric acid, which is stored in granules. Sulfolobus spp. are used in biotechnology for the production of thermostable and acid-resistant proteins called affitins . S. Pacheco et al. “Affinity Transfer to the Archaeal Extremophilic Sac7d Protein by Insertion of a CDR.” Protein Engineering Design and Selection 27 no. 10 (2014):431-438. Affitins can bind and neutralize various antigens (molecules found in toxins or infectious agents that provoke an immune response from the body).