Cellular life on earth has evolved along three major lineages, called Bacteria, Archaea, and Eukarya. Comparative rRNA sequencing revealed these three distinct cellular lineages, also called domains. Bacteria and Archaea are prokaryotic lines whereas Eukarya is the eukaryotic line.
Archaea were discovered by American microbiologist Carl Richard Woese in 1977.
Archaea, is the domain of phylogenetically related prokaryotes distinct from Bacteria and known for tolerance to physiochemical extremes.
Some unique features
Archaea lack peptidoglycan in their cell walls so they are naturally resistant to the activity of lysozyme and the antibiotic penicillin.
All are chemotrophic, Halobacterium can use light to make ATP but the mechanism is quite distinct from that of phototrophic organisms. Most are chemolithotrophs, with hydrogen gas (H2 ) being a widely used inorganic substance. Chemolithotrophic metabolisms are particularly widespread among hyperthermophilic archaea.
Most archaea are anaerobes. E.g., methanogens.
They can thrive on extreme environmental conditions such as hot springs (to temperatures above the boiling point of water), extremely salty bodies of water, and high acidic or alkaline soils and water.
The organism Pyrolobus, for example, is a hyperthermophile capable of growth at up to 113°C, and the methanogen Methanopyrus can grow up to 122°C. Hyperthermophiles do not contain fatty acids in their membranes but instead have C 40 hydrocarbons composed of repeating units of isoprene bonded by ether linkage to glycerol phosphate.
Archaea comprises most of the current “record holders” for growth at a particular extreme condition. These examples of extremophiles that lies in archaea are as follows:
| Genus/Species | Extremophile | Habitat | Minimum | Optimum | Maximum |
| Methanopyrus kandleri | Hyperthermophile | Under sea hydothermal vents | 90°C | 106 °C | 122 °C |
| Picrophilus oshimae | Acidophile | Acidic hot springs | pH -0.06 | pH 0.7 | pH 4 |
| Natronobacterium gregoryi | Alkaliphile | Soda lakes | pH 8.5 | pH 10 | pH 12 |
| Halobacterium salinarum | Halophile | Salterns (salt evaporation ponds) | 15% salt concentration | 25% | 32% |
Certain archaea also show unusual biochemical features, such as the production of methane (natural gas)as an integral part of their energy metabolism. Strictly anaerobic methanogenic archaea can produce methane from carbon dioxide and hydrogen or from acetate or methanol.
Similarities Between Bacteria and Archaea
- Prokaryotic cell structure
- The cytoplasmic membrane has inner and outer hydrophilic surfaces and a hydrophobic interior.
- Both show chemotaxis, and many proteins that control chemotaxis in bacteria are also present in motile archaea.
- Presence of a single circular chromosome that carries 500 to a few thousand genes
- Absence of membrane-enclosed nucleus
- Presence of 70S ribosomes
- Introns are present in tRNA genes but absent in other genes
- Presence of operons
- Absence of capping and poly-A tailing of mRNA.
- Presence of plasmids
- Gas vesicles are present mainly in aquatic bacteria and archaea but never in eukaryotic microorganisms.
- Can synthesize carbon storage granules composed of poly-beta-hydroxyalkanoate
- Chemolithotrophy (Fe, S, H2)
- Has the ability of nitrogen fixation as well as denitrification
Major Difference Between Bacteria and Archaea
Though both bacteria and archaea are prokaryotes, molecular sequencing criteria suggest that Archaea are more closely related to Eukarya than Bacteria.
| Characteristic | Bacteria | Archaea |
| Peptidoglycan | It is a key biomarker of bacteria. It is thicker in gram-positive and thinner in gram-negative. | Peptidoglycan is absent in archaea. They may instead contain pseudomurein or polysaccharides. |
| Composition of peptidoglycan/pseudomurein | The backbone of peptidoglycan contains alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) connected in β-1,4 linkage. Both L and D- amino acids and diaminopimelic acid (DAP) is present. | Some archaeal cell-wall contains a polysaccharide called pseudomurein. The backbone of pseudomurein is composed of alternating units of NAG but contains N-acetyltalosaminuronic acid in place of NAM. The glycosidic bonds between the sugar derivatives are β-1,3 and the amino acids are all of the L stereoisomer. DAP is absent in archaea. |
| Paracrystalline surface layer (S-layer) | Present in several species of bacteria. | S-layers have been found in representatives of all major lineages of Archaea. |
| Endospore formation | Bacteria form endospores. For example, Bacillus and Clostridium. | No archaea have been shown to form endospores. |
| Flagella | Bacterial flagella are made up of a single type of protein, and their diameter is 15-20 nm. | Archaeal flagella are made up of multiple proteins, and their diameter is roughly half the diameter of bacterial flagella. |
| Membrane lipids | In bacteria and eukaryotic microorganisms ester linkages bond glycerol to the fatty acids. | Archaea contain ether bonds between glycerol and their hydrophobic side chains. Archaeal lipids lack true fatty acids; instead, they contain branched side chains constructed of multiples of isoprene. |
| Replication of DNA | In DNA gyrase helps in the packaging of very large DNA by supercoiling. Histones are absent. | DNA replication apparatus of archaea is much more similar to that of eukaryotes. Archaea possess both DNA gyrase and histones. |
| Antimicrobial Susceptibility | Bacteria are susceptible to antimicrobial agents, but the susceptibility differs among bacteria depending on cell-wall composition and acquired resistance. | Most antibiotics do not affect archaea. For example, penicillins do not affect archaea because their cell walls lack peptidoglycan. |
| Methanogenesis | No | Yes |
| Cholorophyll based photosynthesis | Yes | No |
| Pathogenicity | No pathogenic archaea have been identified, but evidence suggests that archaea may play a role in certain opportunistic infections. | Not all bacteria are pathogens. Some are saprophytic, some are beneficial and are used to make yogurt and beer, and also used as probiotics, but some bacteria are pathogenic and cause diseases like tuberculosis, pneumonia, and diarrhea, among others. |
| Cell-wall lacking organisms | Mycoplasma and Ureaplasma are cell-wall-lacking bacteria | Thermoplasma and Ferroplasma are cell-wall lacking archaea. |
| Examples | Examples of bacteria include E. coli, Staphylococcus aureus, Neisseria gonorrhoeae, H. influenzae, etc. | Examples of archaea include Methanobacterium, Methanococcus, Thermococcus, Methanopyrus, Thermoplasma, etc. |
References and further readings
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
- Berthold, E. (2018, August 13). What are archaea? Curious. https://www.science.org.au/curious/earth-environment/what-are-archaea
- Bruslind, L. (n.d.). Archaea. In General Microbiology. Oregon State University. Retrieved June 10, 2021, from https://open.oregonstate.education/generalmicrobiology/chapter/archaea/