Archaea and bacteria are two different domains of cellular life. They are both prokaryotes, as they are unicellular and lack a nucleus. They also look similar (even under a microscope). However, DNA analysis reveals that archaea are as different from bacteria as they are from human beings. Discovered during the 1970s as a unique life form, archaea play an important role in our daily lives, including as part of the human gut microbiome.
Archaea are a domain of single-celled microorganisms. They are extremophiles, capable of surviving in extreme environments where no other organisms would survive. The domain Archaea contains a diverse set of organisms that share properties with both bacteria and eukaryotes (the two other domains). Both bacteria and Archaea are microorganisms that live in a wide range of habitats, including the human body. They look very similar to one another, even under a microscope. Their chemical makeup and physical characteristics, however, are quite different from one another. Some of their key differences include:
Before the discovery of archaea, scientists believed that all prokaryotes were a single type of organism called bacteria. In the late 1970s, a biologist named Dr. Carl Woese conducted genetic experiments on organisms believed to be bacteria. The results were startling: One group of so-called bacteria were radically different from the rest. This unique group of microorganisms lived in extremely high temperatures and produced methane. Woese termed these microorganisms Archaea. Their genetic makeup was so different from the bacteria that he proposed a major change to the way life on Earth is organized. Instead of organizing life into two domains (prokaryotes and eukaryotes), Woese organized life into three domains: eukaryotes, bacteria, and archaea. Archaea, like bacteria, exist in a huge range of environments, including the human body. And, like bacteria, Archaea play an important role in many biological processes. Some of those roles include:
Perhaps the most fascinating aspect of Archaea is their ability to live in incredibly extreme environments. They are capable of thriving where no other organism can survive. For example, according to one study, the archaeal Methanopyrus kandleri strain can grow at 252 degrees F, while Picrophilus torridus can thrive at the incredibly acidic PH of 0.06. These are both records for extremophile environments. Other examples of Archaea in extremophile environments include:
Additionally, archaea may be able to survive in toxic waste and heavy metals. Scientists have found that Archaea, particularly those that thrive in extreme heat, are genetically close to the “universal ancestor” of all organisms on Earth. This finding suggests that Archaea may be the key to understanding the evolutionary origins of life on Earth. Some scientists also believe that Archaea's ability to survive in extraordinarily harsh environments could provide insight into extraterrestrial life. The nature of extremophiles makes them a natural focus for researchers exploring the question of what, if anything, can survive in interstellar space or on planets where typical Earth-based plants and animals would quickly die. One study subjected Archaea to temperature, UV radiation, humidity, and pressure resembling conditions on Mars and on the moon Europa; not surprisingly, the microorganisms lived and thrived.
Learning Outcomes
Prokaryotes are divided into two different domains, Bacteria and Archaea, which together with Eukarya, comprise the three domains of life (Figure 1). The composition of the cell wall differs significantly between the domains Bacteria and Archaea. The composition of their cell walls also differs from the eukaryotic cell walls found in plants (cellulose) or fungi and insects (chitin). The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Some bacteria have an outer capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others. For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea. Phylum Proteobacteria is one of up to 52 bacteria phyla. Proteobacteria is further subdivided into five classes, Alpha through Epsilon (Table 1).
Chlamydia, Spirochetes, Cyanobacteria, and Gram-positive bacteria are described in Table 2. Note that bacterial shape is not phylum-dependent; bacteria within a phylum may be cocci, rod-shaped, or spiral.
Archaea are separated into four phyla: the Euryarchaeota, Crenarchaeota, Nanoarchaeota, and Korarchaeota.
The Plasma MembraneThe prokaryotic plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the cell and separates the inside from the outside. Its selectively permeable nature keeps ions, proteins, and other molecules within the cell and prevents them from diffusing into the extracellular environment, while other molecules may move through the membrane. Recall that the general structure of a cell membrane is a phospholipid bilayer composed of two layers of lipid molecules. In archaeal cell membranes, isoprene (phytanyl) chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Some archaeal membranes are lipid monolayers instead of bilayers (Figure 2). The Cell WallThe cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Therefore, the osmotic pressure within the cell is relatively high. The cell wall is a protective layer that surrounds some cells and gives them shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis (bursting due to increasing volume). The chemical composition of the cell wall varies between Archaea and Bacteria, and also varies between bacterial species. Bacterial cell walls contain peptidoglycan, composed of polysaccharide chains that are cross-linked by unusual peptides containing both L- and D-amino acids including D-glutamic acid and D-alanine. (Proteins normally have only L-amino acids; as a consequence, many of our antibiotics work by mimicking D-amino acids and therefore have specific effects on bacterial cell-wall development.) There are more than 100 different forms of peptidoglycan. S-layer (surface layer) proteins are also present on the outside of cell walls of both Archaea and Bacteria. Bacteria are divided into two major groups: Gram positive and Gram negative, based on their reaction to Gram staining. Note that all Gram-positive bacteria belong to one phylum; bacteria in the other phyla (Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and others) are Gram-negative. The Gram staining method is named after its inventor, Danish scientist Hans Christian Gram (1853–1938). The different bacterial responses to the staining procedure are ultimately due to cell wall structure. Gram-positive organisms typically lack the outer membrane found in Gram-negative organisms (Figure 3). Up to 90 percent of the cell-wall in Gram-positive bacteria is composed of peptidoglycan, and most of the rest is composed of acidic substances called teichoic acids. Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan (only 10 percent of the total cell wall), surrounded by an outer envelope containing lipopolysaccharides (LPS) and lipoproteins. This outer envelope is sometimes referred to as a second lipid bilayer. The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes.
Bacteria are divided into two major groups: Gram positive and Gram negative. Both groups have a cell wall composed of peptidoglycan: in Gram-positive bacteria, the wall is thick, whereas in Gram-negative bacteria, the wall is thin. In Gram-negative bacteria, the cell wall is surrounded by an outer membrane that contains lipopolysaccharides and lipoproteins. Porins are proteins in this cell membrane that allow substances to pass through the outer membrane of Gram-negative bacteria. In Gram-positive bacteria, lipoteichoic acid anchors the cell wall to the cell membrane. Which of the following statements is true?
Archaean cell walls do not have peptidoglycan. There are four different types of archaean cell walls. One type is composed of pseudopeptidoglycan, which is similar to peptidoglycan in morphology but contains different sugars in the polysaccharide chain. The other three types of cell walls are composed of polysaccharides, glycoproteins, or pure protein. Other differences between Bacteria and Archaea are seen in Table 4. Note that features related to DNA replication, transcription and translation in Archaea are similar to those seen in eukaryotes.
Bacteria and Archaea differ in the lipid composition of their cell membranes and the characteristics of the cell wall. In archaeal membranes, phytanyl units, rather than fatty acids, are linked to glycerol. Some archaeal membranes are lipid monolayers instead of bilayers. The cell wall is located outside the cell membrane and prevents osmotic lysis. The chemical composition of cell walls varies between species. Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have peptidoglycan, but they may have pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based cell walls. Bacteria can be divided into two major groups: Gram positive and Gram negative, based on the Gram stain reaction. Gram-positive organisms have a thick cell wall, together with teichoic acids. Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins. Contribute!Did you have an idea for improving this content? We’d love your input. Improve this pageLearn More |