Archaea and viruses likely had a relationship as early as 2 billion years before present day, some researchers positing that co-evolution may have been occurring between these groups at such an early time. It has been further suggested that the last common ancestor of the bacteria and archaea was a thermophile, raising the likelihood that low temperatures are really the extreme environments viewed from an ancestral archaea point of view, and organisms that can tolerate cold conditions appeared only later in evolutionary time.
It was not until that archaea were recognized as a separate domain of prokaryotes through the work of Woese and Fox. Until the chief techniques of distinguishing microorganisms were use of morphology and metabolic functions. Woese and Fox culminated a research direction begun by a number of researchers started in the early s, in which gene coding of DNA material was viewed as a more fundamental technique for organism relatedness.
By the close of the 20th century, an enhanced understanding of the significance and ubiquity of archaea arose by using the polymerase chain reaction to detect prokaryotes in samples of water or soil based solely upon their nucleic acid. The greatest remaining puzzle is whether to acknowledge species within the domain of archaea. While morphological and DNA findings support the recognition of species, it is not clear that significant gene transfer is prohibited, thereby annihilating the validity of species.
In any case, in the present treatment we shall allow the attribution of species, if for no other reason than to follow published research designations and for simplicity of naming. Archaea and bacteria are superficially similar in size and shape, although some archaea species have remarkable geometric shapes, such as the flat and square-shaped cells of some genus Haloquadra members.
Despite this visual similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes: notably the enzymes involved in gene transcription and translation. Other aspects of archaean biochemistry are unique, such as the occurrence on ether lipids within their cell membranes. As with bacteria, archaea have no interior membranes or organelles. Cell membranes are typically bounded by a cell wall and motility is achieved using one or more flagellar tail structures.
Archaea most resemble gram-positive bacteria. Most archaea exhibit a single plasma membrane and cell wall, lacking a periplasmic space; however, Ignicoccus manifests a notably large periplasm with membrane-bound vesicles , enclosed by an outer membrane. Certain archaea aggregate to yield filaments of cells as long as nanometers—such forms are prominent in biofilms. Thermococcus coalescens, on the other hand, have cells that can fuse in culturing to produce monster single cells.
Genus Pyrodictium archaea form an elaborate multicell colony manifesting arrays of slender elongated hollow tubes termed cannulae that protrude from the cellular surface and connect into a dense agglomeration; this protruding form appears to encourage connection or nutrient exchange with neighboring cells of the same genus. Crenarchaeota exhibit a diverse set of geometries: irregularly shaped lobed cells, needle-like filaments that are less than nanometers in cross-section and amazing rectangular rods.
These odd morphologies are likely produced both by their cell walls as well as a prokaryotic cytoskeleton. Proteins associated with cytoskeleton elements of other organisms exist in archaea. Archaeal flagella function like their bacterial counterparts, with elongated stalks driven by rotatory motors at the base. The motors themselves are powered by the electrochemical gradient across the membrane. However, archaeal and bacterial flagella came from different ancestors.
The bacterial flagellum is hollow and is assembled by subunits moving up the central pore to the tip of the flagella, while archaeal flagella are constructed from addition of subunits at the base. The membranes of Archaea are constructed from molecules unlike those in other life forms; this morphology demonstrates the ancestral distance from bacteria and eukaryotes. For every organism, cell membranes are made of phospholipid molecules. These phospholipids exhibit a polar part that dissolves in water a phosphate head , and a hydrophobic non-polar part a lipid tail that is water insoluble.
These dissimilar ends are connected by a glycerol group. In water, phospholipids aggregate, with heads facing the water and tails facing the opposite direction.
The principal structure in cell membranes is a dual layer of phospholipids, often termed a lipid bilayer. In the case of bacteria and eukaryotes, membranes consist chiefly of glycol-ester lipids, but archaea have membranes made of glycerol-ether lipids. Ether bonds are chemically more stable than ester bonds, assisting archaea in survival at extreme temperatures and extreme pH environments. Prokaryotic Diversity. Search for:. 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.
Think about It What types of environments do Crenarchaeota prefer? Think about It Where do Halobacteria live? Finding a Link Between Archaea and Disease Archaea are not known to cause any disease in humans, animals, plants, bacteria, or in other archaea. Key Concepts and Summary Archaea are unicellular, prokaryotic microorganisms that differ from bacteria in their genetics, biochemistry, and ecology.
Some archaea are extremophiles, living in environments with extremely high or low temperatures, or extreme salinity. Only archaea are known to produce methane. Methane-producing archaea are called methanogens. Halophilic archaea prefer a concentration of salt close to saturation and perform photosynthesis using bacteriorhodopsin. Some archaea, based on fossil evidence, are among the oldest organisms on earth. Archaea do not live in great numbers in human microbiomes and are not known to cause disease.
Archaea and Bacteria are most similar in terms of their unicellular structure. Show Answer Answer b. They live in the most extreme environments. Show Answer Sulfolobus is a genus of Archaea. Show Answer Methanobrevibacter oralis was once thought to be the cause of periodontal disease, but, more recently, the causal relationship between this archaean and the disease was not confirmed.
Think about It What accounts for the purple color in salt ponds inhabited by halophilic archaea? What evidence supports the hypothesis that some archaea live on Mars? What is the connection between this methane bog and archaea? Blochl et al. Brock et al. Pacheco et al. Accessed April 7, Archaea typically have a single circular chromosome. The two daughter chromosomes are then separated and the cell divides. This process in Archaea appears to be similar to both bacterial and eukaryotic systems.
The circular chromosomes contain multiple origins of replication, using DNA polymerases that resemble eukaryotic enzymes. However, the proteins involved that direct cell division are similar to those of bacterial systems. DNA replication, similar in all systems, involves initiation, elongation, and termination.
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