Related topics: bacteria

Virus discovery offers clues about origins of complex life

The first discovery of viruses infecting a group of microbes that may include the ancestors of all complex life has been found, researchers at The University of Texas at Austin report in Nature Microbiology. The discovery ...

Methanogenic microbes not always limited to methane

A study led by microbiologists at TU Dresden shows that methanogenic archaea do not always need to form methane to survive. It is possible to bypass methanogenesis with the seemingly simpler and more environmentally friendly ...

First overview of archaea in vertebrates 

Archaea are often mistaken as bacteria, given that both are small, single-cell organisms. However, archaea are as genetically different from bacteria as humans are from bacteria. While archaea are found in most environments, ...

Crucial step identified in the conversion of biomass to methane

Microbial production of methane from organic material is an essential process in the global carbon cycle and an important source of renewable energy. This natural process is based on a cooperative interaction between different ...

Wired for efficiency: How methanogenic microbes manage electrons

Methanogenic archaea use sophisticated enzyme systems to live in energy-limited anoxic environments. A key mechanism for saving energy is electron bifurcation, a reaction that 'splits' the energy of a pair of electrons, making ...

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The Archaea (/ɑrˈkiːə/ ( listen) ar-kee) are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon (sometimes spelled "archeon"). They have no cell nucleus or any other membrane-bound organelles within their cells.

In the past they had been classed with bacteria as prokaryotes (or Kingdom Monera) and named archaebacteria, but this classification is regarded as outdated. In fact, the Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, and so they are now classified as a separate domain in the three-domain system. In this system, the phylogenetically distinct branches of evolutionary descent are the Archaea, Bacteria and Eukaryota.

Archaea are divided into four recognized phyla, but many more phyla may exist. Of these groups, the Crenarchaeota and the Euryarchaeota are the most intensively studied. Classification is still difficult, because the vast majority have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in samples from the environment.

Archaea and bacteria are quite similar in size and shape, although a few archaea have very unusual shapes, such as the flat and square-shaped cells of Haloquadratum walsbyi. 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 transcription and translation. Other aspects of archaean biochemistry are unique, such as their reliance on ether lipids in their cell membranes. Archaea use a much greater variety of sources of energy than eukaryotes: ranging from familiar organic compounds such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon; however, unlike plants and cyanobacteria, no species of archaea is known to do both. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species form spores.

Initially, archaea were seen as extremophiles that lived in harsh environments, such as hot springs and salt lakes, but they have since been found in a broad range of habitats, including soils, oceans, marshlands and the human colon. Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are now recognized as a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle. No clear examples of archaeal pathogens or parasites are known, but they are often mutualists or commensals. One example is the methanogens that inhabit the gut of humans and ruminants, where their vast numbers aid digestion. Methanogens are used in biogas production and sewage treatment, and enzymes from extremophile archaea that can endure high temperatures and organic solvents are exploited in biotechnology.

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