Archaea << ahr KEE uh >>, sometimes called archaebacteria, are a group of single-celled organisms that make up one of three basic divisions of life. Scientists often call these basic divisions domains. The other two domains of living organisms are Eukaryota, which includes animals and plants, and Bacteria, which includes all true bacteria and the algaelike cyanobacteria. Eukaryotes have eukaryotic cells, or cells with a nucleus, while bacteria and archaea have prokaryotic cells, or cells that lack a nucleus. Scientists had traditionally classified archaea with bacteria because of their similar cell structures. But beginning in the 1970’s, close analyses of their genes revealed that archaea and bacteria are too different to be grouped together. In many respects, archaea more closely resemble eukaryotes than they do bacteria.
Archaea rank among the oldest forms of life on Earth. Some scientists believe these organisms are similar to the original ancestors of all modern life. Archaea have developed unusual properties. For example, various kinds of archaea can consume acetic acid, hydrogen, or sulfur. Some archaea can even produce the gas methane. Other kinds have developed an unusual form of photosynthesis, a process by which they make food. Unlike photosynthesis in plants and bacteria, archaeal photosynthesis does not use the green pigment chlorophyll.
Archaea live in a wide variety of habitats. Many live in such harsh conditions as oil wells, deep-sea hydrothermal vents (hot springs), and anaerobic (oxygen-free) environments. Other archaea grow in soils and within various living organisms. People have archaea living harmlessly in their digestive systems.
See also Cell (Inside a living cell); Extremophile; Prokaryote.
