Archaea

Archaea or Archaebacteria, common name for a group of one-celled organisms, many of which do not require oxygen or sunlight to live. Before the discovery of archaebacteria, scientists divided all living organisms into prokaryotes (organisms without a cellular nucleus), which consisted primarily of bacteria, and eukaryotes (organisms with a cellular nucleus), which consisted of fungi, plants, and animals. Archaebacteria were initially grouped with bacteria because like bacteria, they lack a well-defined nucleus. Recent evidence, however, has demonstrated that archaebacteria have a genetic makeup that more closely resembles the eukaryotes, organisms that have a well-defined nucleus. This unique structure means that archaebacteria cannot be accurately grouped with either the prokaryotes or the eukaryotes. Instead, scientists have proposed that these microorganisms be classified in a new branch of life, or domain, called archaea.

Archaebacteria often live in extreme conditions that were once considered inhospitable to life. Some archaebacteria live in deep-sea hydrothermal vents in the Pacific Ocean. Located at depths of 3 km (2 mi), the hot vents provide a dark environment with extremely high temperature and pressure where few creatures can survive. Instead of deriving energy from the sun, these microorganisms obtain energy by oxidizing inorganic chemicals that spew from the hot vents. In a process known as chemosynthesis, archaebacteria harvest energy from chemical reactions involving hydrogen sulfide and other inorganic compounds. These deep-sea archaebacteria make up the bottom of the food chain for clams, tube worms, mussels, and other animals that live near the vents (see Marine Life).

Researchers recently discovered archaebacteria that can fix nitrogen—that is, chemically combine nitrogen with hydrogen and oxygen to make ammonia or nitrates. Other types of living things can then use such nitrogen compounds to make amino acids and proteins. The ability to fix nitrogen was previously thought to be restricted to bacteria. The enzyme that archaebacteria use to fix nitrogen appears to be very ancient and may come from a life form that predates archaebacteria and true bacteria. Known nitrogen-fixing archaebacteria can survive in extreme high-temperature conditions such as deep-sea hydrothermal vents or geothermally heated sand.

Scientists initially found archaebacteria only in harsh environments, but recently these microorganisms have been found in the guts of animals, compost piles, saturated marshes, and other common places. Knowledge gleaned from studying a third branch of life could provide insight on the common ancestry of all living organisms.

Scientific classification: Archaebacteria are members of the domain archaea. The archaebacterium found near deep-sea hydrothermal vents is classified as Methanococcus jannaschii.