ARCHAEBACTERIUM

also spelled Archaeobacterium, plural Archaebacteria, or Archaeobacteria, any member of a group of bacteria that differ in certain morphological, physiological, and genetic features from the eubacteria. Archaebacteria do have some features in common with the eubacteria, however, suggesting to some researchers that the two groups shared a common ancestor early in their evolutionary history. The archaebacteria are aquatic or terrestrial microorganisms that exhibit a diversity of shapes, including spherical, rod-shaped, and spiral forms. Archaebacteria lack murein (ester lipids) in the cell walls, which is characteristic of eubacteria; instead, they have ether lipids, as well as a number of different cell-wall constituents. Archaebacteria also differ from eubacteria in the structure of their ribosomal RNAs, which are used in genetic testing to assess the degree of genetic relatedness among different species. The archaebacteria reproduce using a wide variety of mechanisms, including binary and multiple fission, budding, and fragmentation. Archaebacteria survive in a number of extreme environments, including very hot or saline ones. Archaebacteria may be aerobic, anaerobic, or facultatively anaerobic in their metabolic requirements. Some archaebacteria, such as Halobacterium, require a highly saline environment. Others, such as Methanobacterium, produce methane (CH4) as an end product, while still others are dependent on sulfur for their metabolism. The latter group are among the most thermophilic of the archaebacteria, surviving in temperatures higher than 45 to 50 C (113 to 122 F).