See Arthropods, Terrestrial Archaebacteria

Archaebacteria ("old bacteria") are one of two fundamentally different kinds of bacteria, the other being eubacteria ("true bacteria"). Archaebacteria, made of prokaryotic cells, are distinguished by several biochemical features that include characteristic 5S and 16S ribo-somal RNAs, lipids linked with ethers (R-O-R) rather than esters (R-C-OH), and a lack of peptidoglycan in their cell walls. They may be an ancient life form, little changed from their original ancestors, primarily because of the similarities among their long-chain RNA molecules; since these are found in the ribosomes of all organisms, they probably have not changed much over evolutionary time.

Although bacteria do not fossilize well and thus have historically proved difficult organisms to classify on the basis of evolutionary history, advances in molecular biology and its study of the macromolecules making up life have clarified the ancestral relationships among these organisms. In the latter part of the twentieth century, American biologist Carl Woese and German biologists Otto Kandler and Wolfram Zillig led the charge to reclassify living things based on this genetic revolution. It is not surprising to learn that, reminiscent of what are thought to be the earliest life forms, these bacteria contain some of the most resistant beings on earth, the so-called extremophiles. Many are able to withstand temperatures and conditions quite inhospitable to the majority of modern surface dwellers, including many other bacteria, all plants, animals, fungi, and protoctists. Among the archae-bacteria are the methanogenic bacteria that are poisoned by oxygen. Other archaebacteria are halophils, salt lovers that thrive in high concentrations of sodium chloride (salt like that prevailing in Great Salt Lake near Salt Lake City, Utah). Still others are thermoacidophils that are able not only to live but also to reproduce in boiling hot springs under sulfuric acid conditions. Some (Koreabacteria) are detected in moderate marine waters solely on the basis of their ribosomal RNA sequences. That the archaebacteria can grow in dry salty oceansides, in boiling muds, in and around erupting volcanoes, in Old Faithful at Yellowstone National Park, and under the water at ocean bottom in submarine vents pouring forth sul-fide-rich fumes speaks to their ancient heritage: although scarce and inhospitable to the majority of life forms today, such environments were no doubt much more common on the early earth during the Archean Eon. Geologist Jack Corliss argues that deep sea vents, full not only of sea creatures such as giant tube worms (pogonophorans) but also of methanogenic archaebacteria, are those ecosystems most like the environments of life during its beginnings, more than three and a half billion years ago. The chemistry of these zones is less dependent on sunlight and oxygen than modern surface ecosystems, thus agreeing with paleobiological reconstructions of the ancient planet.

The very name archaebacteria, in conjunction with an emphasis on the antiquity of these organisms (rather than on their molecular biological features), seems to argue against Woese and others who would divide life into three kingdoms: one eubacterial, another archae-bacterial, and the last composed of all the nucleated or eukaryotic organisms. Because of our vantage point as humans, we cannot help be the measure of all the things we observe, and thus, important as it is, molecular biological comparisons of organisms is not the only criterion for comprehensive classification. Recombination mode, presence or absence of embryos, of absorptive, heterotrophic, or photosynthetic nutrition, behavior, and morphology must also be taken into account. Thus in the five-kingdom classification system, archaebacteria are not given their own kingdom (or "domain," as Woese calls Archaea) but considered one of two great bacterial subkingdoms. We prefer the term Archaebacteria to Archaea, because the latter downplays the undeniable fact that these microscopic beings share the structural details, much of the physiology, and the genetics of the rest of the bacteria.

—Lynn Margulis and Dorion Sagan

See also: Bacteria; Benthos; Classification, Biological; Evolution; Evolutionary Genetics; Five Kingdoms of Nature; Microbiology; Oxygen, History of Presence in the Atmosphere Bibliography

Madigan, Michael T., John M. Martinko, and Jack Parker. 1997. "Prokaryotic Diversity: Archaea." In Biology of Microorganisms, edited by Thomas D. Brock, ch. 17. Englewood Cliffs, NJ: Prentice Hall; Margulis, Lynn, and Karlene Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3d ed. New York: W. H. Freeman.

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