Archaebacteria

Archaebacteria are primitive, one-celled life forms without a distinct nucleus, different from bacteria in their genetic components. They have been found to be genetically unique and are probably one of the earth's earliest life-forms.

Early Life Forms

The nature of the earth's earliest life forms has always been an intriguing question for the earth sciences (paleontology) and biology. The fossil record shows that one-celled organisms are very ancient, their oldest-known fossils being almost 3.5 billion years old. The long fossil record of prokaryotes consists of preserved fossil cells and distinctive layered mineral structures called stromatolites and microbialites. These structures were produced from the cell metabolisms of colonies of prokaryotes. From at least 3.5 billion years ago to around one billion years ago, microscopic prokaryotes were the earth's only organisms. As they do today, they included a diversity of forms commensurate with their long evolutionary history. The appearance of the eukaryotic cell (more than one billion years ago) ushered in the age of multicellular organisms (metazoans and metaphytes) some 700 million years ago, and these have become the dominant life forms on the earth. What the much earlier prokaryotic organisms were like and which type of one-celled organisms produced the microbialites is unclear. Usually, these oldest fossils are attributed to the life activities of photosynthetic organisms, particularly the cyanobacteria, referred to in many works as the blue-green algae. Several other types of one-celled life forms could have been responsible for some of them, particularly the photosynthetic bacteria and possibly the archaebacteria.

Molecular biologists, utilizing ribonucleic acid (RNA) nucleotide sequencing and other biochemical methods, believe that the nature of early life-forms can be discovered. RNA nucleotide sequences of amino acids can be regarded as a sort of chemical “historical document” that is capable of being “read.” The closer two RNA nucleotide sequences are to each other, the smaller the evolutionary distance between them, and the more recently, in geologic time, they separated from each other. The further one nucleotide RNA sequence is from another, the greater is the evolutionary distance that separates the two organisms. The utilization of nucleotide sequencing can thus produce an evolutionary tree, or phylogeny, for even the most primitive of organisms, and therefore, it becomes possible to determine which organisms out of the great variety of primitive life forms currently living were some of the first to appear. Through information obtained by such sequencing, archaebacteria have been recognized by molecular biologists as some of the most primitive and biochemically unique of organisms. Archaebacteria RNA sequences turn out to be distinctly different from those of other bacteria, even though the various organisms that comprise archaebacteria look like and they were previously placed with the bacteria.

Archaebacteria is a polyphyletic group that once included species from two domains, Archaea and Bacteria. There are now considered to be three domains: Archaea, Bacteria, and Eucaryota. Although there are similarities in cell structure and function among Bacteria and Archaea, and some trees group the Archaea and Bacteria, the relationship is currently unclear. In their tolerance of extreme ecological conditions and their metabolism, the archaebacteria differ from all other monerans, a condition that has led biologists to consider these organisms as particularly well suited to the adverse conditions of the early earth. The very earliest eras of geologic time may well have been the age of archaebacteria.

Molecular Characteristics

On a fundamental molecular level, archaebacteria are different in their biochemistry from the other prokaryotes. Nucleotide RNA sequences and other biochemical differences that exist between the various types of eubacteria (bacteria exclusive of the archaebacteria) are minor when compared with the differences between eubacteria and archaebacteria. Nongenetic differences include such features as cell walls, those of all eubacteria being composed of a complex polymer called peptidoglycan, which is a sugar derivative. In contrast, the cell walls of the various types of archaebacteria are composed of a variety of other materials, none of which is peptidoglycan. The lipids (fats) in archaebacteria cells are also fundamentally distinct from the lipids in the cells of eubacteria and eukaryotes. Ribosomal RNA is what ultimately distinguishes the archaebacteria, for it is markedly different in its sequences of bases from any eubacteria. In higher eukaryotic organisms, where the fossil record is good, greater RNA ribosomal differences exist between those organisms that are separated by long periods of geologic time than between those that are separated by shorter periods. Ribosomal RNA differences and the biochemical differences that exist between the eubacteria and the archaebacteria suggest that an evolutionary distance of great magnitude separates them.

The eukaryotic cell has long been observed to be a sort of combination between prokaryotic-type cells, functioning within the cell as chloroplasts and mitochondria, and another cell type that “ingested” the prokaryotes and incorporated them to become a more complex entity in symbiotic collaboration. Archaebacteria have some genetic characteristics that suggest a link with the eukaryotes, which has led some scientists to propose a predecessor to them both: The “other” cell type that linked up with prokaryotes underwent substantial further evolution and eventually became the eukaryotic cell. The pre-eukaryotic other cell type is known as a urkaryote. All three cell types—the prokaryote, the urkaryote, and the eukaryote—are hypothesized to have arisen from a common cell ancestor, the progenote. The progenote may have been biochemically simpler than any of the three fundamental life forms that arose from it, an event that might have taken place during the first one billion years of Earth's history.

Thermoacidophiles

Archaebacteria are represented by three classes: the thermoacidophiles, the extreme halophiles, and the methanogens. The thermoacidophiles occupy hot, acid environments, often rich in metallic ions and in sulfur compounds, such as hot springs and fumaroles. These organisms are viable under the acidic hot conditions are intolerable to other life forms, with temperatures as high as 75 degrees Celsius. The extreme halophiles, or halobacteria, live only in extremely salty environments. The methanogens are anaerobes that metabolize organic material to form methane; they were the first of the archaebacteria to be discovered.

The igneous activity of various sorts appears from the geologic record to have been much more intensive and widespread on the early earth (between two and four billion years ago) than during more recent geologic times or at present. A terrestrial geologic record for the first one billion years of the earth's history is unknown, as the widespread igneous and tectonic activity during this time seems to have destroyed the evidence; an actual record begins nearly four billion years ago. During the next 1.5 billion years (the Archean eon), igneous phenomena and massive tectonism were still dominant. Hot spring and fumarolic activity would have been more commonplace during these early times than during later geologic times, and these environments favor the thermoacidophiles. Although the fossil and sedimentation record of the Archean eon does not negate the possibility that archaebacteria were some of the most widespread and dominant life forms of that time, a determination that a particular organic—or presumed organic—structure of the early earth was produced by archaebacteria or by some other moneran is quite difficult and may well be impossible. Many puzzling structures, seemingly of biogenic origin, have been reported from Archean strata. Some stromatolite-like or microbialite-like structures are associated with what appear to be hot spring deposits. These structures may well represent minerals deposited as a consequence of life activities of thermoacidophiles associated with geothermal activity. Like so many stromatolite-like and microbialite-like structures, unequivocal proof of their biogenic origin is difficult to obtain. Structures similar to them, however, are produced today in hot springs, in the hottest waters of which live communities of thermoacidophile archaebacteria.

Sometimes, hot spring deposits and structures contain carbon-rich sediments or graphite. In addition, some hydrothermal veins of various ages have associated carbonaceous or graphitic material. Such material may have originated from thermophyllic archaebacteria. Stratified metallic element deposits are known from Archean strata, some of which have been thought to be of biogenic (possibly archaebacterial) origin. Some deposits yield microbialites exhibiting distinctive dome, finger, or layered structures containing metallic oxides or carbonates. Today, such structures are produced by the cyanobacteria and photosynthetic bacteria, but these younger stromatolites lack components like the oxidized metals. Other Archean stromatolites or microbialites and stromatolite-like structures associated with geothermally active environments have a distinctive “signature” different from later forms, and their origin by thermoacidophile archaebacteria cannot be ruled out.

Extreme Halophiles

The second group of archaebacteria, the extreme halophiles, requires an intensely saline environment. Shallow, marginal marine areas, evaporite basins, and salt flats are the niches in which these organisms generally flourish. Physiologically, the extreme halophiles are photosynthetic; however, the photosynthetic pigment is not chlorophyll but rather a light-sensitive red pigment, bacterial rhodopsin. The cell walls of the extreme halophiles differ from those of other bacteria in the presence of compounds that prevent the destruction of the walls in the high salt concentration conditions under which they live. The chemical similarities of ribosomes and lipids of the extreme halophiles and the methanogens suggest a common origin.

Again, the fossil record of these organisms is difficult to interpret; some biologists suggest that the halophiles were more prevalent early in the earth's history than they are today. Fossil rod-shaped bacterial cells have been found as far back as the mid-Archean (3.2 billion years ago); however, as the gross morphology of archaebacteria differs little from that of eubacteria, the evidence remains inconclusive.

Peculiar and distinct microbialites of Archean age associated with radial sprays of gypsum crystals were described from western Ontario in the 1910s by Charles Doolittle Walcott. Walcott, a pioneer North American paleontologist who concentrated on the early (Precambrian and Cambrian) fossil record, made many finds of peculiar structures resembling fossils in Precambrian strata, many of which remain a mystery. Walcott thought that the radiating gypsum crystals were the rays, or spicules, of a type of spongelike organism he called atikokania. Associated with Walcott's atikokania are distinctive microbialites that contain “lenses” of gypsum that almost certainly originated in a very saline environment. These microbialites could represent the product of physiological activity of the extreme halophiles when, during the process of photosynthesis, they locally removed carbonic acid from the saline water. The white lenses that characterize these distinctive microbialites are gypsum fillings between the black calcium carbonate bands, possibly precipitated by photosynthesis of halophilic archaebacteria.

Methanogens

Methanogens produce their metabolic energy by breaking down organic compounds incorporated into sediments or reducing carbon dioxide in the presence of elemental hydrogen, releasing methane. Without the methanogens, organic carbon would eventually become incorporated into the sediments of the earth's crust, where it would accumulate and could not be recycled back into the biosphere. The methanogens facilitate this carbon recycling. Methanogens, like the other archaebacteria, have biochemical features distinct from all other bacteria, suggesting that they evolved separately. Like the other archaebacteria, methanogens differ from other prokaryotes in the sequences of nucleotides that make up the RNA in their ribosomes and protein. However, fossil methanogens are more difficult to distinguish within the geologic record than other archaebacteria, as they leave no distinctive chemical “footprint,” as the others can. The abundance of black, carbon-rich sediments in strata of the Archean eon suggests that the oxygen-free, anaerobic environment where the methanogens flourish was commonplace during that time. The methanogens' biochemical uniqueness, and thus presumed great geologic age, along with the anaerobic Archean Earth environment, suggests that they may have been a dominant part of the Archean biosphere and not restricted as they are today.

Principal Terms

eubacteria: minute, prokaryotic organisms that inhabit a range of habitats considerably greater in diversity than those occupied by other organisms; exclusive of archaebacteria, they constitute the majority of monerans

eukaryotic cell: the cell type present in all animals, plants, fungi, and protists; each cell has a distinct nucleus and mitochondria, chloroplasts, and other subcellular structures absent in prokaryotic cells

microbialite: a biogenic sedimentary structure that is found fossilized in sedimentary rock strata of various ages and that is attributed to the life activities of monerans

monerans: generally, single-celled organisms that often grow in colonies and that are made of prokaryotic cells

prokaryotic cell: the cell type found in the kingdom monera, characterized by several criteria, including the absence of a cell nucleus, mitochondria, and chloroplasts

ribosome: a large multienzyme complex made up of protein and ribonucleic acid (RNA) molecules that carry and process information stored in deoxyribonucleic acid (DNA); this information is also carried by RNA to synthesize proteins

stromatolite: a biogenic sedimentary layered structure produced by sediment trapping, binding, or precipitation as a result of the photosynthesis of microorganisms, principally cyanobacteria (blue-green algae)

Bibliography

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