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Prokaryote is a kingdom, or division, in the classification scheme devised for all life on Earth. This kingdom, which is also designated as Monera, includes all bacteria and blue-green algae (which are also called Cyanobacteria). There are four other kingdoms in the classification system. The classification is based on the structure of a subunit of the ribosome. This criterion was selected because the structure of the subunit seems to have been maintained with little change throughout the millions of years that life has existed on Earth.

Besides the kingdom Prokaryotae, there are the Protista (eukaryotic organisms' organisms that have a nucleus enclosed in a well-defined membrane), Fungi , Animalia (eukaryotes organized into complex organisms), and Plantae.

The use of kingdoms in the classification of organisms arose with the work of Carolus Linneus who, in the mid-1700s, devised the system that is still used today. The Linnean system of classification has kingdoms as the highest level, with six other subdivisions down to the species level. Bacteria are divided into various genera. A group of bacteria derived from a single cell is called a strain. Closely related strains constitute a bacterial species. For example, the complete classification of the bacterium Escherichia coli is as follows:

  • Kingdom: Prokaryotae (Monera)
  • Division (also called Phylum): Gracilicutes
  • Class: Scotobacteria
  • Order: Enterobacteriales
  • Family: Enterobacteriaceae
  • Genus: Escherichia
  • Species: Escherichia coli

The Prokaryotae are further divided into two subkingdoms. These are called the Eubacteriobonta (which contains the so-called Eubacteria ) and the Archaebacteriobonta (which contains the so-called Archaebacteria). This split arose from the research of Carl Woese. He showed that the so-called 16 S ribosomal subunit of bacteria divide bacteria into two groups; the Eubacteria and the Archaeobcteria.

Archaebacteria are a very diverse group of bacteria and have several features that set them apart from the other Prokaryotae. Their cell walls lack a structure called the peptidoglycan , which is a rigid and stress-bearing network necessary for the survival of other bacteria. Archaebacteria live in extreme environments such as deep-sea vents, hot springs, and very salty water. Finally, some metabolic processes of Archaebacteria are different from other bacteria.

The feature that most distinguishes the bacteria and blue-green algal members of the Prokaryote from the members of the other kingdoms is the lack of membrane-bound structure around the genetic material. The genetic material, deoxyribonucleic acid (DNA ), is dispersed through the inside of the microorganism, a region that is typically referred to as the cytoplasm . In contrast, eukaryotic organisms have their genetic material compartmentalized inside a specialized membrane.

A second distinctive feature of the Prokaryotae concerns their method of reproduction. Most bacteria reproduce by growing and then splitting in two. This is called binary fission. Eukaryotic organisms have a more complex process that involves the replication of their differently organized genetic material and the subsequent migration of the material to specific regions of the cell.

Blue-green algae and some bacteria are able to manufacture their own food from sunlight through the process of photosynthesis . Green plants likewise have this capability. This type of bacteria are the photoautotrophs. Other bacteria are able to utilize elements like nitrogen, sulphur, hydrogen, or iron to make their food. This type of Prokaryote are the chemoautotrophs. But the bulk of the Prokaryotae exists by decomposing and using compounds made by other organisms. This decomposition is a vital process. Without this bacterial activity, the wastes of other organisms would blanket Earth.

The relative simplicity of the Prokaryotae, as compared to eukaryotes, extends to the genetic level. The prototypical bacterial species Escherichia coli contains approximately 5,000 genes. On average, about one in every 200 bacteria is likely to have a mutation in at least one of the genes. In a 100 ml culture containing one million bacteria per milliliter, this translates to 500,000 mutant bacteria. This ability of members of the Prokaryotae to mutant and so quickly adapt to a changing environment is the principle reason for their success through time.

The ecological distribution of the Prokaryotae is vast. Bacteria have adapted to live almost everywhere, in environments as diverse as the thermal deep-sea vents to the boiling hot springs of Yellowstone National Park, from the soil to the intestinal tract of man and animals. The diversity of bacteria led to the design of a classification system just for them. David Hendricks Bergey spearheaded this classification scheme in the first half of the twentieth century. His efforts culminated in the publication (and ongoing revisions) of the Bergey's Manual of Systematic Bacteriology.

See also Bacterial kingdoms; Evolutionary origin of bacteria and viruses