The expression (from ancient Greek πρό pro 'before' and κάρυον karyon 'core') refers to the seedlessness. The term Prokarya is used less often, the name Monera is out of date.
The division of living beings into prokaryotes and eukaryotes was first clearly emphasized by Edouard Chatton in 1925 for unicellular forms of life when distinguishing between protists . The more recent division of cellular organisms into three domains corresponds to the division of prokaryotes into the two domains bacteria and archaea . The third domain are the eukaryotes.
Characteristics compared to eukaryotes
In their cells ( eucytes ), eukaryotes have a “real” nucleus , separated from the surrounding cytoplasm by a double membrane , in which the DNA is organized in chromatin . In prokaryotic cells ( protocytes ), however, the DNA is freely located in the cytoplasm.
- The DNA of the prokaryotes usually consists of a single, double-stranded, densely structured molecule that is self-contained, i.e. has no ends and is called a bacterial chromosome . The DNA molecule is arranged in a small area, which is therefore also referred to as the core equivalent or nucleoid. Except for certain archaea ( Euryarchaeota and Proteoarchaeota ), the DNA is not associated with histone proteins , as is the case with the nuclear DNA in the chromosomes of eukaryotic organisms. In the case of Escherichia coli and other bacteria examined afterwards, the DNA double strand is an approximately 1 mm long, self-contained molecule, which is why one speaks of a ring DNA.
- Only some prokaryotes contain linear, i.e. double-ended DNA strands (e.g. Borrelia ).
- Some bacteria also have smaller double-stranded, closed, or linear DNA molecules called plasmids .
In contrast to eukaryotes, prokaryotes do not contain any membrane-bounded organelles such as plastids and mitochondria , nor do they contain dictyosomes , centrioles and mitotic spindles. They also have no vacuoles and no endoplasmic reticulum (ER).
Size and complexity
The size of prokaryotes (with elongated ones the diameter) is between 0.2 and 700 µm ( Thiomargarita namibiensis about 700 µm).
The shapes are not very complex, except for myxobacteria . The structure of the cell envelope, on the other hand, is complex, sometimes with a second cell membrane . Murein (a compound made up of sugars and amino acids ) is only found in the cell walls of bacterial cells .
Metabolism and physiology
Metabolic performance of the prokaryotes Anoxygenic photosynthesis available Oxygen photosynthesis available Aerobic breathing available fermentation available Anaerobic breathing available Nitrogen fixation available Chemolithotrophy available
The physiological diversity is very high. Some prokaryotes are viable under extreme conditions : temperature range up to over 100 ° C; oxic or anoxic environment; acidic environment (pH 1-4); high hydrostatic pressures (1000 bar).
The multiplication takes place as simple asexual multiplication , mostly by division into two.
Record of prokaryote species
There are very different opinions about the number of all prokaryote species: Some assume that only around 1% of prokaryote species ( taxonomically distinguishable types) have been discovered and described, around 99% are still unknown. Others estimate the number of prokaryote species present on earth at 10 8 to 10 17 , which is only a fraction of a percentage of known species. The fact is, however, that around 500 to 800 prokaryote species are newly discovered and described each year.
The respective type strain (the cultivated prokaryote strain on which the new description was based) must be stored in at least two strain collections and thus made accessible to other scientists. In Germany this is possible with the Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH ( DSMZ ), in France with the Institut Pasteur , in Belgium with the BCCM (Belgian coordinated collections of microorganisms).
The reason that so far only such a small proportion of species has been recorded is that traditionally the organisms are described using a culture , but that culture methods are only known for a small part of the prokaryote species. Nucleic acids of the prokaryotes located therein can be obtained from habitats and characterized. Based on the data obtained in this way, the prokaryotic society in it can be characterized and the number of species it contains can be estimated. From the results of these investigations in comparison with the results of cultural investigations, one concludes that there is a very large proportion of prokaryote species that were previously not cultivable in the prokaryote societies of natural habitats. In addition, many habitats are hardly accessible for investigations: Prokaryotes have been detected up to 77 km in height in the atmosphere and up to 4 km deep in the earth's crust , i.e. far below the ground . More than half of all prokaryotes live in the soil between 10 and 100 m depth (also underwater), although this depth is difficult to access.
Total amount and distribution on earth
The number of prokaryotes on earth can only be estimated from a multitude of data on different areas of life on earth. According to Whitman et al. (1998) there are said to be 4 - 6 · 10 30 prokaryotic cells on earth that contain 3.5 - 5.5 · 10 17 g carbon, 0.85 - 1.3 · 10 17 g nitrogen and 0.09 - 0, 14 · 10 17 g of phosphorus. For comparison: the carbon content of all plants on earth is 1.0 to 1.7 times, their nitrogen and phosphorus content only about a tenth. The number of prokaryotic cells is distributed over four large areas of life on earth as indicated in the table.
|habitat||Number of prokaryote cells|
|Water bodies and water sediments up to 10 cm below the sediment surface||0.12 x 10 30|
|Ocean sediments deeper than 10 cm below the sediment surface||3.5 · 10 30|
|terrestrial area up to 8 m below the earth's surface||0.26 x 10 30|
|terrestrial subsurface deeper than 8 m below the earth's surface||0.25 - 2.5 · 10 30|
|Total earth||4 - 6 · 10 30|
The number of prokaryotes in animals, plants, polar ice and the atmosphere is so small that it can be neglected for the total number on earth. The vast majority of prokaryotes are found in the aquatic and terrestrial subsurface.
It is estimated that around 1.7 · 10 30 cells are newly formed each year . This value appears low with regard to the total number of cells, it can be explained by the fact that the turnover time at terrestrial depths is estimated at 1000–2000 years.
Since the dry matter of prokaryotes contains on average about 50% carbon, it can be assumed that the 3.5-5.5 · 10 17 g carbon of all prokaryotes corresponds to about 7-11 · 10 17 g dry matter. With a water content of about 80% in microorganisms, this results in a total mass of prokaryotes of 3.5-5.5 · 10 18 g (3.5-5.5 billion tons).
The prokaryotes are the earliest known cellular creatures in evolutionary history . The study of prokaryotes in water bodies and in the earth, including the sea and sea floors, is one of the research areas of geobiology and geomicrobiology .
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