Cercomonas
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Cercomonas sp. |
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| Scientific name | ||||||||||||
| Cercomonas | ||||||||||||
| Dujardin |
Cercomonas is a genus of wild flagellates . They are the most species-rich and eponymous genus of the Cercomonadida and occur worldwide in waters and in the soil.
features
Light microscopic features
The representatives are flagellates, which form various forms of pseudopodia , including flat, thin, thread-like and branched. The cells are between four and 65 µm in size, but most species measure around 10 µm. The pseudopodia can arise at any point in the cell. Most species form flat or finger-like pseudopodia. However, some species also form reticulopodia (rhizopodia). The pseudopodia are all formed on the solid substrate or on the surface film of the water and are almost always in contact with the substrate.
They have two always present, differently striking flagella , which are also preserved in the amoeba stage . The front flagella is directed forward and strikes slowly with beating movements. The posterior flagella points backwards on the ventral side. It lies in a pit or inside a cytoplasmic canal, but it can also be exposed. Cells in motion usually have a pseudopodium at the rear end. The movement of the anterior flagella should not affect the sliding movement of the cell. The undulating movements of the posterior flagella should push the cell forward.
Sliding flagellates can quickly transform into the amoeboid form, which then no longer has an anterior-posterior alignment. In the amoeboid form, the cell is usually immobile and flattened. This is the form in which food is normally consumed.
In the mobile cells, the nucleus is located in the front area of the cell near the flagella. In some species, the microtobule cone can already be seen in the light microscope. The contractile vacuoles are usually not restricted to a specific area of the cell. In Cercomonas metabolicus , Cercomonas norrvicensis and Cercomonas ovatus , however, they are always found in the anterior cell area. The food vacuoles are in the central and rear areas of the cell.
Electron microscopic features
The cell surface consists of the plasmalemma , in some species this is covered by a thick glycocalyx . Each species has one or two types of extrusomes , trichocysts , osmiophilic granules, and microtoxicysts. The nucleus is located in the front area near the basal body . Microbodies are found all over the cell. The mitochondria are scattered throughout the cell, their cristae are usually flattened tubules.
The two basal bodies of the flagella are approximately at right angles to each other. They are connected to one another by fibrillar bridges. In the transition zone to the flagellum, the posterior basal body has a transverse plate in the area of the cell surface. The anterior basal body has a fibrous, striped spur. The anterior basal body has a proximal wheel-shaped structure, the posterior one does not. Associated with the basal bodies are two to four lateral microtubule roots. The most conspicuous part of the flagellum apparatus is a fibrillary root that starts from the anterior or both basal body (s) and extends to the cell nucleus, where it ends in a granule. From this, microtubule rays emanate, which reach back along the surface of the cell nucleus and form what is known as the microtubule cone.
Furthermore, the genus is defined by three characteristic sequence sections in the gene for the 18S rRNA.
Life cycle
Some species have a complex life cycle. It consists of vegetative, mononuclear cells, which can either be mobile flagellates or rather immobile amoebas, of multinucleated plasmodia and mononuclear cysts .
The plasmodia are flattened, have multiple nuclei, flagella and contractile vacuoles. A plasmodium arises from nuclear divisions without subsequent cell divisions or through the fusion of several mononuclear cells. The core number can reach 100 or more. The mature Plasmodium breaks down again into mononuclear cells.
Cysts are formed from the flagellate form and are mononuclear, spherical cells with - depending on the type - smooth or wrinkled surface.
The switch between cyst and flagellate and between flagellate and amoeba can take place in both directions. However, a plasmodium can only arise from amoeba and itself can only form flagellates. With the different types there are different strongly reduced forms of the complete life cycle.
Distribution and habitats
The genus is distributed worldwide. It is a common representative in rivers and lakes, as well as in fresh water reservoirs. They also occur in brackish water , as well as in a wide variety of soils. It is the second most common genus of zoo flagellates in soils. Although they are actually aerobes , they occur in all areas of wastewater treatment, including anoxic areas, but also in natural anaerobic habitats.
ecology
Cercomonas feeds primarily on bacteria . Ground-dwelling representatives are able to kill the much larger nematode larvae of the species Caenorhabditis elegans . The mechanism is unclear, but requires direct cell-to-cell contact. A perforation of the nematode cuticle could not be observed. Adhesion takes place through pseudopodia structures on the front part of the cell. The death of the nematodes occurs within a few hours. Cercomonas does not feed directly on the dead nematodes. There are three ways in which Cercomonas can benefit from the death of the nematodes: (a) by eliminating a direct competitor for the bacterial food, (b) eliminating potential predators , since adult C. elegans can eat Cercomonas , and (c ) the dead nematodes serve as a substrate for the growth of bacteria, which Cercomonas then feed on. The phytopathogenic nematode Heterodera schachtii were not killed.
Systematics
Cercomonas is the most species-rich genus of the Cercomonadida and belongs to the family Cercomonadidae within this group . The genera Dimastigamoeba, Prismatomonas, Reptomonas, Cercomastix, Cercobodo and Dimorpha are all assigned to Cercomonas today.
The genus was first described by Félix Dujardin in 1841, the type species is the Cercomonas longicauda, also first described by Dujardin . The genus name is therefore feminine, although several authors use species epithets with male endings, for example for species that were transferred from Cercobodo to Cercomonas .
According to Mylnikov and Karpov (2004), the genus consists of the following species
Cercomonas jutlandica wasdescribedas a separate genus Neocercomonas in2004, but was transferred to Cercomonas in 2006. Analysis of DNA samples from different habitats suggests that there are over 100 species.
In molecular genetic studies of the 18S rRNA sequences, Cercomonas breaks down into two clearly separated clades (A and B), whereby the two together may not form a monophyletic group. The two clades in turn split into two clearly separate subclades (1 and 2). Karpov et al. therefore divided Cercomonas into three genera in 2006 , although most of the known species could not yet be definitely assigned to any of the genera. However, you should remain in Cercomonas . The genus Neocercomonas described in 2004 was discarded again.
The simplified cladogram looks like this:
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supporting documents
- Alexander P. Mylnikov, Serguei A. Karpov: Review of diversity and taxonomy of cercomonads . Protistology, Vol. 3, 2004, pp. 201-217. ISSN 1680-0826
- Serguei A. Karpov, David Bass, Alexander P. Mylnikov, Thomas Cavalier-Smith : Molecular Phylogeny of Cercomonadidae and Kinetid Patterns of Cercomonas and Eocercomonas gen. Nov. (Cercomonadida, Cercozoa) . Protist, Volume 157, 2006, pp. 125-158, doi : 10.1016 / j.protis.2006.01.001
Individual evidence
- ↑ a b c Serguei A. Karpov, David Bass, Alexander P. Mylnikov, Thomas Cavalier-Smith: Molecular Phylogeny of Cercomonadidae and Kinetid Patterns of Cercomonas and Eocercomonas gen. Nov. (Cercomonadida, Cercozoa) . Protist, Volume 157, 2006, pp. 125-158, doi : 10.1016 / j.protis.2006.01.001
- ↑ Lisa Bjørnlund, Regin Rønn: 'David and Goliath' of the soil food web - Flagellates that kill nematodes . Soil Biology & Biochemistry, Volume 40, 2008, pp. 2032-2039, doi : 10.1016 / j.soilbio.2008.04.011
- ^ A b Alexander P. Mylnikov, Serguei A. Karpov: Review of diversity and taxonomy of cercomonads . Protistology, Vol. 3, 2004, pp. 201-217. ISSN 1680-0826
- ^ Félix Dujardin: Histoire Naturelle des Zoophytes Infusoires . Roret, Paris.
- ^ David Bass, Thomas Cavalier-Smith: Phylum-specific environmental DNA analysis reveals remarkably high global biodiversity of Cercozoa (Protozoa) . International Journal of Systematic and Evolutionary Microbiology, Volume 54, 2004, pp. 2393-2404, doi : 10.1099 / ijs.0.63229-0
