Euglenida

from Wikipedia, the free encyclopedia
Euglenida
Euglena viridis

Euglena viridis

Systematics
Domain : Eukaryotes (eukaryota)
without rank: Excavata
without rank: Discoba
without rank: Discicristata
without rank: Euglenozoa
without rank: Euglenida
Scientific name
Euglenida
Stein , 1878

The Euglenida form a class of eukaryotic unicellular organisms , with around 1000 known species, which are found worldwide and in almost every habitat . The best-known genus are the eye animals ( Euglena ). They combine animal and plant properties.

Distribution and way of life

Much of the Euglenida live in shallow fresh water, which is rich in organic matter such as dead animals and plants. Some species live in the sea (e.g. Eutreptia viridis ) or brackish water. They also colonize extreme locations such as snow and salt lakes. Reports of parasitic representatives of the Euglenida could never be finally confirmed and are therefore considered dubious.

Under certain conditions, some representatives, for example Euglena sanguinea , occur in such large quantities that they color a pool of water red or green. One then speaks of an algal bloom .

The green color of many species is caused by chlorophylls (chlorophyll a and b) of the chloroplasts , with the help of which they carry out photosynthesis ( phototrophic nutrition ). Photosynthesis products are stored as paramylon grains.

The propagation takes place by longitudinal division . The entire process takes about 2 to 4 hours. Sexual reproduction is not known in Euglenida, it is possible that they separated from the main Protist tribe before sexual reproduction was developed.

nutrition

The diet of the Euglenida is very diverse and depends in part on special cellular structures.

The phototrophic Euglenida have functional chloroplasts with the chlorophylls a and b and photosynthesis. However, these green euglenida are not completely photoautotrophic either, since they too are dependent on the absorption of dissolved organic compounds, such as vitamins, from the surrounding medium. This pinocytotic uptake of dissolved substances is called osmotrophy. Since the green Euglenida feed phototrophically and osmotrophically, they are also called mixotrophic.

The heterotrophic euglenida do not have functional chloroplasts and are therefore exclusively dependent on the absorption of organic substances. Within the heterotrophic euglenida, two groups can be distinguished: The phagotrophic euglenida have special ingestion devices with whose help they can ingest larger prey organisms such as other unicellular organisms. These food particles are then digested. The phagotrophic Euglenida include, for example, representatives of the genera Peranema , Entosiphon or Petalomonas . But even these organisms do not feed exclusively phagotrophically, but also osmotrophically. Organisms that feed exclusively osmotrophically, i.e. neither photosynthesize nor have specialized ingestion apparatus, are referred to as osmotrophic euglenoids. Primarily osmotrophic euglenida descend from phagotrophic ancestors. These include, for example, representatives of the genus Distigma . Secondary osmotrophic euglenida can clearly be traced back to phototrophic ancestors. In the course of evolution, these organisms lost the ability to photosynthesize. A classic example of this is Euglena longa (formerly Astasia longa ). In this heterotrophic organism, rudimentary plastids were found that contain remnants of their DNA but no longer photosynthesize.

construction

Euglena sp. with spirally striped cell envelope (pellicula)

Euglenida have elongated cells with a helical structure. They usually have one or two flagella that they can swim with. But they can also crawl on surfaces.

The Euglenida do not have a cell wall , but a so-called pellicula (lat. Fellchen), which consists of a cell membrane and a protein-containing layer underneath, which can vary in thickness depending on the species. Below there are microtubules as cytoskeletal elements . As a result, their surface is not even; at a higher resolution, spiral stripes can be seen under the light microscope.

At the front end of the cell, the pellicle turns into a channel complex, which is also known as an ampoule and is used to absorb nutrients (cell mouth). In their vicinity there is a pulsating bubble (contractile vacuole) that pumps out excess water that was absorbed by osmosis .

The flagella arise at the front of the cell and run through the reservoir / channel complex. There are three flagella roots, so-called basal bodies , but only one or two of the flagella protrude from the ampoule. In the species with only one protruding flagella, the second flagella is shortened within the canal complex. On the flagella are hair-like protein structures, which are also called mastigonemata .

Secondary endosymbiosis

The question of the origin of the chloroplasts of phototrophic euglenida was long controversial. From cytological and molecular biological findings the following hypothesis could be made: Initially, the taxon was composed exclusively of phagotrophic species. A representative of this group "ate" a green alga one day, which was not digested, but remained in him. Also in the further course of development, during the reproduction of the euglenida through cell division, this green alga was preserved in that it also reproduced and its offspring lived on in the daughter individuals. Eventually, the green algae lost some of their independence and were established as the cell's own chloroplasts . This process is known as secondary endocytobiosis because, according to the endosymbiotic theory, the green algae themselves are said to have originated through endosymbiosis: Endosymbiotic cyanobacteria have therefore developed into chloroplasts in host cells by giving up part of their autonomy. The endosymbiosis of the green algae in Euglenida is therefore a second stage of endosymbiosis. Many indications can be found for this assumption: The chloroplasts of the phototrophic eye animals are surrounded by three covering membranes and not by two membranes as in green algae and plants. Molecular biological studies also show a close relationship between the chloroplast genes and the corresponding genes in green algae. The Euglenida themselves, represented by their cell nuclei, are clearly related to the other, non-phototrophic representatives of the Euglenozoa. After photosynthesis was established within the group, some of these phototrophic species reduced their chloroplasts again or lost them completely. These relationships can only be reconstructed through systematic molecular studies.

Systematics

Phacus suecicus (middle)

Together with the Diplonemida , the Kinetoplastida and the Symbiontida , the Euglenida are grouped together in the Euglenozoa .

Within the Euglenida, a split took place in the course of evolution: One group includes the phototrophic and those heterotrophic taxa that have lost their chloroplasts again. Together with Peranema trichophorum , a species that can ingest eukaryotic cells by phagocytosis , the taxa of this group form a monophylum .

The evolution of the second group proceeded independently of this. It is formed from primarily heterotrophic representatives. The species within this monophylum show no evidence of secondary loss of plastids .

The group is divided as follows:

swell

Individual evidence

  1. ↑ Huber-Pestalozzi, G. (1955): The phytoplankton of freshwater: Euglenophyceen, Volume 4
  2. ↑ Michajlow, W. (1972): Euglenoidina parasitic in Copepoda. PWN-Polish Scientific Publishers.
  3. ^ Peter H. Raven, Ray F. Evert, Susan E. Eichhorn: Biologie der Pflanzen , 3rd edition, 2000, ISBN 3-11-015462-5 , p. 376.
  4. ^ A b Leedale GF (1967) Euglenoid Flagellates. Prentice-Hall, Inc., Englewood Cliffs, New Jersey.
  5. ↑ Farmer MA (1988) A re-evaluation of the taxonomy of the Euglenophyceae based on ultrastructural characteristics. (Dissertation) Rutgers University, Piscataway, New Jersey
  6. ↑ a b Busse, I. & Preisfeld, A. 2003. Systematics of primary osmotrophic euglenids: A molecular approach to the phylogeny of Distigma and Astasia (Euglenozoa). Int. J. Syst. Evol. Microbiol. 53 (Pt 2): 617-24
  7. ↑ Gockel, G. & Hachtel, W. (2000): Complete gene map of the plastid genome of the nonphotosynthetic euglenoid flagellate Astasia longa. Protist 151 (4): 347-351.
  8. ↑ a b Adl, SM, Simpson, AGB, Lane, CE, Lukeš, J., Bass, D., Bowser, SS, Brown, MW, Burki, F., Dunthorn, M., Hampl, V., Heiss, A., Hoppenrath, M., Lara, E., le Gall, L., Lynn, DH, McManus, H., Mitchell, EAD, Mozley-Stanridge, SE, Parfrey, LW, Pawlowski, J., Rueckert, S. ., Shadwick, L., Schoch, CL, Smirnov, A. and Spiegel, FW: The Revised Classification of Eukaryotes. Journal of Eukaryotic Microbiology , 59: 429-514, 2012, The Revised Classification of Eukaryotes PDF Online
  9. ↑ Gommers-Ampt JH, Van Leeuwen F, de Beer AL, Vliegenthart JF, Dizdaroglu M, Kowalak JA, Crain PF, Borst P (1993): Beta-D-glucosyl-hydroxymethyluracil: a novel modified base present in the DNA of the parasitic protozoan T. brucei. Cell 75: pp. 1129-1136

Web links

Commons : Euglenida  - collection of images, videos and audio files