Euglenid
Encyclopedia
Euglenoids are one of the best-known groups of flagellate
s, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Most euglenids are unicellular. Many euglenids have chloroplast
s and produce energy through photosynthesis
, but others feed by phagocytosis or strictly by diffusion. They belong to the phylum Euglenophyta, and their cell structure is typical of that group.
Euglenids are believed to descend from an ancestor that took up green algae
by secondary endosymbiosis
.
, which is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubule
s. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids the strips can slide past one another, causing an inching motion called metaboly. Otherwise they move using the flagella.
in 1884 as the flagellate order Euglenida. Botanists subsequently treated the algal division Euglenophyta; thus they were classified as both animals and plants, as they share characteristics with both. This conflict is an example of why the kingdom Protist
a was adopted. However, they retained their double-placement until the flagellates were split up, and both names are still used to refer to the group.
As with other Euglenozoa, the primitive mode of nutrition is phagocytosis. Prey such as bacteria
and smaller flagellates are ingested through a cytostome
, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and in Entosiphon form an extendable siphon. Most phagotrophic euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the substrate. In some, such as Peranema, the leading flagellum is rigid and beats only at its tip.
) and Hyalophacus (colourless Phacus
). Since they lack a developed cytostome, these forms feed exclusively by absorption.
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Most euglenids are unicellular. Many euglenids have chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s and produce energy through photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, but others feed by phagocytosis or strictly by diffusion. They belong to the phylum Euglenophyta, and their cell structure is typical of that group.
Euglenids are believed to descend from an ancestor that took up green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
by secondary endosymbiosis
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
.
Structure and locomotion
Euglenids are distinguished mainly by the presence of a pelliclePellicle (biology)
The pellicle is a thin layer supporting the cell membrane in various protozoa, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more hydrodynamic. They vary from flexible and elastic to rigid. Although stiff, the pellicle is...
, which is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids the strips can slide past one another, causing an inching motion called metaboly. Otherwise they move using the flagella.
History and classifications
The euglenids were first defined by Otto BütschliOtto Bütschli
Johann Adam Otto Bütschli was a German zoologist and professor at the University of Heidelberg. He specialized in invertebrates and insect development...
in 1884 as the flagellate order Euglenida. Botanists subsequently treated the algal division Euglenophyta; thus they were classified as both animals and plants, as they share characteristics with both. This conflict is an example of why the kingdom Protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
a was adopted. However, they retained their double-placement until the flagellates were split up, and both names are still used to refer to the group.
Classification and nutrition
The classification of euglenids is still variable, as groups are being revised to conform with their molecular phylogeny. To some extent, however, the results support the traditional groups based on differences in nutrition and number of flagella; at any rate these provide a starting point for considering euglenid diversity.As with other Euglenozoa, the primitive mode of nutrition is phagocytosis. Prey such as bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and smaller flagellates are ingested through a cytostome
Cytostome
A cytostome or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles. Only certain groups of protozoa, such as the ciliates and excavates, have cytostomes. Such...
, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and in Entosiphon form an extendable siphon. Most phagotrophic euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the substrate. In some, such as Peranema, the leading flagellum is rigid and beats only at its tip.
Osmotrophic euglenids
In many cases exposure to certain chemicals or prolonged absence of light may kill off the chloroplasts without otherwise harming the organism. There are a number of species where chloroplasts are absent, formerly treated in separate genera such as Astasia (colourless EuglenaEuglena
Euglena is a genus of unicellular flagellate protists. It is the best known and most widely studied member of the phylum Euglenozoa , a diverse group containing some 44 genera and at least 800 species. Species of Euglena are found in fresh and salt waters...
) and Hyalophacus (colourless Phacus
Phacus
Phacus is a genus of unicellular protists, of the phylum Euglenozoa ....
). Since they lack a developed cytostome, these forms feed exclusively by absorption.