Euglenid

The euglenids (or euglenoids) are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Many euglenids have chloroplasts and produce energy through photosynthesis, but others feed by phagocytosis or strictly by diffusion. They belong to the phylum Euglenozoa, and their cell structure is typical of that group.

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Euglenids are distinguished mainly by the presence of a pellicle, which is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubules. 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.

The euglenids were first defined by Otto Bütschli 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 Protista 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 & 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 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.

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Chloroplasts presumably originated from some ingested green alga. They are pigmented with chlorophylls a and b, giving them a bright green colour. Often they are associated with granules of paramylon, a storage carbohydrate that is unique to the euglenids. Most coloured euglenids also have a stigma or eyespot, which is a small splotch of red pigment on one side of the flagellar pocket. This shades a collection of light sensitive crystals near the base of the leading flagellum, so the two together act as a sort of directional eye. The cytostome is a vestigial, although nutrients may still be obtained by absorption.

A few coloured euglenids have two roughly equal flagella, such as Eutreptia, and some have four. In most, however, the trailing flagellum is shortened so that it does not emerge from the flagellar pocket. The emergent flagellum typically undergoes a complex looping motion that pulls the euglenid along a slightly helical path. These include the common genera Euglena, Phacus, and Trachelomonas, which produces an organic lorica that encases the cell. There is also one genus, Colacium, in which the mature cells are non-motile and form branched colonies supported by mucous stalks.

Whereas chloroplasts in green algae and higher plants are surrounded by two membranes, those in euglenids are surrounded by three. The extra membrane presents a barrier to the import of chloroplast precursor proteins not present in the simpler two-membrane chloroplasts, where chloroplast proteins are synthesized on free polysomes and then post-translationally imported directly into the chloroplasts. In euglenids protein import is fundamentally different, with three steps:

1. Synthesis on polysomes bound to the rough endoplasmic reticulum (ER) with co-translocation into the ER;
2. Transport from the ER into the Golgi apparatus; and
3. Import across the three chloroplast membranes.

The extra membrane reflects the secondary origin of euglenid chloroplasts from another eukaryotic algae, rather than from prokaryotes as in plants and other Archaeplastida. A third membrane is also found in the chloroplasts of most dinoflagellates, while many other algae have chloroplasts with four membranes.

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 have been lost, formerly treated in separate genera such as Astasia (colourless Euglena) and Hyalophacus (colourless Phacus). Since they lack a developed cytostome, these forms feed exclusively by absorption.