EUGLENA

Euglena is a common genus of flagellate protozoa, typical of the euglenids, and commonly found in nutrient-rich freshwater, with a few marine species. The cells vary in length from around 20 to 300 μm, and are typically cylindrical, oval, or spindle-shaped with a single emergent flagellum. There are usually many bright green chloroplasts, although some species are colorless. The name comes from the Greek words eus and glêne, or good eyeball, referring to the light-sensitive eyespot.

Over 100 species of Euglena have been described. Marin et al. (2003) revised the genus so that it forms a monophyletic group, moving several species with rigid pellicles to the genus Lepocinclis and including several species without chloroplasts, formerly classified as Astasia and Khawkinea.

The Euglenids are one example of why the Kingdom Protista (a.k.a. Protoctista) makes so much sense in comparison to the older taxonomy, in which protozoa were grouped with the animals and chloroplast containing algae were considered plants. In this group of single celled organisms, the Eugeloids, which are obviously derived from the same ancestral organism, there are both organisms containing chloroplasts that synthesize their own sugars using sunlight and dissolved carbondioxide, and organisms that do not have the capability to perform photosynthesis and thus must ingest or absorb their food.

Complex chloroplasts

The complex chloroplasts of Euglena (and those of photosynthetic dinoflagellates) are surrounded by three membranes. This is unlike chloroplasts of higher plants and most green algae, which are surrounded by two membranes. The additional membranes present a barrier to import of chloroplast precursor proteins not present in the simpler double membrane chloroplast envelope. In higher plants and most green algae, chloroplast proteins are synthesized on free polysomes and then posttranslationally imported directly into the chloroplast. Protein import into these complex chloroplasts is fundamentally different and occurs in three distinct 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.

This fundamental difference in import mechanism is proposed to reflect the different evolutionary origins of these chloroplast types. The endosymbiotic theory postulates that choroplasts are a vestige of ancient symbiotic relationships between host cells and internalized photosynthetic organisms. Chloroplasts with a two-membrane envelope are thought to have derived from an association between a eukaryotic host and a photosynthetic prokaryote, while the complex chloroplasts of Euglena are believed to have resulted from a secondary endosymbiotic association between a eukaryotic host and a photosynthetic eukaryote.

References

• Marin B, Palm A, Klingberg M, Melkonian M (2003). "Phylogeny and taxonomic revision of plastid-containing euglenophytes based on SSU rDNA sequence comparisons and synapomorphic signatures in the SSU rRNA secondary structure." Protist 154(1):99–145.

External links

• Euglena
• Prostist Images: Euglena
• Euglena at Droplet - Microscopy of the Protozoa
• Pictures of Euglena gracilis
• Effect of Light Intensity on the Lipid Composition of Euglena gracilis
• Mitochondrial trans-2-Enoyl-CoA Reductase of Wax Ester Fermentation from Euglena gracilis Defines a New Family of Enzymes Involved in Lipid Synthesis
• Aliphatic Chains of Esterified Lipids in Isolated Eyespots of Euglena gracilis var. bacillaris1 (pdf)
• Nature, intracellular distribution and formation of terpenoid quinones in Euglena gracilis. (pdf)
• Tryptophan Synthetase in Euglena gracilis Strain G (pdf)
• Lipid Metabolism of Manganese-deficient Algae: I. Effect of Manganese Deficiency on the Greening and the Lipid Composition of Euglena Gracilis Z. (pdf)
• A hydroxy fatty acid dehydrogenase in Euglena gracilis (pdf)
• Comparative studies of biosynthesis of galactolipids in Euglena gracilis strain Z (pdf)