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  Section: Algae » An Overview
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Summaries of the Ten Algal Divisions
  Cyanophyta and Prochlorophyta
Endosymbiosis and Origin of Eukaryotic Algae

Euglenophyta include mostly unicellular flagellates (Figure 1.39) although colonial species are common. They are widely distributed, occurring in freshwater, brackish and marine waters, most soils, and mud. They are especially abundant in highly heterotrophic environments. The flagella arise from the bottom of a cavity called reservoir, located in the anterior end of the cell. Cells can also ooze their way through mud or sand by a process known as metaboly, a series of flowing movements made possible by the presence of the pellicle, a proteinaceous wall which lies inside the cytoplasm. The pellicle can have a spiral construction and can be ornamented. The members of this division share their pigmentation with prochlorophytes, green algae, and land plants, because they have chlorophylls a and b, β- and γ-carotenes, and xanthins. However, plastids could be colorless or absent in some species. As in the Dinophyta the chloroplast envelope consists of three membranes. Within the chloroplasts, the thylakoids are usually in groups of three, without a girdle lamella and pyrenoids may be present. The chloroplast DNA occurs as a fine skein of tiny granules.
Unicell of Euglena mutabilis. (Bar: 10 µm.)
FIGURE 1.39 Unicell of Euglena mutabilis. (Bar: 10 µm.)

The photoreceptive system consisting of an orange eyespot located free in the cytoplasm and the true photoreceptor located at the base of the flagellum can be considered unique among unicellular algae. The reserve polysaccaccharide is paramylon, β-1,3-glucan, stored in the granules scattered inside the cytoplasm and not in the chloroplasts like the starch of the Chlorophyta. Though these possess algae chlorophylls, they are not photoautotrophic but rather obligate mixotrophic, because they require one or more vitamins of the B group. Some colorless genera are phagotrophic, with specialized cellular organelle for capture and ingestion of prey; some others are osmotrophic. Some of the pigmented genera are facultatively heterotrophic. Only asexual reproduction is known in this division. Euglenophyta posses unique cellular and biochemical features that place these microorganisms closer to trypanosomes than to any other algal group.


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