Euglenophyta - Structural characteristics of algal protists

Euglenophyta

This is a group of unicellular flagellated organisms, which probably represent the most ancient group of algal protists. Individuals range in size from 10−500 µm. Euglenophytes are commonly found in fresh water, par-ticularly that with a high organic content, and to a lesser extent, in soil, brackish water and salt water. Members of this group have a well-defined nucleus, and chloro-plasts containing chlorophylls a and b (Figure 9.1). Thestorage product of photosynthesis is a β-1,3-linked glucan called paramylon, found al-most exclusively in this group. Euglenophytes lack a cellulose cell wall but have instead, situated within the plasma membrane, a flexible pellicle made up of interlocking protein strips, a characteristic which links them to certain protozoan species. A further similarity is the way in which locomotion is achieved by the undulation of a terminal flagellum. Movement towards a light source is facilitated in many euglenids by two structuressituated near the base of the flagellum; these are the paraflagellar body and the stigma or eyespot. The latter is particularly conspicuous, as it is typically an orange-red colour, and relatively large.

Reproduction is by binary fission (i.e. by asexual means only). Division starts at the anterior end, and proceeds longitudinally down the length of the cell, giving the cell a characteristic ‘two-headed’ appearance. During mitosis, the chromosomes within the nucleus replicate, forming pairs that split longitudinally. Since the euglenophyte is usually haploid, it thus becomes diploid for a short period. As fission proceeds, one daughter cell retains the old flagellum, while the other one generates a new one later. As in the binary fission of bacteria, the progeny are genetically identical, i.e. clones. When conditions are unfavourable for survival due to failing nutrient supplies, the cells round up to form cysts surrounded by a gelatinous covering; these have an increased complement of paramylon granules, but no flagella. An important respect in which euglenids may be at variance with the notion of ‘plant-like protists’ is their ability to exist as heterotrophs under certain conditions. When this happens, they lose their photosynthetic pigments and feed saprobically on dead organic material in the water.