Chapter 6

Euglenophyta

EUGLENOPHYCEAE

Euglenoid flagellates occur in most freshwater habitats: puddles, ditches, ponds, streams, lakes, and rivers, particularly waters contaminated by animal pollution or decaying organic matter (Buetow, 1968). Usually larger bodies of purer water, such as rivers, lakes, and reservoirs, have sparser populations of less common euglenoids as planktonic organisms. Marine euglenoids are more common than supposed, with Eutreptia, Eutreptiella (Figs. 6.11, 6.14(c)), and Klebsiella occurring exclusively in marine or brackhish water, and many other genera having one or a few marine species. These occur in the open sea, in tidal zones among seaweeds, and as sand inhabitants on beaches. Brackish species of Euglena (Figs. 6.1, 6.2, 6.3, 6.7, 6.14(c)) often color estuarine mud flats green when light intensity is low, the green color

disappearing in full sunlight as the euglenoids creep away from the surface. There are also several parasitic euglenoid flagellates, mostly species of Khawkinea, Euglenamorpha, and Hegneria.

Euglenoids are characterized by chlorophylls a and b, one membrane of chloroplast endoplasmic reticulum, a mesokaryotic nucleus, flagella with fibrillar hairs in one row, no sexual reproduction, and paramylon or chrysolaminarin as the storage product in the cytoplasm.

Euglenoid cells have two basal bodies and one or two emergent flagella (Fig. 6.2). The flagella are similar to those of trypanosomes in having a paraxonemal rod (paraxial rod) that runs the length of the flagellum inside the flagellar membrane (Ngô and Bouck, 1998; Bastin and Gul, 1999; Talke and Preisfeld, 2002). The paraflagellar rod is composed of two major proteins forming an elongated alpha-helical stalk that parallels the axoneme. The one emergent flagellum in Euglena

Fig. 6.1 Light micrographs of Euglena stellata. (From Shin

and Triemer, 2004.)

246 CHLOROPLAST E.R.: EVOLUTION OF ONE MEMBRANE

Fig. 6.2 A semidiagrammatic drawing of the fine structure of the anterior part of a Euglena cell. (C) Canal; (CER) chloroplast endoplasmic reticulum; (CV) contractile vacuole; (E) eyespot; (LF) long flagellum; (M) mastigonemes; (MB) muciferous body; (Mt) microtubules; (N) nucleus; (P) paraflagellar swelling; (Pa) paramylon; (PG) pellicle groove; (Pl) plasmalemma; (PS) pellicle strip; (Py) pyrenoid; (R) reservoir; (SF) short flagellum. (Adapted from Mignot, 1966; Jahn and Bovee, 1968.)

has helically arranged fibrillar hairs (no microtubules) attached along the length of the flagellar membrane. The fibrillar hairs are of two lengths: there is a single helical row of long (3 m) hairs and two helical rows of short (1.5 m) hairs in Euglena (Bouck et al., 1978). Other genera have flagella similar to Euglena (Hilenski and Walne, 1985). There are two basic types of flagellar movement in the class. The first group (including the Eutreptiales and Euglenales) has the flagellum continually motile from base to apex, resulting in cell gyration with the anterior end of the cell tracing a wide circle. The swimming rate of Euglena gracilis (Figs. 6.7, 6.14(a)) depends on the temperature: at 10 C it is 15 m s 1 while at 30 C it is 84m s 1 (Lee, 1954). The second group (including

Peranema (Fig. 6.13), Entosiphon, and Sphenomonas) has the flagellum held out straight in front of the cell with just the tip motile, resulting in smooth

swimming or gliding locomotion in contact with the substratum or air–water interface at rates up to 30 m s 1 (Saito et al., 2003).

Euglenoid cells are surrounded by a pellicle that has four main components: the plasma membrane, repeating proteinaceous units called strips, subtending microtubules, and tubular cisternae of endoplasmic reticulum (Figs. 6.2, 6.3, 6.4, 6.5) (Leander and Farmer, 2000, 2001a). The strips are arranged in parallel, are characteristic of the species, and are composed primarily of the proteinaceous articulins. Below each strip is a set of parallel microtubules, where each microtubule occupies a discrete position relative to the strip. The cisterna of endoplasmic reticulum is also intimately associated with each strip and appears to function as a reservoir for calcium.

Each strip of the pellicle has a thick side and a thinner flange side (Figs. 6.5, 6.6). In the

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Fig. 6.3 Euglena rustica. Transmission electron micrograph of a mid sagittal section of a cell. (Arrow) Paramylon granule; (cp) chloroplast; (Py) pyrenoid; (Nu) nucleus. (From Brown et al., 2002.)

construction of the pellicle, the thick end of one strip fits under the thin flange end of the second strip. This structure gives the pellicle an alternating pattern of ridges and grooves. In Euglena gracilis (Figs. 6.5, 6.7, 6.14(a)) there are about 40 of the S-shaped strips overlapping at their lateral margins, under the plasma membrane. The region of strip overlap is occupied by a set of microtubuleassociated bridges and microtubule-independent bridges.

Muciferous bodies occur under the pellicle strips and contain a water-soluble mucopolysaccharide. The muciferous bodies open to the

outside through pores between the strips of the pellicle. The muciferous bodies function in the formation of the stalk in Colacium, lorica formation in Trachelomonas, cyst formation and lubrication during euglenoid movement (Leander and Farmer, 2000).

Some euglenoids have a flexible pellicle that allows the cells to undergo a flowing movement known as euglenoid movement (Fig. 6.7). This type of movement occurs only when the cells are not swimming and results from sideways movement of the pellicle strips. The movement may be induced by the binding of Ca2 from the endoplasmic reticulum directly under the strips of the pellicle (Murata et al., 2000).

Euglena gracilis changes its shape two times per day when grown under the synchronizing effect of a daily light–dark cycle. At the beginning of the light period, when photosynthetic capacity is low (as measured by the ability of the cells to evolve oxygen), the population of cells is largely spherical. The mean cell length of the population increases to a maximum in the middle of the light period when photosynthetic capacity is greatest, and then decreases for the remainder of the 24-hour period. The population becomes spherical by the end of the 24-hour period when the cycle reinitiates. These changes are also observed under dim light conditions and are therefore controlled by a biological clock and represent a circadian rhythm in cell shape (Lonergan, 1983).

The locomotory flagellum or flagella emerge from an anterior invagination of the cell, which consists of a narrow tubular portion, the canal, and a spherical or pyriform chamber, the reservoir (Figs. 6.2, 6.8). The canal is a rigid structure, whereas the reservoir easily changes shape and is regularly distorted by the discharge of the contractile vacuole. The rigidity of the canal is maintained by microtubules that form a flat helix around the canal, in much the same position as hoops on a barrel. The pellicle lines the canal but not the reservoir, the reservoir being the only part of the cell covered solely by the plasmalemma. A cylindrical pocket arises as an infolding of the plasma membrane of the reservoir in Euglena, Colacium, and Peranema and is probably common in the euglenoids (Willey and Wibel, 1985a; Surek

248 CHLOROPLAST E.R.: EVOLUTION OF ONE MEMBRANE

Fig. 6.4 Scanning electron micrographs of euglenoids showing the helical pellicle of Phacus triqueter (lower left),

Euglena acus (middle), and Lepocincilis ovata (upper right). (From Leander and Farmer, 2001b.)

and Melkonian, 1986). The reservoir pocket is similar to the cytosome found in protozoa assigned to the Kinetoplastida (Willey and Wibel, 1985b). In the Kinetoplastida (bodonids and trypanosomatids), the cytosome is associated with the uptake of food organisms by phagotrophy, and the reservoir pocket has a similar function in the phagotrophic euglenoids (e.g., Peranema; Fig. 6.13) where the pocket is called a cytosome.

The contractile vacuole (Figs. 6.1, 6.2, 6.13, 6.14), in the anterior part of the cell next to the reservoir, has an osmoregulatory function, expelling excessive water taken into the cell. The contractile vacuole fills and empties at regular intervals of 15 to 60 seconds. It empties into the reservoir, from which the water is carried out through the canal (Leedale, 1967).

Mitochondria are of typical algal type. Colorless euglenoids always have more mitochondria

than do equivalent-sized green ones. When green cells are decolorized by heat or streptomycin, they have a sevenfold increase in mitochondrial volume, reflecting a change from autotrophic to heterotrophic nutrition. The formation of two mitochondrial enzymes, fumarase and succinate dehydrogenase, necessary for dark respiration of substrates, is repressed by light (Davis and Merrett, 1974).

Nucleus and nuclear division

The euglenoid nucleus is of the mesokaryotic type, having chromosomes that are permanently condensed during the mitotic cycle, a nucleolus (endosome) that does not disperse during nuclear division, no microtubules from chromosomes to pole spindles, and a nuclear envelope that is intact during nuclear division (Fig. 6.9). The chromosome number is usually high, and polyploidy probably occurs in some genera (Gravilaˇ, 1996).

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(a)

(b)

(c)

Fig. 6.5 Euglena gracilis. Diagrams of a whole swimming cell (a) and transverse sections of the cell surface (b) and (c), illustrating details of the articulating S-shaped strips of the membrane skeleton and the infrastructure associated with strip overlap. The position of the skeleton and bridges seems well suited to mediate the sliding of adjacent strips that is presumed to occur during shape changes. (MAB1, MAB2) Microtubule-associated bridges between the pellicle strips; (MIB-A, MIB-B) microtubule-independent bridges between the pellicle strips; (PM) plasma membrane; (T) transverse fiber. (From Dubreuil and Bouck, 1985.)

Mitosis in euglenoids (Leedale, 1970; Chaly et al., 1977) begins during early prophase with the nucleus migrating from the center of the cell to an anterior position. Microtubules appear in the nucleus, but they do not attach to the chromosomes. At metaphase, bundles of microtubules are among the chromosomes, and the nucleolus has started to elongate along the division axis. In anaphase, the intact nuclear envelope elongates along the division axis, the nucleolus divides, and