Fig. 6.13 Peranema trichophorum. (a) General cell structure. (b),(c) Two stages in the ingestion of a cell of Euglena (stippled cell). (c) Canal; (cv) contractile vacuoles; (cy) rim of cytosome; (fv) food vesicle; (g) Golgi; (ir) ingestion rods; (lf) leading flagellum; (m) mitochondrion; (n) nucleus; (p) paramylon; (tf) trailing flagellum. (After Leedale, 1967.)

undergo a shift from carbohydrate to fat oxidation, as evidenced by an increase in malate synthetase, the enzyme involved in the glycolate bypass, important in the oxidation of fats.

The euglenoids belong to the osmotrophic acetate flagellates, having the ability to grow photosynthetically in the light or heterotrophically in the dark. In either state, the fixed carbon is used as a source of energy or as building blocks for cell constituents. The substrates that can be used for heterotrophic growth vary from one species to another, with the permeability of the substrate into the cell probably being the most important factor. As a rule, the most readily utilized substrates are acetic and butryic acids and the corresponding alcohols (e.g., ethanol). The two most commonly used substrates are acetate and ethanol.

the phototrophic and osmotrophic species. The phototrophic species evolved from a single secondary endosymbiotic event involving a chloroplast from a green alga in the Prasinophyceae (Marin, 2004). Osmotrophic genera such as Astasia (Fig. 6.14(b)) contain chloroplast genetic material indicating they evolved from photosynthetic euglenoids (Sekiguchi et al., 2002).

Three orders of euglenoids are presented here. Investigations utilizing nucleic acid sequencing have shown the organisms in the Euglenales to have evolved most recently (Marin, 2004).

Order 1 Heteronematales: two emergent flagella, the longer flagellum directed anteriorly and the shorter one directed posteriorly during swimming; special ingestion organelle present.

Order 2 Eutreptiales: two emergent flagella, one directed anteriorly and the other laterally or posteriorly during swimming; no special ingestion organelle.

Order 3 Euglenales: two flagella, only one of which emerges from the canal; no special ingestion organelle.

Classification

Studies on rRNA of euglenoids support a monophyletic origin of the Euglenophyceae with the kinetoplastids as a sister clade (Preisfeld et al., 2000; Nudelman et al., 2003). The phagotrophic euglenoids (e.g., Peranema; Fig. 6.13) evolved before

Heteronematales

Here the colorless cells have a special ingestion organelle (Triemer, 1997), and are phagocytic, taking up food particles whole and digesting them in food vesicles. Peranema trichophorum is a euglenoid that ingests other cells and detritus (Leedale, 1967) (Fig. 6.13). The ingestion apparatus consists

Fig. 6.14 (a) Euglena gracilis. (b) Astasia klebsii. (c)

Eutreptiella marina. (d) Trachelomonas grandis. (e) Phacus triqueter. (C) Chloroplast; (Ca) canal; (CV) contractile vacuole; (E) eyespot; (Ev) envelope; (F) emergent flagellum; (FS) flagellar swelling; (M) mitochondrion; (N) nucleus; (P) paramylon grains or paramylon sheath around chloroplast;

(R) reservoir. (After Leedale, 1967.)

of two parallel tapering rods, the hooked anterior ends of which are attached to the stiffened rim of the cytosome. The latter is a permanent “mouth” situated in a subapical position independent of the canal opening. There is no permanent “gullet,” and food vacuoles are formed at the cytosome only when feeding takes place. Peranema normally ingests food particles and living organisms by engulfing them whole into food vacuoles. The ingestion rods are protruded and attached to the surface of the prey, which is then pulled through the cytosome in connection with a wave of euglenoid movement from the Peranema cell. With a large prey, such as Euglena, the rods are detached, moved, and attached again farther along the prey,

so that more of it can be pulled into the predator. By repeated pullings, the whole Euglena is engulfed, the process taking up to 15 minutes. A second form of attack, reserved for larger algal cells, consists of cutting and sucking rather than engulfing. Several Peranema cells converge on their prey, with their ingestion rods protruded and used to rasp a way through the prey’s wall or periplast. Euglena spirogyra pellicle is cut through in about 10 minutes, with the cell contents sucked out into a temporary food canal below the cytosome. If the prey is large enough, the predators finally enter the cell and engulf what remains of the prey. The food vacuoles decrease in size as digestion proceeds, the indigestible remains being finally ejected through a “defecation area” of constant position at the posterior end of the cell. It is possible to show that chemotaxis is important in directing Peranema to its prey by bursting open living algal cells in a suspension of Peranema, the peranemas streaming in from all directions for the meal.

Fig. 6.15 (a) Larva of the damselfly Ischnura verticalis with a plug of Colacium libellee in the rectum. (b), (c) Colony and single swimming cell of Colacium vesiculosum. ((a) adapted from Rosowski and Willey, 1975; (b), (c) after Stein and Johnson in Huber-Pestalozzi, 1955.)

Eutreptiales

The organisms in the Eutreptiales have two emergent flagella and no special ingestion organelles. Eutreptia and Eutreptiella (Figs. 6.11, 6.14(c)) are estuarine or marine genera, while Distigma is characteristic of acid freshwaters.

Euglenales

In this primarily freshwater order, the flagellum without the paraflagellar swelling has been reduced so that it does not emerge from the canal. Common genera in the order are the green photosynthetic Euglena (Figs. 6.1, 6.2, 6.3, 6.7, 6.14(a)), Trachelomonas (Figs. 6.12, 6.14(d)), and Phacus (Figs. 6.4, 6.14(d)), as well as the colorless osmotrophic Astasia (Fig. 6.14(b)).

Colacium libellee is a member of this order that establishes itself in the rectum of damselfly nymphs during the winter in colder lakes (Figs.