SYNUROPHYCEAE

The Synurophyceae are closely related to the Chrysophyceae (Ariztia et al., 1991). The Synurophyceae differ, however, from the Chrysophyceae in the following: the Synurophyceae have chlorophylls a and c1 (Andersen and Mulkey, 1983), the flagella are inserted into the cell approximately parallel to one another (Fig. 11.1), there is a photoreceptor near the base of each flagellum, there is no eyespot, and the contractile vacuole is in the posterior portion of the cell (Lavau et al., 1997; Andersen et al., 1999). Chloroplast endoplasmic reticulum is present in a few species, but absent in most. The cells usually are covered by bilaterally symmetrical scales.

In the Synurophyceae, scales composed of silica commonly occur outside the cell (Figs. 11.1, 11.2). The scales are bilaterally symmetrical and are formed in a silica deposition vesicle. The membrane of the silica deposition vesicle (the silicalemma) controls the shape of the scale along with proteins and glycoproteins that adhere the developing scale to the silicalemma (Schultz et al., 2001). The presence of germanium in the medium results in inhibition of scale formation (Klaveness and Guillard, 1975). The scales are carried in the scale vesicle to the plasma membrane where the plasma membrane and the scale vesicle fuse, releasing the scales outside the cell (Beech et al., 1990). The scales are held next to the cell in an organic envelope (Ludwig et al., 1996), which is either hyaline or yellow-brown, the latter appearance being due to the impregnation of iron salts. The scales of the Synurophyceae are commonly

composed of a number of parts, such as the dome, shield, and bristle of Mallomonas (Lavau and Wetherbee, 1994) (Fig. 11.2). The scales of the Synurophyceae are overlapped precisely so that the anterior end of one scale overlaps the right margin of the scale to its left (Leadbeater, 1990).

Fig. 11.1 Semidiagrammatic drawing of the cytology of

Synura, showing the characteristic cytology of the Synurophyceae. (CV) Contractile vacuole; (F) flagella;

(G) Golgi; (L) chrysolaminarin vesicle; (N) nucleus;

(P) photoreceptor; (S) scale; (SV) scale vesicle. (Adapted from Andersen, 1985.)

Fig. 11.2 Mallomonas zellensis. (a) Whole cell. (b) Scales. (c) A scale with the bristle removed. (b) Bristle; (d) dome;

(f) flange; (h) hood; (r) ribs; (ri) rim; (s) shield; (sc) scale; (st) strut. (After Fott, 1962.)

The scales are cemented together to form a scale case by the organic envelope (Fig. 11.4). This precise arrangement of scales differs from the loosely arranged scales of the Chrysophyceae.

Tessellaria volvocina (Fig. 11.3) is a colonial member of the class that has a large number of cells encased within a multilayer covering of siliceous scales. The individual cells of T. volvocina do not have a covering of scales (Tyler et al., 1989; Pipes and Leedale, 1992).

Analysis of lake sediments often reveals the presence of the silicified scales of the Synurophyceae as well as the silicified frustules of diatoms (Smol et al., 1984; Dixit et al., 1999). Analysis of the species that produced these silicified deposits can often result in a history of

environmental conditions in the lake. Diatoms usually do not grow in waters below a pH of 5.8 to 6, whereas several members of the Synurophyceae thrive in acidic lakes (Saxby-Rouen et al., 1997). As environmental concerns over the acidification of lakes by acid rains increase, these species will probably be more widely used as indicators of lake acidification.

Unlike the Chrysophyceae, the Synurophyceae are not phagocytotic, probably because the fixed scale-case is solidly formed and bacteria and food particles cannot easily reach the cell membrane. The cysts (statospores or stromatocysts) of the Synurophyceae are usually more elaborate than those of the Chrysophyceae.

Classification

The Synurophyceae share affinities with the Bacillariophyceae, Chrysophyceae, and Phaeophyceae (Lavau et al., 1997; Andersen et al., 1999). Because of their silicified outer covering, they have been called “flagellated diatoms”; yet they have many of the characteristics of the Chrysophyceae such as statospores.

There is a single class, the Synurales. All of the organisms are flagellates. Siphonaceous forms do not exist, as true filaments cannot be formed by organisms with a silica scale-case, and coccoid forms would be similar to diatoms. The diatoms

Corethron, Rhizosolenia, and Attheya are similar in appearance to coccoid Synurophyceae (Andersen, 1987).

Synura (Fig. 11.4) is a widely distributed colonial member of the class. The life cycle of Synura petersenii (Figs. 11.5(a), 11.6) has been described by Sandgren and Flanagin (1986). Vegetative colonies of S. petersenii normally consist of a number of biflagellate cells joined at their posterior end. The cells are covered with silicified scales (Leadbeater, 1990). Each cell has two parietal chloroplasts, a central nucleus, and a large posterior chrysolamarin vacuole. Sexual reproduction in S. petersenii is isogamous and heterothallic. Gametes appear when two compatible clones are mixed. No special culture conditions are required to induce sexual behavior. Actively growing cell populations are continually

Fig. 11.3 Tessellaria volvocina. (a) Light micrograph. Arrowhead points to a scale case. (b) Scanning electron micrograph showing overlapping of plate scales and rare spine scales (arrowhead). (From Tyler et al., 1989.)

Fig. 11.4 Synura uvella. (a) Light micrograph. (b) Scanning electron micrograph of a colony of cells. (c) Scanning electron micrograph of a single cell showing the two flagella emerging from an apical pore in the scale case. Bar 10 m. (From Andersen, 1985.)

receptive to mating when mixed with a sufficient number of cells from a compatible clone. Male gametes are solitary biflagellate cells that appear similar to vegetative cells. The solitary male

gametes are probably released from the colonies of the male clone. The female gametes occur in colonies or as solitary cells, and are also similar in appearance to vegetative cells. Male and female gametes initially contact each other anteriorly; this is followed by lateral fusion. The silica scale layers mingle so that the resulting fusion cell maintains a complete scale layer. The biflagellate planozygote undergoes minor shape changes