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 c 1 (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, 11.3). 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) (Figs. 11.2, 11.3(c), (d)). 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). The scales are cemented together to form a scale case by the organic envelope. This precise arrangement of scales differs from the loosely arranged scales of the Chrysophyceae.
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).