Heterokontophyta

XANTHOPHYCEAE

The Xanthophyceae contain primarily freshwater and terrestrial algae with a few marine representatives. The class is characterized by motile cells with a forwardly directed tinsel flagellum and a posteriorly directed whiplash flagellum (Figs. 19.1, 19.5(c)). The chloroplasts contain chlorophylls a and c (Sullivan et al., 1990), lack fucoxanthin, and are colored yellowish-green. The eyespot in motile cells is always in the chloroplast (Figs. 19.1, 19.5(c)), and the chloroplasts are surrounded by two membranes of chloroplast endoplasmic reticulum. The outer membrane of the chloroplast E.R. is usually continuous with the outer membrane of the nucleus. In most non-motile cells the wall is composed of two overlapping halves (Figs. 19.2 (d), (e), (f), 19.3, 19.4). Molecular data have shown the Xanthophyceae is most closely related to the Phaeophyceae (Ariztia et al., 1991; Potter et al., 1997). Although the class is commonly called the Xanthophyceae, the proper name is the Tribophyceae since there is no genus in the class that can lend its name to Xanthophyceae (Hibberd, 1981).

Cell structure

Cell wall

The cell walls of two Xanthophyceae, Tribonema (Figs. 19.2, 19.3) (Cleare and Percival, 1973) and Vaucheria (Figs. 19.7, 19.8), are composed of cellulose (Parker et al., 1963). In Vaucheria cellulose comprises 90% of the wall, with the remaining

Fig. 19.1 Light and electron microscopical drawing of a zoospore of a typical member of the Xanthophyceae,

Mischococcus sphaerocephalus. (C) Chloroplast; (CV) contractile vacuole; (E) eyespot; (FS) flagellar swelling; (LF) long flagellum with hairs; (N) nucleus; (SF) short flagellum;

(V) vacuole. (Adapted from Hibberd and Leedale, 1971.)

portion being amorphous polysaccharides composed primarily of glucose and uronic acids.

Many of the algae in the class have walls composed of two overlapping halves that fit together as do the two parts of the bacteriologist’s Petri dish (Figs. 19.2(d), (e), (f), 19.3, 19.4). The two-part nature of the wall cannot be delineated with the light microscope unless the cells have been treated with certain reagents such as concentrated potassium hydroxide. A typical example is the wall of Ophiocytium majus (one study places Ophiocytium in the closely related Eustigmatophyceae based on the sequence of

mitochondrial cytochrome oxidase (Ehara et al., 1997)), which is tubular in shape (Fig. 19.4). The wall is composed of two parts, a cap of constant size fitted over a tubular basal portion. As the cell grows and increases in length, the tubular basal portion elongates, but the cap remains the same size. The rims of both the cap and the tube are finely tapered and overlap for some distance so that the bipartite condition is not normally apparent in living cells. A layer of intercalary material, presumably functioning as a cementing substance, separates the cap from the tubular part of the wall. Filamentous genera, such as