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

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 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).