PHAEOTHAMNIOPHYCEAE

Recent nucleotide sequencing has uncovered an evolutionary line of golden-brown algae not related to other golden-brown algae (Bailey et al., 1998). These algae have been placed in the class Phaeothamniophyceae, a class that is most closely related to the Xanthophyceae and Phaeophyceae. The cytology of these three classes is similar (Fig. 20.1). The cells have two membranes of chloroplast endoplasmic reticulum with the outer membrane of chloroplast E.R. continuous with the outer membrane of the nuclear envelope. The chloroplasts have a ring-shaped genophore and girdle lamellae. The flagella are inserted laterally into the motile cells. The anterior tinsel flagellum has tripartite hairs that lack lateral filaments. The posterior flagellum lacks hairs. New daughter cells are formed by eleutheroschisis (parent cell wall is completely cast off and new daughter cell walls are formed). Vesicles under the plasma membrane appear similar to the physodes that occur in the Phaeophyceae. The Phaeothamniophyceae is the only class of algae where fucoxanthin and heteroxanthin occur together. Endogenous siliceous cysts (statospores) are not produced by these algae.

Phaeothamnion is a filamentous brown alga that produces zoospores that settle to produce new filaments (Fig. 20.1) (Andersen et al., 1998). Tetrachrysis occurs in environments such as peat ponds and has cells embedded in a common mucilage (Dop et al., 1980) (Fig. 20.2). Tetrasporopsis is a colonial freshwater alga that consists of a brown, gelatinous, bladdery sac (Entwisle and Andersen, 1990) (Fig. 20.2).