DICTYOCHOPHYCEAE

These golden-brown algae are characterized by tentacles or rhizopodia on basically amoeboid vegetative cells (Moestrup, 1995; Preisig, 1995). Amoeboid cells are relatively rare among the algae, being mostly restricted to the Dictyochophyceae and the Xanthophyceae (Hibberd and Chretiennot-Dinet, 1979). The algae in the Dictyochophyceae have been previously classified in the Chrysophyceae, although molecular evidence shows them to be most closely related to the Pelagophyceae (Cavalier-Smith et al., 1995) or Eustigmatophyceae (Daugbjerg and Andersen, 1997).

Classification

The Dictyochophyceae can be divided into three orders (Preisig, 1995):

1. Order 1 Rhizochromulinales: marine and freshwater unicells with tentacles.

2. Order 2 Pedinellales: unicells with a long anterior flagellum and a second flagellum reduced to a basal body, usually three to six chloroplasts (if chloroplasts are present), marine and freshwater.

3. Order 3 Dictyocales: marine unicells with an external silicified skeleton.

Rhizochromulinales

This order contains the more primitive organisms in the order (O'Kelly and Wujek, 1995). Rhizochromulina (Fig. 14.1(a), (b)) has amoeboid non-flagellated vegetative cells with many fine beaded-filipodia and a single golden-brown chloroplast (Hibberd and Chretiennot-Dinet, 1979). The fusiform zoospore has a single tinsel flagellum with a second basal body in the protoplasm (Fig. 14.1(b)). Chrysoamoeba (Fig. 14.1(d)) lives as a solitary amoeba for the greater part of its life cycle, transforming into swimming cells with a single long flagellum only for short periods. In Phaeaster (Fig. 14.1(c)), the anterior portion of the cell is drawn out into rhizopodia.