PRYMNESIOPHYCEAE

The Prymnesiophyta are a group of uninucleate flagellates characterized by the presence of a haptonema between two smooth flagella. The Prymnesiophyta have two membranes of chloroplast endoplasmic reticulum, as do the Cryptophyta and the Heterokontophyta, but differ in having flagella without mastigonemes. Molecular data also show that the Prymnesiophyta are distinct from the Cryptophyta and Heterokontophyta (Bhattacharya and Ehlting, 1995; Medlin et al., 1994). Until 1962, the organisms were considered part of the Chrysophyceae, at which time Christensen split them off into a separate class, the Haptophyceae (named after the presence of the haptonema). The name Haptophyceae was a descriptive name and not based on a genus in the class; thus the name was later changed to Prymnesiophyceae, based on the genus Prymnesium (Fig. 22.7) (Hibberd, 1976). The fossil record of the Prymnesiophyceae is known from the Carboniferous (approximately 300 000 000 years ago) (Faber and Preisig, 1994; Jordan and Chamberlain, 1997).

The cells are commonly covered with scales. In many cases, the scales are calcified, thereby producing coccoliths. The chloroplasts lack girdle lamellae and most contain chlorophylls a and c1/c2, β-carotene, diadinoxanthin, and diatoxanthin (Zapata et al., 2004). The storage product is chrysolaminarin (leucosin) in vesicles in the posterior end of the cell (Janse et al., 1996). The anterior end of the cell has a large Golgi apparatus and sometimes a contractile vacuole.

The Prymnesiophyceae are primarily marine organisms, although there are some freshwater representatives. They make up a major part of the marine nannoplankton and constitute about 45% of the total phytoplankton cells in the middle latitudes of the South Atlantic.