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 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. 22.6) (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 c 1 / c 2, β-carotene, diadinoxanthin, and diatoxanthin (Zapata et al., 2004 ; Zhao et al., 2015). The storage product is primarily mannitol (Tsuji et al., 2015). 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 (Shalchian-Tabrizi et al., 2011). While diatoms dominate in nutrient-rich coastal waters, Prymnesiophyceae dominate offshore where nutrients are limiting. Prymnesiophyceae 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. They decrease in frequency toward the poles although some still occur in polar waters (Manton et al., 1977).

Cell Structure

Flagella

Most of the Prymnesiophyceae have two smooth flagella of approximately the same length (Figs. 22.1, 22.2(a)). The Pavlovales is the exception, where one flagellum is longer than the other and is usually covered by small cylindrical to club-shaped hollow scales 70 nm long and 20 nm wide (Fig. 22.2(b)) (van der Veer, 1969 ; Green and Manton, 1970).