Prymnesiophyceae

A class of algae (also known as Haptophyceae) in the chlorophyll a-c phyletic line (Chromophycota). In protozoological classification these organisms constitute an order, Prymnesiida or Haptomonadida, in the class Phytamastigophora. Most of the approximately 300 species of prymnesiophytes are biflagellate monads. See also Chromophycota.

This class has been segregated from the Chrysophyceae, with which it shares many biochemical and ultrastructural characters. Prymnesiophytes differ from chrysophytes, however, in several significant characters: (1) the typical monad bears a filiform organelle, the haptonema, between the two flagella; (2) except in the order Pavlovales, the flagella are of equal length and smooth; (3) organic scales, which may be calcified, cover most mottle cells.

Calcified scales (cellulosic scales impregnated with calcite) are characteristic of many prymnesiophytes. These scales, which are given the general term coccoliths, were discovered in marine sediment before they were observed on living cells. Coccoliths are classified into several morphological types (such as rhabdoliths, discoliths, zygoliths, and ceratoliths), which are of great diagnostic value in the taxonomy of the prymnesiophytes that bear them (coccolithophorids).

In most prymnesiophytes a nonmotile phase alternates with a motile phase. The nonmotile phase is a free-living unicell or a palmelloid or pseudofilamentous colony. In some cases the alternation is mediated by sexual reproduction. Usually, however, reproduction is effected by binary fission or the production of zoospores.

Prymnesiophytes are primary marine, with coccolithophorids constituting one of the three major components of phytoplankton (the others being diatoms and dinoflagellates). See also Algae; Coccolithophorida; Phytoplankton; Protozoa.