The haptophytes, classified either as the Haptophytina, Haptophyta or Prymnesiophyta (named for Prymnesium), are a clade of algae that can produce minerals. The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at the class rank rather than as a division. Although the phylogenetics of this group has become much better understood in recent years, there remains some dispute over which rank is most appropriate.
Characteristics
The chloroplasts are pigmented similarly to those of the heterokonts, but the structure of the rest of the cell is different, so it may be that they are a separate line whose chloroplasts are derived from similar red algal endosymbionts. Haptophyte chloroplasts contain chlorophylls a, c1, and c2 but lack chlorophyll b. For carotenoids, they have beta-, alpha-, and gamma- carotenes. Like diatoms and brown algae, they have also fucoxanthin, an oxidized isoprenoid derivative that is likely the most important driver of their brownish-yellow color. The cells typically have two slightly unequal flagella, both of which are smooth, and a unique organelle called a haptonema, which is superficially similar to a flagellum but differs in the arrangement of microtubules and in its use. The name comes from the Greek hapsis, touch, and nema, round thread. The mitochondria have tubular cristae. Most haptophytes reportedly produce chrysolaminarin rather than starch as their major storage polysaccharide, but some Pavlovaceae produce paramylon. The chain length of the chrysolaminarin is reportedly short (polymers of 20–50 glycosides, unlike the 300+ of comparable amylose), and it is located in cytoplasmic membrane-bound vacuoles.
Significance The best-known haptophytes are coccolithophores, which make up 673 of the 762 described haptophyte species, and have an exoskeleton of calcareous plates called coccoliths. Coccolithophores are some of the most abundant marine phytoplankton, especially in the open ocean, and are extremely abundant as microfossils, forming chalk deposits. Other planktonic haptophytes of note include Chrysochromulina and Prymnesium, which periodically form toxic marine algal blooms, and Phaeocystis, blooms of which can produce unpleasant foam which often accumulates on beaches. Haptophytes are economically important, as species such as Pavlova lutheri and Isochrysis sp. are widely used in the aquaculture industry to feed oyster and shrimp larvae. They contain a large amount of polyunsaturated fatty acids such as docosahexaenoic acid (DHA), stearidonic acid and alpha-linolenic acid. Tisochrysis lutea contains betain lipids and phospholipids.
Classification
The haptophytes were first placed in the class Chrysophyceae (golden algae), but ultrastructural data have provided evidence to classify them separately. Both molecular and morphological evidence supports their division into five orders; coccolithophores make up the Isochrysidales and Coccolithales. Very small (2–3μm) uncultured pico-prymnesiophytes are ecologically important. Haptophytes was discussed to be closely related to cryptomonads. Haptophytes are closely related to the SAR clade.
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