Haptophyta

The great majority of Haptophyta are unicellular, motile, palmelloid, or coccoid (Figure 1.35), but a few form colonies or short filaments. These algae are generally found in marine habitats, although there are a number of records from freshwater and terrestrial environments. Flagellate cells bear two naked flagella, inserted either laterally or apically, which may have different length. A structure apparently found only in algae of this division is the haptonema, typically a long thin organelle reminiscent of a flagellum but with a different ultrastructure. The chloroplast contains only chlorophylls a, c₁, and c₂. The golden yellow brown appearance of the chloroplast is due to accessory pigments such as fucoxanthin, β-carotene, and other xanthins. Each chloroplast is enclosed within a fold of endoplasmic reticulum, which is continuous with the nuclear envelope. Thylakoids are stacked in threes, and there are no girdle lamellae. The nucleic DNA is scattered throughout the chloroplast as numerous nucleoids. When present as in Pavlova, the eyespot consists in a row of spherical globules inside the chloroplast; no associated flagellar swelling is present. The most important storage product is the polysaccharide chrysolaminarine. The cell surface is typically covered with tiny cellulosic scales or calcified scales bearing spoke-like fibrils radially arranged. Most haptophytes are photosynthetic, but heterotrophic nutrition is also possible. Phagotropy is present in the forms that lack a cell covering. A heteromorphic diplohaplontic life cycle has been reported, in which a diploid planktonic flagellate stage alternates with a haploid benthic filamentous stage.