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
FIGURE 1.35 Unicell of Helicosphaera carteri.