Page 40 - Algae

General Overview 23

FIGURE 1.35 Unicell of Helicosphaera carteri.

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.

CRYPTOPHYTA
The unicellular flagellates belonging to the division Cryptophyta are asymmetric cells dorsiven-
trally constructed (Figure 1.36). They bear two unequal, hairy flagella, subapically inserted, emer-
ging from above a deep gullet located on the ventral side of the cell. The wall of this gullet is lined
by numerous ejectosomes similar to trichocysts. Cryptophytes are typically free-swimming in
freshwater and marine habitats; palmelloid phases can also be formed, and some members are
known to be zooxanthellae in host invertebrates or within certain marine ciliates. Cryptophyta
possess only chlorophylls a and c 2 . Phycobilins are present in the thylakoid lumen rather than in
phycobilisomes. The chloroplasts, one or two per cell, are surrounded by a fold of the endoplasmic
reticulum. In the space between these membranes a peculiar organelle, the nucleomorph, is located.
This organelle can be interpreted as the vestigial nucleus of the red algal endosymbiont that gave
rise to the chloroplasts of the Cryptophyta. Thylakoids are arranged in pairs, with no girdle lamel-
lae. The pyrenoid projects out from the inner side of the chloroplast. The chloroplast DNA is con-
densed in small nucleoids scattered inside the chloroplast. The reserve polysaccharide accumulates