Page 55 - Algae

38 Algae: Anatomy, Biochemistry, and Biotechnology

FIGURE 2.3 Palmelloid phase of Euglena gracilis. (Bar: 10 mm.)

the cell. They occur only in eukaryotic algae, in the divisions of Heterokontophyta, Haptophyta,
and Chlorophyta. They can be as large as the scales of Haptophyta (1 mm), but also as small as
the scales of Prasynophyceae (Chlorophyta) (50 nm). There are at least three distinct types of
scales: non-mineralized scales, made up entirely of organic matter, primarily polysaccharides,
which are present in the Prasynophyceae (Chlorophyta); scales consisting of calcium carbonate
crystallized onto an organic matrix, as the coccoliths produced by many Haptophyta; and scales
constructed of silica deposited on a glycoprotein matrix, formed by some members of the
Heterokontophyta.
Most taxa of the Prasinophyceae (Chlorophyta) possess several scale types per cell, arranged in
1–5 layers on the surface of the cell body and flagella, those of each layer having a unique mor-
phology for that taxon. These scales consist mainly of acidic polysaccharides involving unusual
2-keto sugar acids, with glycoproteins as minor components. Members of the order Pyramimona-
dales exhibit one of the most complex scaly covering among the Prasinophyceae. It consists of three
layers of scales. The innermost scales are small, square, or pentagonal; the intermediate scales are
either naviculoid, spiderweb-shaped, or box shaped (Figure 2.4); the outer layer consists of large
basket or crown-shaped scales. It is generally accepted that scales of the Prasinophyceae are syn-
thesized within the Golgi apparatus; developing scales are transported through the Golgi apparatus
by cisternal progression to the cell surface and released by exocytosis. In some Prasynophyceae
genera such as Tetraselmis and Scherffelia, the cell body is covered entirely by fused scales. The
scale composition consists mainly of acidic polysaccharides. These scales are produced only
during cell division. They are formed in the Golgi apparatus and their development follow the
route already described for the scales. After secretion, scales coalesce extracellularly inside the par-
ental covering to form a new cell wall.
In the Haptophyta, cells are typically covered with external scales of varying degree of com-
plexity, which may be unmineralized or calcified. The unmineralized scales consist largely of
complex carbohydrates, including pectin-like sulfated and carboxylated polysaccharides, and
cellulose-like polymers. The structure of these scales varies from simple plates to elaborate,
spectacular spines and protuberances, as in Chrysochromulina sp. (Figure 2.5) or to the unusual
spherical or clavate knobs present in some species of Pavlova.
Calcified scales termed coccoliths are produced by the coccolithophorids, a large group of
species within the Haptophyta. In terms of ultrastructure and biomineralization processes, two