Page 37 - Algae

20 Algae: Anatomy, Biochemistry, and Biotechnology

RHODOPHYTA
The red algae mostly consist of seaweeds but also include the genera of free-living unicellular
microalgae. The class Bangiophyceae (Figure 1.24) retains morphological characters that are
found in the ancestral pool of red algae and range from unicells to multicellular filaments or
sheet-like thalli. The Floridophyceae (Figure 1.29) includes morphologically complex red algae
and are widely considered to be a derived, monophyletic group. Rhodophyta inhabit prevalently
marine ecosystems but they are also present in freshwater and terrestrial environment. The lack
of any flagellate stages and the presence of accessory phycobiliproteins organized in phycobili-
somes (shared with Cyanobacteria, Cryptophyta, and Glaucophyta) are unique features of this div-
ision; chlorophyll a is the only chlorophyll. Chloroplasts are enclosed by a double unit membrane;
thylakoids do not stack at all, but lie equidistant and singly within the chloroplast. One thylakoid is
present around the periphery of the chloroplast, running parallel to the chloroplast internal mem-
brane. The chloroplastic DNA is organized in blebs scattered throughout the whole chloroplast.
The most important storage product is floridean starch, an a-1,4-glucan polysaccharide. Grains
of this starch are located only in the cytoplasm, unlike the starch grains produced in the Chloro-
phyta, which lie inside the chloroplasts. Most rhodophytes live photoautotrophically. In the great
majority of red algae, cytokinesis is incomplete. Daughter cells are separated by the pit connection,
a proteinaceous plug that fills the junction between cells; this connection successively becomes a
plug. Species in which sexual reproduction is known generally have an isomorphic or hetero-
morphic diplohaplontic life cycle; haplontic life cycle is considered an exception.

HETEROKONTOPHYTA

One of the defining features of the members of this division is that when two flagella are present,
they are different. Flagellate cells are termed heterokont, that is, they possess a long mastigone-
mate flagellum, which is directed forward during swimming, and a short smooth one that points
backwards along the cell. Chrysophyceae contain single-celled individuals (Figure 1.2) as well as
quite colonial forms. Xanthophyceae can be unicellular (coccoids or not) filamentous, but the most
distinctive species are siphonous (Figure 1.14). All known species of Eustigmatophyceae are
green coccoid unicells either single (Figure 1.1), in pairs or in colonies. Bacillariophyceae are
a group of unicellular brown pigmented cells that are encased by a unique type of silica wall, com-
posed of two overlapping frustules that fit together like a box and lid (Figure 1.30 and
Figure 1.31). Raphidophyceae are unicellular wall-less heterokonts (Figure 1.32). Dictyochophy-
ceae, known as silicoflagellates, are unicells that bear a single flagellum with mastigonemes

FIGURE 1.29 Frond of Rhodophyllis acanthocarpa. (Bar: 5 cm.)