A cell wall, defined as a rigid, homogeneous and often multilayered structure, is present in both
prokaryotic and eukaryotic algae.
In the Cyanophyta the cell wall lies between the plasma membrane and the mucilaginous
sheath; the fine structure of the cell wall is of Gram-negative type. The innermost layer, the
electron-opaque layer or peptidoglycan layer, overlays the plasma membrane, and in most cyanobacteria
its width varies between 1 and 10 nm, but can reach 200 nm in some Oscillatoria species.
Regularly arranged discontinuities are present in the peptidoglycan layer of many cyanobacteria;
pores are located in single rows on either side of every cross wall, and are also uniformly distributed
over the cell surface. The outer membrane of the cell wall appears as a double track structure tightly
connected with the peptidoglycan layer; this membrane exhibits a number of evaginations representing
sites of extrusion of material from the cytoplasm through the wall into the slime. The
cell wall of Prochlorophyta is comparable to that of the cyanobacteria in structure and contains
Eukaryotic algal cell wall is always formed outside the plasmalemma, and is in many respects
comparable to that of higher plants. It is present in the Rhodophyta, Eustigmatophyceae
(Figure 2.15a and 2.15b), Phaeophyceae (Heterokontophyta), Xanthophyceae (Heterokontophyta), Chlorophyceae, and Charophyceae (Chlorophyta). Generally, cell walls are made up of two components,
a microfibrillar framework embedded in an amorphous mucilaginous material composed
of polysaccharides, lipids, and proteins. Encrusting substances such as silica, calcium carbonate, or
sporopollenin may be also present. In the formation of algal cell walls the materials required are
mainly collected into Golgi vesicles that then pass it through the plasma membrane, where
enzyme complexes are responsible for the synthesis of microfibrils, in a pre-determinate direction.
|FIGURE 2.15 Transmission electron microscopy image of Nannochloropsis sp. in transversal section
(a). Arrows point to the cell wall. Negative staining of the shed cell walls (b). (Bar: 0.5 µm.)