Page 53 - Algae

36 Algae: Anatomy, Biochemistry, and Biotechnology

FIGURE 2.1 Schematic drawing of a simple cell membrane.

Type 2: Cell Surface with Additional Extracellular Material
Extracellular matrices occur in various forms and include mucilage and sheaths, scales, frustule,
cell walls, loricas, and skeleta. The terminology used to describe this membrane-associated material
is quite confusing, and unrelated structures such as the frustule of diatoms, the fused scaled cover-
ing of some prasynophyceae, and the amphiesma of dinoflagellates have been given the same name,
that is, theca. Our attempt has been to organize the matter in a less confusing way (at least in our
opinion).

Mucilages and Sheaths
These are general terms for some sort of outer gelatinous covering present in both prokaryotic and
eukaryotic algae. Mucilages are always present and we can observe a degree of development of
a sheath that is associated with the type of the substrate the cells contact (Figure 2.2). All cyano-
bacteria secrete a gelatinous material, which, in most species, tends to accumulate around the
cells or trichome in the form of an envelope or sheath. Coccoid species are thus held together to
form colonies; in some filamentous species, the sheath may function in a similar manner, as in
the formation of Nostoc balls, or in development of the firm, gelatinous emispherical domes of
the marine Phormidium crosbyanum. Most commonly, the sheath material in filamentous species
forms a thick coating or tube through which motile trichomes move readily. Sheath production is a
continuous process in cyanobacteria, and variation in this investment may reflect different physiologi-
cal stages or levels of adaptation to the environment. Under some environmental conditions the
sheath may become pigmented, although it is ordinarily colorless and transparent. Ferric hydroxide
or other iron or metallic salts may accumulate in the sheath, as well as pigments originating within
the cell. Only a few cyanobacterial exopolysaccharides have been defined structurally; the sheath of
Nostoc commune contains cellulose-like glucan fibrils cross-linked with minor monosaccharides,
and that of Mycrocystis flos-aquae consists mainly of galacturonic acid, with a composition
similar to that of pectin. Cyanobacterial sheaths appear as a major component of soil crusts
found throughout the world, from hot desert to polar regions, protecting soil from erosion,
favoring water retention and nutrient bio-mobilization, and affecting chemical weathering of the
environment they colonize.