(cid:2)1), the viscosity of Porphyridium sp. polysaccharide was found to be stable over a wide range of pH values (29), temperatures (30120(cid:5)C) and salinities. From the dependence of the intrinsic viscosity on ionic strength, it was estimated that the stiffness of the Porphyridium sp. polysaccharide chains is in the same range as that of xanthan gum and DNA (Eteshola et al., 1996). On the basis of rheological studies, it was hypothesized that the biopolymer chain molecules adopt an ordered conformation in solution and that the polysaccharide has the form of double or triple helix.

In concentrated solutions (12 g l There are no known carbohydrolases capable of cleaving red microalgal two crude activities capable of degrading polysaccharides. However, the polysaccharide have been found: in the dinoflagellate Crypthecodinium cohnii, which preys specifically on Porphyridium sp. and degrades its polysaccharide (Ucko et al., 1989), and the second in a mixture of soil bacteria (Arad et al., 1993b). Although the molecular weights of the degradation products were similar to that of the native polysaccharide, the viscosities of these products were significantly lower than the viscosity of the native polysaccharide (Simon et al., 1992, 1993; Arad et al., 1993b). the first 15.2 Environmental effects on polysaccharide production

Ambient environmental conditions influence polysaccharide production, for example, nitrate and sulfate starvation of Porphyridium sp. not only enhance production and solubilization of the polysaccharide (Kost et al., 1984; Wanner & Kost, 1984; Thepenier et al., 1985; Adda et al., 1986; Arad et al., 1988, 1992), but also influence its chemical composition (Ucko et al., 1994). In Porphyridium sp., carbon dioxide concentration in the medium (bubbling in air, or 0.03 or 3% CO2) influenced growth, cell-wall polysaccharide production, and the ratio of soluble to bound polysaccharide. The latter ratio was highest when carbon dioxide was withheld from the culture. Carbon dioxide concentration also influenced the chemical composition of the cellwall polysaccharide, the ratio of galactose to xylose being doubled when carbon dioxide was supplied at a lower concentration. It was thus suggested that carbon dioxide concentration affects polysaccharide composition by changing the partitioning of

, 2000). Kroen & Ramus (1990) suggested that polysaccharide production is controlled at the carbon fixation level. Studies in the laboratory of S. Arad on the effect of environmental conditions on the polysaccharide indicate, however, that exocellular (soluble) polysaccharide production is controlled at the level of carbon partitioning rather than by total photosynthetic loading (Friedman et al., 1991; Arad et al., 1992; Li et al., 2000).