Alginic acid, derived from algae, is insoluble in water, but its sodium salt is readily soluble. Alginate solutions form gels upon contact with Ca2+. A single alginate molecule typically contains three regions: the "M region" (rich in mannuronic acid), the "G region" (rich in guluronic acid), and the MG region (containing both uronic acids). Ca²⁺ and other divalent cations readily bind to the G region; therefore, calcium alginate gel is considered a three-dimensional network molecule in which Ca cross-links the G regions of long-chain molecules. The composition of alginate varies among different organisms. For example, some alginates contain 93% M region and only 3% G region (e.g., Ascophyllum modosum), while others contain only 62% M region (e.g., Laminaria digitata). Differences in the M/G ratio significantly affect their physicochemical properties. Azotobacter vinelandii can also produce alginate, and because it can be produced artificially, without geographical or seasonal limitations, it has the potential to become a new source of alginate.
