Azotobacter vinelandii lacking the Na+-NQR activity: a potential source for producing alginates with improved properties and at high yield

The mutant ATCN4 strain of Azotobacter vinelandii, which lacks the Na(+)-NQR activity and results in an alginate overproduction (highly mucoid phenotype), was cultured in shake flasks in minimal and rich medium, and the chemical composition and rheological properties of the alginate were determined. Mutant ATCN4 exhibited a high efficiency for sucrose conversion to alginate and PHB accumulation, reaching yields that were 3.6- and 1.6-fold higher than those obtained from the wildtype cultures in minimal medium (Burk's sucrose, BS). The alginate produced by ATCN4 in the minimal medium had a high degree of acetylation (≥4 %) and a low G/M ratio (=2) with respect to the polymer synthesised in the rich medium (BS with yeast extract) (degree of acetylation = 0 % and G/M ratio of 4.5). The alginate produced in the minimal medium exhibited a pronounced pseudoplastic behaviour and a higher G* module in comparison to that observed in the alginate obtained in the cultures using a rich medium. The ATCN4 mutant culture in the minimal medium promoted the synthesis of a polymer of improved rheological quality in terms of its mechanical properties. These characteristics make this mutant a valuable source for producing alginates with improved or special properties.