Sucrose Concentration Affects Respiratory Activity and Alginate Molecular Characteristics in Azotobacter vinelandii ATCN4, Influencing the Water-Retention and Mechanical Properties of Hydrogels

The Azotobacter vinelandii ATCN4 strain is a strict aerobic bacterium that lacks the Na+-translocating NADH:ubiquinone oxidoreductase complex and can produce alginate as an extracellular polysaccharide. Bioprocess conditions for bacterial alginate are typically selected to maximize polymer yield, assuming that the most productive condition also yields the most suitable material. Here we tested this assumption by evaluating how initial sucrose concentration affects respiratory activity, alginate production and molecular characteristics, and the performance of ionically crosslinked hydrogels prepared from the resulting alginates. A sustained respiratory quotient value observed during the stationary phase using 20 g L−1 sucrose was associated with an increase in alginate production during this period. Using 10 g L−1 sucrose, the highest alginate concentration (3.5 g L−1) and a 2.9-fold higher alginate yield on sucrose were obtained, compared with 20 g L−1 sucrose. Sucrose concentration also affected molecular weight, G/M ratio, and degree of acetylation, with a G/M ratio of about 2 when 10 g L−1 sucrose was used as the carbon source. Hydrogels prepared from the alginate obtained under the least productive condition (20 g L−1) showed the highest water-retention capacity together with intermediate apparent stiffness. The results show that the condition most adequate for alginate production was not the condition optimal for hydrogel performance, indicating that bioprocess design for alginate-based materials should be guided by the target material property rather than polymer yield alone.