Repeated xylitol production by immobilized Gluconobacter oxydans using diatom frustule-alginate composite carriers

Xylitol, a common sugar substitute in food and pharmaceuticals, can be produced microbially using glucose as the primary substrate. Our earlier study established an efficient one-step biotransformation system in Gluconobacter oxydans that converts D-arabitol to xylitol. To improve the reusability of resting cells, recombinant G. oxydans was immobilized in a composite matrix of Cyclotella meneghiniana frustules and solium alginate. The combined approach not only enhanced the mechanical strength and stability of the beads but may also have facilitated oxygen transfer within the immobilization matrix. Compared with frustules adsorption and alginate encapsulation alone, the combined immobilized cells exhibited increases in catalytic activity of 32.8% and 17.3%, respectively. An in-situ immobilization protocol, adding 2.0 g/L frustules to the fermentation medium with a 22-h culture time, further improved immobilized cell activity by 17.3%. Moreover, frustule-adsorbed cells cross-linked with 0.5% glutaraldehyde (GA) exhibited enhanced catalytic performance in continuous batch operation. Finally, the repeated biotransformation was carried out by immobilized G. oxydans, achieving an average xylitol titer of 15.44 g/L over six consecutive batch cycles. This study presents a practical immobilization strategy for xylitol production and offers a reference for microbial production of other biochemicals.