Carbon Resource Feeding Strategies For Enhancing Poly (Vinyl Alcohol) Degradation By A Mixed Microbial Culture

Poly(vinyl alcohol) (PVA) is a kind of biodegradable synthetic polymer, but in the natural environment the efficiency of PVA degradation is low, and PVA is produced by the hydrolysis of poly(vinyl acetate) in industry. Its main chains are made up of sole carbon atoms and its functional group is hydroxyl. Owing to its structure, PVA has many prominent features, such as thermostable, adhesive, tolerance towards solvents et al.. Nowadays, PVA is massively used in industry. However, disposed PVA has caused serious pollution in natural environment. To eliminate PVA pollution, it is necessary to enhance PVA-degrading enzymes (PVADE) production and PVA-degrading efficiency.This study was mainly focused on three aspects: PVA degradation by the mixed microbial culture; enhancement of PVADE by fermentation optimization; enhancement of PVA-degrading efficiency by 1,4-butanediol. The main results are as follows.(1) PVA degradation by the mixed microbial culture was confirmed by GPC, Iodometry and IR spectra. Compared with other PVA-degrading strains, the mixed microbial culture had higher PVA-degrading rate. It was found that the mixed microbial culture could degrade PVA completely. By detecting fermentation products, it was found that its structure was similar with PVA, but the amount of hydroxyl groups was low and a new type of chemical bond (C-O-C) was formed. In the mixed microbial culture, Pseudomonas sp. and Bacillus sp. were existed. During PVA degradation, the relative abundance of the mixed microbial culture significantly changed. These results reflected a significant shift in microbial community structure. PVA oxidase was not detected in the PVA-degrading supernatant. However, PVADE and PVA dehydrogenase were detected.(2) In this study, it was found that 1,4-butanediol could enhance PVADE production. High PVADE activity (3.43 U/ml), which was 4.6 folds of the control (0.75 U/ml), was achieved with 1,4-butanediol as carbon source. Specifically, diauxic growth coupled with increased PVA-degrading rate was observed. Based on this phenomenon, two-stage fermentation, adding another carbon source at a proper time, was designed. By applying this strategy, high PVADE productivity (60.8 U/L/h) was achieved. Further, two-stage fermentation was extended to three-stage fermentation by adding PVA to improve PVADE production. The PVADE activity per unit biomass (YPVADE/X) was significantly enhanced over two-stage fermentation and the maximum increment was 418 U/g.(3) PVA-degrading efficiency was enhanced by optimizing 1,4-butanediol feeding strategies to improve cell vitalities. Specifically, the average PVA degrading rate was improved to 0.150 g/L/ h by adjusting 1,4-butanediol addition time to 21 h. It was found that 1,4-butanediol (above 15 g/L ) and PVA (above 30 g/L) at high concentration were toxic to cells. Based on the above results, continuous fed-batch fermentation was applied to control 1,4-butanediol concentration below 5.0 g/L. Consequently, cell vitalities for PVA degradation and cell growth were significantly improved. Meanwhile, high average PVA degrading rate (0.447 g/L/h) was achieved.