New insight into selective Na+ stress on acidogenic fermentation of waste activated sludge from microbial perspective: Hydrolase secretion, fermentative bacteria screening, and metabolism modification

[Display omitted] •Selective Na+ stress improved protease activity and promoted DOMs biodegradation.•Na+ stress induced bacteria screening with reduced species richness and diversity.•Selective Na+ stress facilitated “SCFAs-producing” microbial community formation.•Salt-tolerant acidogens were enriched and vulnerable methanogens were inhibited.•Na+ stress is prime inducement to modify microbial community for SCFAs accumulation. The short-chain fatty acids (SCFAs) production was facilitated in Na+ assistant anaerobic fermentation of waste activated sludge, whereas the relevant micro-ecosystem characteristics were unclear. This work explored the microbial community and hydrolases shifts under Na+ stress. It demonstrated that the selective Na+ stress increased protease activities to 126–160% while inhibiting α-glucosidase activities to 83.1–91.3%. Correspondingly, the biodegradation rates of protein and glucose were improved from 25.6% and 45.8% to 39.0% and 55.2%, respectively. Furthermore, the microbial species richness and biodiversity gradually decreased, attributing to Na+-induced cell lysis. Selective Na+ stress was the prime inducement for microbial community evolution and promoted “SCFAs-producing” bacterial community formation by “bacteria-screening” roles. Overall, the hydrolytic bacteria and acidogens became dominant bacteria, which were resistive to Na+ stress, while the SCFAs consumers were restrained as they are salinity-sensitive. Such microbial community shift modified functional metabolisms for enhancing anaerobic fermentation performance. The pathways for metabolism and genetic information processing were improved, positively relating to the facilitated catabolism of fermentation substrates. Consequently, the mechanism of maximized SCFAs accumulation by selective Na+ stress was proposed, i.e. microbial modification of fermentation ecosystem for facilitating sludge hydrolysis and acidification with impeded methanogenesis.