Metagenomics unraveled the characteristics and microbial response to hypersaline stress in salt-tolerant aerobic granular sludge.

In this study, the salt-tolerant aerobic granular sludge (SAGS) was cultivated with the increased salinity (0-9% NaCl), showing oval shape, and clear outline. The related sludge characteristics in the formation process of SAGS as well as the effects of salinity on the performance (removal ability, sludge biomass and EPS component) of SAGS were evaluated. Increased salinity accelerated the formation of SAGS, and resulted in the excess secretion of EPS. Relationship between EPS and settling capacity of SAGS was determined, with the increase of salinity, SVI decreased linearly and the sedimentation performance of granular sludge was enhanced. Pearson correlation analysis showed that shorter settling time (3 min) and longer anaerobic influent time (30 min) were beneficial to the operation of SAGS reactor. Metagenomics results showed that the SAGS was dominated by Candida, Halomonas and other salt-tolerant bacteria, the enrichment of these salt-tolerant microbes played an important role in maintaining the stability of granular sludge system and improving the overall salt-tolerant performance. Compared with S9 samples, the proteome regulation in S0 sample was more active and the abundance of Cell motility related proteins was 5 times higher than that in S9 samples. Extracellular structure related proteins was more active in S9, and its abundance was 3.6 times that of S0.