Mariculture wastewater treatment by salinity-acclimated activated sludges from different sources: Performance and...

The discharge of mariculture wastewater poses a serious threat to coastal ecosystems. Activated sludges from three sources, including Zhanjiang (ZJ), Nanjing (NJ), and Shenzhen (SZ), was used to investigate the effectiveness and mechanisms of salinity gradient acclimation for treating mariculture wastewater. All three sludges were subjected to a stepwise salinity increase (5‰) from 0‰ to 30‰. At the high salinity of 30‰, NJ had the optimal removal rates for COD (89.3%) and TN (82.1%), which were higher than those for ZJ (COD: 85.8%; TN: 79.4%) and SZ (COD: 82.3%; TN: 74.0%). For all three sludges, the total extracellular polymeric substance (EPS) content gradually increased along with elevated salinity, and the protein-to-polysaccharide ratio (PN/PS) performed a decreasing trend due to the dramatic increase of PS. Compared with ZJ and SZ, NJ exhibited a higher EPS content (173.6 mg/g-MLSS) and a lower PN/PS (3.63), along with more secretion of tightly bound EPS (TB-EPS) for cellular structure protection. Microbial community analysis revealed that some phyla, such as Pseudomonadota (in ZJ, NJ, and SZ), Cyanobacteria (in ZJ), and Patescibacteria (in NJ), were notably enriched during the salinity gradient acclimation. In NJ, halotolerant Massilia and unclassified Saccharimonadales were specifically enriched at the high salinity of 30‰, and they might play important roles for COD and TN removal. The prediction results suggest that NJ might harbor a more complete nitrogen-related metabolic network than ZJ and SZ, and the higher abundances of denitrification genes in NJ might contribute to its superior nitrogen-removing performance.