Microbial succession and pollutant removal metabolic pathways in anaerobic treatment of saline organic wastewater based on metagenomic technology

In this study, anaerobic granular sludge was subjected to domestication using saline organic wastewater to investigate the impact of salt-tolerant domestication and the addition of compatible solutes on effluent quality, microbial community succession and organic matter metabolic pathways. After 200 days, the relative abundance of Euryarchaeota significantly increased from 7.99% to 33.54%, with Methanomicrobia dominating and accounting for 93.12% of Euryarchaeota. To adapt to the high osmotic pressure, enzymes involved in ATP binding and active ion transport across the membrane exhibited a substantial increase in relative abundance, with five representative enzymes surpassing 0.3%. Notably, key enzymes in the glycolytic metabolic pathway (gpmI, ENO and DLAT) reached final relative abundances of 0.0690, 0.0903 and 0.0571%, respectively. Similarly, the relative abundances of fwdA and cdhA, key enzymes in the methanogenic metabolic pathway, increased to 0.0857% and 0.1228%. These changes were reflected in the treated effluent, as the average chemical oxygen demand (COD) removal decreased from 82.10% to 64.55% with a gradual increase in salinity (Nacl concentration) from 2 to 20 g/L, but increased to 70.13% after the addition of 2 mmol/L betaine. The study demonstrates that the gradual elevation of salinity during domestication and the exogenous addition of betaine have a positive effect on the stability of anaerobic systems in high salinity environments, providing a viable strategy for the anaerobic treatment of saline organic wastewater.