Effect of nitrate on indole degradation characteristics and methanogenesis under mixed denitrification and methanogenesis culture

•First investigated the high COD/NO3−-N ratio of indole wastewater in mixed culture.•Through nitrate to observe the denitrification and methanogenesis co-existence.•Nitrate was found to facilitate indole degradation and inhibit methane formation.•The metabolites were identified based on 13C-indole isotope analysis.•A dynamic community analysis was presented to study the microbial structures.