Diverse aromatic-degrading bacteria present in a highly enriched autotrophic nitrifying sludge

Biotransformation of refractory organics by ammonia-oxidizing microorganisms in nitrifying sludge have been widely reported, while the contribution of heterotrophic bacteria in nitrifying sludge in the biotransformation and degradation process might be overlooked. Here, we provide metagenomic and metatranscriptomic evidences showing that heterotrophic bacteria in a highly enriched autotrophic nitrifying sludge could significantly contribute to the aromatic biotransformation and biodegradation. Diverse genes encoding enzymes for aromatic degradation were observed in an enriched autotrophic nitrifying sludge. These genes are involved in the degradation of at least 15 complex aromatics. Genome binning results showed that these genes were mainly carried by species in Bacteroidetes (Flavobacteriaceae and Sphingobacteriales), Alphaproteobacteria (Rhodobacter) and Betaproteobacteria (Bordetella, Acidovorax, Ramlibacter and Pusillimonas). According to the metatranscriptomic analysis, the overall expression of the potential aromatic-degrading genes was significantly upregulated, and almost all genes involved in phenol degradation were over expressed after the nitrifying sludge was exposed to phenol. Overall, our results suggest that certain heterotrophs in nitrifying sludge are involved aromatic biotransformation and biodegradation and advance our knowledge of the underlying properties and metabolic mechanisms of the nitrifying sludge. (C) 2019 Elsevier B.V. All rights reserved.