Heterotrophic nitrification processes driven by glucose and sodium acetate: New insights into microbial communities, functional genes and nitrogen metabolism from metagenomics and metabolomics

[Display omitted] •Heterotrophic microorganisms mainly remove NH4+-N by nitrogen assimilation.•Heterotrophic nitrifying fungi are more competitive in glucose environments.•Lower biomass and higher gas production for NH4+-N removal using CH3COONa.•Signal molecule NO production is an important heterotrophic nitrification pathway.•The demand for NO and energy is the driving force for heterotrophic nitrification. Heterotrophic nitrification (HN) bacteria use organic carbon sources to remove ammonia nitrogen (NH4+-N); however, the mechanisms of carbon and nitrogen metabolism are unknown. To understand this mechanism, HN functional microbial communities named MG and MA were enriched with glucose and sodium acetate, respectively. The NH4+-N removal efficiencies were 98.87 % and 98.91 %, with 88.06 % and 69.77 % nitrogen assimilation for MG and MA at 22 h and 10 h, respectively. Fungi (52.86 %) were more competitive in MG, and bacteria (99.99 %) were dominant in MA. Metagenomic and metabolomic analyses indicated that HN might be a signaling molecule (NO) in the production and detoxification processes when MG metabolizes glucose (amo, hao, and nosZ were not detected). MA metabolizes sodium acetate to produce less energy and promotes nitrogen oxidation reduction; however, genes (hao, hox, and NOS2) were not detected. These results suggest that NO and energy requirements induce microbial HN.