Metagenomic analysis revealed the evolution of microbial communities, metabolic pathways, and functional genes in the heterotrophic nitrification-aerobic denitrification process under La stress.

Trivalent lanthanum (La) exists widely in ammonia nitrogen (NH-N) tailing water from ionic rare earth mines; however, its effect on heterotrophic nitrification-aerobic denitrification (HN-AD) is unknown, thereby limiting the application of the HN-AD process in this field. In this study, we conducted an HN-AD process using a sequencing batch reactor (5 L) that was continuously operated to directly treat acidic (NH)SO wastewater (influent NH-N concentration of approximately 110 mg/L and influent pH of 5) containing different La concentrations (0-100 mg/L). The NH-N removal efficiency of the reactor reached 98.25 % at a La concentration of 100 mg/L. The reactor was in a neutral-to-alkaline environment, which favored La precipitation and complexation. Metagenomic analysis revealed that the relative abundance of Thauera in the reactor remained high (88.62-92.27 %) under La stress. The relative abundances of Pannonobacter and Hyphomonas significantly increased, whereas that of Azoarcus significantly decreased. Metabolic functions in the reactor were mainly contributed by Thauera, and the abundance of metabolic functions under low La stress (≤5 mg/L) significantly differed from that under high La stress (≥10 mg/L). The relative abundance of ammonia assimilation-related genes in the reactor was high and significantly correlated with ammonia removal. However, traditional ammonia oxidation genes were not annotated, and unknown ammonia oxidation pathways may have been present in the reactor. Moreover, La stimulated amino acid biosynthesis and translocation, the citrate cycle, sulfur metabolism, and oxidative phosphorylation and promoted the overproduction of extracellular polymeric substances, which underwent complexation and adsorbed La to reduce its toxicity. Our results showed that the HN-AD process had a strong tolerance to La, stable NH-N removal efficiency, the potential to recover La, and considerable application prospects in treating NH-N tailing water from ionic rare earth mines.