The microbial detoxification mechanisms and nitrogen transformation pathways in the anaerobic ammonia oxidation...

Thiourea can significantly inhibit the aerobic ammonia oxidation process, yet it can also serve as a multi‑electron donor in denitrification. Understanding the perturbational influence of thiourea on nitrogen removal in anaerobic ammonium oxidation (Anammox) systems is crucial for its application in treating high NH₄⁺-N wastewater containing thiourea. Therefore, the present inquiry investigated its effects on nitrogen degradation pathways and microbial metabolism in Anammox systems. The findings reveal that when the influent thiourea concentration remains below 139.7 mg/L, both sulfur-oxidizing autotrophic denitrifiers (SOB) and their heterotrophic denitrifiers residing in the Anammox reactor fully metabolize this compound. They use NO₂^--N/NO₃^--N as electron acceptors, converting thiourea into SO₄^2- and NH₄⁺-N. The activity of Anammox bacteria (AnAOB) was not significantly affected. However, the activity of the associated ammonia-oxidizing bacteria (AOB) was significantly inhibited, and nitrite-oxidizing bacteria (NOB) took over to mitigate the dissolved oxygen toxicity in the influent. The denitrifying microbial community in the Anammox system, originally dominated by AnAOB, shifted toward a multifunctional denitrifying community dominated by heterotrophic denitrifiers, SOB, and AnAOB. As thiourea concentrations increase, denitrifying microorganisms struggle to grow synergistically, leading to thiourea accumulation. Oxidative stress in the system rises, and catalase (CAT) activity is inhibited, resulting in cellular damage. Accumulated thiourea reduced the relative abundances of core genes hzs and hdh in AnAOB and sqr in SOB. Although the relative abundances of fccA/fccB in SOB and nirS in denitrifiers were upregulated, these potential functional changes could not counteract thiourea toxicity. AnAOB activity is restricted and cannot be restored in the short-term. Thiobacillus and Candidatus_Brocadia, as functional genera of SOB and AnAOB, respectively, exhibit abundance that varies with changes in nitrogen-sulfur transformation performance. Enhancing sulfur-autotrophic and heterotrophic denitrification to prevent thiourea accumulation is a key strategy for alleviating the denitrification stress imposed on the Anammox system.