Comment on “Role of Ammonia Oxidation in Organic Micropollutant Transformation during Wastewater Treatment”: Overlooked Evidence to the Contrary

Abstract

Metrics & MoreArticle RecommendationsI n their critical review, 1 Su, Smets, and coauthors extensively summarized studies on the role played by ammoniaoxidizing bacteria (AOB) in organic micropollutant (OMP) transformation using three levels of evidence: molecular, cellular, and community. They also comprehensively covered the abiotic reactions with the N-species formed from nitrification. We agree with the authors that some studies do support the important role played by AOB in the transformation of specific OMPs. However, we find that the authors' conclusion, "AOB are the main drivers of OMP biotransformation during wastewater treatment processes" (p 2173), does not stand up to proper scientific scrutiny. In the following, we will present our main arguments and provide the overlooked evidence contradicting the authors' conclusion. (In the following, all page numbers and references to graphical elements refer to Su et al. 2020. 1 ) Molecular Level. One argument the authors used to demonstrate the dominance of AOB in OMP biotransformation was that many observed OMP biotransformation reactions could be catalyzed by AOB, including (1) hydroxylation, (2) O-dealkylation, (3) thioester oxidation, (4) dehydrogenation, and (5) nitration. However, except for nitration, all these reactions may also be catalyzed by enzymes other than ammonia monooxygenase (AMO), including monooxygenases, dioxygenases and other oxidoreductases from heterotrophs. 2,3 Many of those enzymes commonly occur in activated sludge communities. 4,5 Thus, detecting the same transformation products in sludge communities as in AOB pure cultures does not provide conclusive evidence for the dominant role of AOB in oxidative biotransformation of OMPs in wastewater treatment.Cellular Level. The authors state that OMPs exhibiting

Toward Characterizing the Genetic Basis of Trace Organic Contaminant Biotransformation in Activated Sludge: The Role of Multicopper Oxidases as a Case Study

“…The fact that those enzymes seem to be distributed in both AOB and non-AOB strains, as well as their inhibition by ATU that depletes copper ions (essential for MCO activity), renders MCOs good candidates for this role. This observation highlights the long-debated discussion on the role of AOB bacteria in TrOCs elimination and the need to investigate enzymes other than AMO in our efforts to clarify the extent of their contribution. , …”

“…This observation highlights the long-debated discussion on the role of AOB bacteria in TrOCs elimination and the need to investigate enzymes other than AMO in our efforts to clarify the extent of their contribution. 24,75 Elucidating the Genetic Basis of TrOCs Biotransformations. The need to improve our understanding of the genetic and biochemical basis of pollutant degradation reactions has been highlighted repeatedly before.…”

Emerging investigator series: contributions of reactive nitrogen species to transformations of organic compounds in water: a critical review

“…Other microbial metabolisms also contribute to transformation in these systems, including heterotrophs with diverse metabolic capabilities. 137 In contrast, RNS reactions appear to contribute significantly to transformation of some organic contaminants with primary aromatic amine or phenol functional groups in treatment systems with high NO 2 − concentrations, such as nitritation/denitritation systems treating high-strength wastewaters. The relevance of biologically-derived RNS in these systems depends heavily on reactor conditions affecting RNS concentrations, including microbial activity and pH.…”

“…Other microbial metabolisms also contribute to transformation in these systems, including heterotrophs with diverse metabolic capabilities. 137 In contrast, RNS reactions appear to contribute signicantly to transformation of some organic contaminants with primary aromatic amine or phenol functional groups in treatment systems with high NO 2…”

Response to “Comment on ‘Role of Ammonia Oxidation in Organic Micropollutant Transformation during Wastewater Treatment’: Overlooked Evidence to the Contrary”

“…W e thank Profs. Fenner and Men for their critical comments 1 on our manuscript that examined the role of ammonia oxidation in organic micropollutant (OMP) transformation during wastewater treatment. 2 Their comments highlight the challenge in unraveling the contribution of different functional microorganisms in OMP transformation as well as their individual contributions at the community and system level.…”