Overlooked siderophore producers favor ammonium oxidation in global wastewater treatment plants

Iron is essential for biological nitrogen removal in wastewater treatment plants (WWTPs), as a significant portion of microbial nitrogen-transforming enzymes require iron. However, iron bioavailability is a global challenge for nitrogen removal microbes in WWTPs, where it often exists in insoluble forms due to its complexation with various wastewater constituents.

Combined laboratory experiment and metagenomic analysis of 52 global WWTPs, we found that siderophore-producing bacteria (SPB) were previously uncharacterized dominant members in activated sludge. SPB enhance the iron uptake of activated sludge microbial communities by facilitating the transport of iron ions from insoluble sources into the cells. Of the 1328 total recovered metagenome-assembled genomes (MAGs) from global WWTPs, 6.2% were identified as SPB, while 79.3% of MAGs could utilize siderophores, indicating widespread sharing of siderophores in WWTPs. Interestingly, nearly all ammonium-oxidizing bacteria (AOB) from WWTPs lacked siderophore-producing capacity, and exogenous siderophore (20 µM pyochelin) addition boosted ammonium oxidation rates by 28.2%. Moreover, strong indications were found for an association between AOB and the SPB in global WWTPs, suggesting their symbiotic interaction is a common and critical process to maintain ammonium oxidation performance. SPB in WWTPs were predominantly aerobic or facultative anaerobic heterotrophic bacteria, exhibiting low taxonomic diversity but high abundance.

This study reveals SPB as previously overlooked but crucial contributors to biological nitrogen removal in global WWTPs, providing foundational insights into iron-based microbial cooperation within engineered systems. Modulating SPB activity based on their metabolic characteristics is a promising strategy to cope with low iron bioavailability issue for biological processes in WWTPs. Video Abstract.