Genomic potential for complete denitrification by Desulfobacillus in anammox bioreactors

Desulfobacillus are frequently observed as abundant flanking denitrifying microorganisms in anammox bioreactors and wastewater treatment systems. Previous studies suggested that Desulfobacillus lacked canonical nitric oxide reductase, restricting them to incomplete denitrification. As a result, they were thought to provide NO to anammox bacteria and act as an N₂O sink. In this study, 48 Desulfobacillus genomes from GTDB were analyzed, including 21 from anammox-related bioreactors. Multiple potential nitric oxide reductases (eNOR, nNOR, and sNOR) were identified, and all 21 anammox-associated genomes encode at least one candidate nitric oxide reductase, with 5 also encoding a potential nitric oxide dismutase (NOD). The phylogeny, gene context, conserved active site residues, and 3D structures of these NO-transforming enzymes were further investigated. The findings indicated most Desulfobacillus possess the genomic potential for complete denitrification in anammox bioreactors. Furthermore, a discrepancy between 16S rRNA gene amplicon analysis and metagenomic analysis was observed. A proportion of Desulfobacillus 16S rRNA genes may be misclassified as Denitratisoma by the SILVA database. As Denitratisoma was previously detected in especially high abundance (>20%) in anammox bioreactors, this misclassification even highlights the importance of Desulfobacillus in anammox and other nitrogen removal processes.