Horizontal gene transfer drives DNRA capacity in wastewater treatment systems

Anammox bacteria require large amounts of iron for hydrazine synthase (HZS) and hydrazine oxidoreductase (HZO). By analyzing 8 anammox genomes across four genera, we found that only Candidatus Brocadia sinica harbors a compact genomic island (100 kb separation). In the dispersed species Ca. Kuenenia stuttgartiensis, transcriptomic data revealed a 300- to 1500-fold excess of hzs over iron genes, indicating severe expression uncoupling. Thus, physical co-localization of iron support genes with anammox core enzymes is rare but exists in one Brocadia lineage, potentially enabling better co-regulation. These findings provide a genomic basis for predicting iron responsiveness across anammox species in engineered systems. Significance StatementOnly Candidatus Brocadia sinica harbors a compact 100 kb separation). Dispersed species exhibit 3001500-fold expression mismatch between hzs and iron genes. Physical co-localization may enable co-regulation of iron supply with anammox metabolism. Genomic architecture predicts lineage-specific iron responsiveness in anammox engineering.