A unique compact genomic island co-localizing iron and anammox genes in Candidatus Brocadia sinica, but not in other species

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 (<10kb) where hzs co-localizes with iron uptake (TonB, FeoAB) and Fe-S cluster assembly (NifU/NifS) genes. All other species show dispersed architectures (>100kb 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.