Weak magnetic field promotes denitrification by stimulating ferromagnetic ion-containing metalloprotein expression.

Weak magnetic field (WMF) has been recognized to promote biological denitrification processes; however, the underlying mechanisms remain largely unexplored, hindering the optimization of its effectiveness. Here, we systematically investigated the effects of WMF on denitrification performance, enzyme activity, microbial community, and metaproteome in packed bed bioreactors treating high nitrate wastewater under different WMF intensities and C:N ratios. Results showed that WMFs significantly promoted denitrification by consistently stimulating the activities of denitrifying reductases and NAD/NADH biosynthesis across decreasing C:N ratios. Reductases and electron transfer enzymes involved in denitrification were overproduced due to the significantly enriched overexpression of ferromagnetic ion-containing (FIC) metalloproteins. We also observed WMFs' intensity-dependent selective pressure on microbial community structures despite the effects being limited compared to those caused by changing C:N ratios. By coupling genome-centric metaproteomics and structure prediction, we found the dominant denitrifier, Halomonas, was outcompeted by Pseudomonas and Azoarcus under WMFs, likely due to its structural deficiencies in iron uptake, suggesting that advantageous ferromagnetic ion acquisition capacity was necessary to satisfy the substrate demand for FIC metalloprotein overproduction. This study advances our understanding of the biomagnetic effects in the context of complex communities and highlights WMF's potential for manipulating FIC protein-associated metabolism and fine-tuning community structure.