Iron Modulates the Growth and Activity of Nitrate-Dependent Methanotrophic Bacteria by Reprogramming Carbon Metabolism

Iron is indispensable for literally all microorganisms, yet becomes toxic at elevated levels. Protein-based iron storage compartments, such as ferritins, play a key role in maintaining iron homeostasis when the iron level surpasses microbial requirements. However, the energy-intensive nature of iron storage raises questions about how microbes balance this bioprocess between growth and metabolism. Here, using nitrate-dependent methanotrophic bacteria with the simplified metabolic system as a model, we propose a novel metabolic reprogramming pathway regulated by iron storage that controls the balance between growth and activity. Isotopic labeling and meta-omics analyses revealed a striking contrast between bacterial abundance and methane-dependent denitrification activity in "Ca. M. sinica". Using microscopy and energy dispersive spectroscopy, we identified iron-rich nanoparticles within cells exposed to 40 μM Fe2+, alongside increased expression of genes involved in iron metabolism and methane oxidation coupled with denitrification. Additionally, we observed a shift from the energy-demanding Calvin cycle to the more energy-efficient serine pathway for carbon fixation, promoting the synthesis of glycine and succinyl-CoA, which serve as key precursors for iron storage proteins. These metabolic adjustments highlight a strategy for coordinating both substance and energy metabolism in nitrate-dependent methanotrophic bacteria, thereby enhancing their capacity for simultaneous nitrogen and carbon removal. Our findings reveal that iron may act as a metabolic "switch" in microorganisms, offering new insights into the targeted manipulation of microbial metabolism to maximize their beneficial functions in both engineered and natural environments.