Sunlight-activated semiconducting iron minerals in anammox granules: Stimulating metabolic cooperation, suppressing...

Anaerobic ammonium oxidation (anammox) is a cost-effective biological nitrogen removal technology. An interesting phenomenon has been largely overlooked for anammox whose activity may be regulated by sunlight, which is ubiquitous yet inconspicuous. Here, we demonstrate that sunlight-activated naturally abundant semiconductor iron minerals (hematite, magnetite, and goethite) enhance metabolic cooperation, suppress nitrate accumulation, and mitigate oxidative stress in anammox granules. Under illumination, nitrate accumulation decreased by up to 43 %. Dominant Candidatus Brocadia increased its community activity from 68.2 % to 72.5 %, with hzs and hdh upregulated 1.5- and 1.1-fold. The accumulation of tricarboxylic acid cycle intermediates reflected accelerated carbon flux. Iron minerals mitigated photoinduced oxidative stress by stabilizing granular structures and improving cell viability. Illumination upregulated glutathione-S-transferase, a function likely supported by Pseudomonadota given the absence of glutathione biosynthesis in anammox bacteria. These findings suggest that sunlight-activated iron minerals stimulate anammox-mediated nitrogen removal, offering a sustainable strategy for wastewater treatment.