It has been widely accepted that iron plays an important role in stimulating the activity of anammox bacteria, which contain many iron clusters for electron transport in cells. However, whether anammox bacteria could directly use and how to uptake Fe(III) have been long-time ignored. Here, we found that micrometer-scale magnetite with the size of 10-20 μm significantly promoted the anammox bacterial activity by iron core and iron uptake. Anammox bacteria cannot utilize Fe(III) directly as they are unable to secrete siderophore for the extracellular Fe(III) transfer to intracellular. In anaerobic anammox consortia at the presence of magnetite, siderophore synthesis bacteria belonging to