A flavoprotein. The enzyme catalyses the reduction of bound ferric iron in a variety of iron chelators (siderophores), resulting in the release of ferrous iron. The enzyme from the hyperthermophilic archaeon Archaeoglobus fulgidus is not active with uncomplexed Fe(III). cf. EC 188.8.131.52, ferric-chelate reductase (NADH) and EC 184.108.40.206, ferric-chelate reductase (NADPH).