The chemical reactions and pathways resulting in the formation of siderophores, low molecular weight Fe(III)-chelating substances made by aerobic or facultatively anaerobic bacteria, especially when growing under iron deficient conditions. The complexes of Fe(3+)-siderophores have very high stability constants and are taken up by specific transport systems by microorganisms; the subsequent release of iron requires enzymatic action. [ http://www.ncbi.nlm.nih.gov/pubmed/20376388 ]