Occurrence of hydroxamate siderophore iron chelators in soils

Abstract

Although iron is abundant in soils (1–6%), it is often unavailable to plants because its solubility is dependent on pH and controlled by the low solubility of ferric oxides. Iron availability to plant roots may thus depend on organic chelators which Lindsay reports would maintain an adequate iron supply by diffusion and mass flow at concentrations as low as 10−8 M1,2. Hydroxamate siderophores (HS), microbially produced, ferric-specific, iron transport molecules with stability constants3 as high as 1032, may represent the chelators which maintain these soil iron concentrations. Such peptide derivatives were shown to control iron availability in aquatic ecosystems4, but little is known about their soil role beyond their function as microbial growth factors5,6. We report here the occurrence of HS in aqueous extracts of a variety of soils in concentrations (10−7 M–10−8 M after correction to 10% soil moisture) sufficient to affect plant nutrition1,2.