New Roles for Siderophores In the Biogeochemical Cycling of Contamiant Metals.

The fate and transport of contaminant metals in soils and natural waters is regulated by myriad molecular scale processes, including sorption, dissolution, and complexation reactions. These processes, and thus the mobility and bioavailability of metals, are in turn partially controlled by the presence of biomolecules exuded by plants and microbes. Siderophores, biogenic chelating agents with high affinities for iron that are exuded by microbes and plants, have a potentially strong influence on metal speciation and transport because the also may form strong complexes with specific oxidation states of contaminant metals. To better understand the possible effects of siderophore complexation on metal mobility and bioavailability, we conducted a series of spectroscopic, computational, and chemical experiments to relate the stability and structure of metal-siderophore complexes to the mechanisms of their formation and degradation in the environment. Experiments conducted using a representative suite of siderophores containing differing combinations of hydroxamate, catecholate, and α-hydroxycarboxylate moieties show that all structures may mobilize metals from (hydr)oxide surfaces. Metal siderophore complexes, which may result from dissolution reactions at specific pH, have high stability constants and thus may be environmentally persistent. Despite these high stabilities, metal complexes may still sorb strongly to mineral surfaces as variety of different surface structures. The results suggest that siderophores strongly influence the fate, transport, and bioavailability of metals in terrestrial and aquatic ecosystems.