Effects of chelating agents on trace metal speciation and bioavailability

The effect of chelating agents on metal bioavailability is discussed in terms of interactions between metals, chelating agents, microorganisms, aquatic organisms and plants. The statement "chelating agents are not bioavailable" was found not to hold true for all organisms. Microorganisms that are able to degrade chelating agents have specific, species-dependent transporters for their uptake. Some chelates are also as toxic as the free metal ion to microorganisms (e.g. HgEDTA). Exceptions to the free metal ion activity model (FIAM) are also observed for some higher aquatic organisms. Results from hydroponics experiments also clearly showed that metal uptake into plant shoots did not follow the FIAM model in the presence of chelants. Several possible explanations for the observed departures from the FIAM are discussed: the presence of kinetically distinct transport systems for solutes in higher plants including high-affinity ion specific transport systems and indiscriminate apoplastic transport of solutes via breakages in the endodermis, the possible existence of nonspecific uptake sites, as well as the ability of plants to control and regulate ion uptake via shoot-root communication and modifications in the root plasma membranes.