When are metal complexes bioavailable?

Environmental contextThe concentration of a free metal cation has proved to be a useful predictor of metal bioaccumulation and toxicity, as represented by the free ion activity and biotic ligand models. However, under certain circumstances, metal complexes have been shown to contribute to metal bioavailability. In the current mini-review, we summarise the studies where the classic models fail and organise them into categories based on the different uptake pathways and kinetic processes. Our goal is to define the limits within which currently used models such as the biotic ligand model (BLM) can be applied with confidence, and to identify how these models might be expanded. AbstractNumerous data from studies over the past 30 years have shown that metal uptake and toxicity are often best predicted by the concentrations of free metal cations, which has led to the development of the largely successful free-ion activity model (FIAM) and biotic ligand model (BLM). Nonetheless, some exceptions to these classical models, showing enhanced metal bioavailability in the presence of metal complexes, have also been documented, although it is not yet fully understood to what extent these exceptions can or should be generalised. Only a few studies have specifically measured the bioaccumulation or toxicity of metal complexes while carefully measuring or controlling metal speciation. Fewer still have verified the fundamental assumptions of the classical models, especially when dealing with metal complexes. In the current paper, we have summarised the exceptions to classical models and categorised them into five groups based on the fundamental uptake pathways and kinetic processes. Our aim is to summarise the mechanisms involved in the interaction of metal complexes with organisms and to improve the predictive capability of the classic models when dealing with complexes.