The response of organisms to metal exposure is dependent on the chemical and physical associations (speciation) as well as the concentration of the respective metal. Metal toxicity is regulated by the biogeochemical environment in which it exists, with the metals physical form, kinetic lability, and oxidation state mediating bioavailability. With few exceptions, the uptake rate of a trace metal by an organism is largely dependent on its “free” or hydrated concentration in the growth environment. The “free” or available metal is regulated by a complex series of competition reactions among aqueous ligands and cell membrane associated ligands that transport metals into the organism. Therefore, total and operationally defined “dissolved” metal concentrations are not necessarily predictive of toxicity or potential risk to the environment.
The objective of this project was to advance understanding of metal-ligand binding to further the development of practical and predictive models of trace metal bioavailability. The primary speciation focus was on strong metal-binding ligands and colloidal phases.
In this project, researchers isolated important metal-ligand pools and characterized the ligands and their metal-binding properties in three contrasting marine estuaries, focusing primarily on copper and to a lesser degree zinc. Chemical measures of metal speciation were linked to bioavailability as quantified in parallel laboratory studies with marine algae. Multiple bioavailability/toxicity endpoints were measured, including cellular budgets of trace metals, molecular biomarkers, and growth characteristics. Rigorous trace metal “clean” protocols were integrated throughout all components of the study, including the bioassay studies, and state-of-the-art analytical techniques were applied for speciation measurements. Chemical speciation tools included electrochemical methods, principally voltammetry (cathodic stripping voltammetry and anodic stripping voltammetry), and kinetic separations on chelating-resins. Ultrafiltration at 1 kDa and 10 kDa was also routinely applied to fractionate aquatic ligand pools. Three Department of Defense (DoD) impacted marine estuaries with major contrasts in ligand source, type, and abundance, as well as significant gradients in trace element levels were studied—San Diego Bay, California; Norfolk Harbor, Virginia; and Cape Fear, North Carolina.
Studies of the physical and kinetic speciation of copper and zinc in three impacted marine estuaries have shown that colloidal phases of copper and dissolved organic matter are coupled and dominate the “dissolved” pool in each system; however, in contrast to copper, colloidal fractions of zinc were very small and a super-majority of “dissolved” zinc in each system was kinetically labile. A strong relationship was observed between colloidal copper and kinetically non-labile copper species, implying very slow reactivity of colloidal copper, and the bioassay work validates this conclusion, documenting the protective role of colloidal phases. Large gradients in metal speciation and metal-binding ligand were measured between and within systems; however, the bioavailability studies indicate that copper uptake into algal cells can be accurately predicted from levels of strong ligand.
Current regulatory frameworks and metrics do not address metal speciation. Addressing speciation can enable more realistic estimations of acute and chronic risk from metal exposures. Results of this project strongly support efforts to develop speciation-based water quality criteria. The technology and models underpinning these findings are transferable to scientists within the Environmental Protection Agency (EPA) and DoD.