Assessing the Dietary Bioavailability of Metals Associated with Natural Particles: Extending the Use of the Reverse Labeling Approach to Zinc

Sediment Ballet: Unveiling the Dynamics of Metal Bioavailability in Sediments Following Resuspension and Reequilibration

Assessing the risk of metal-contaminated sediments under disturbed conditions is challenging due to the lack of methods that capture instant changes in metal bioavailability. Existing approaches provide inadequate understandings of the processes regulating metal bioavailability under nonequilibrium conditions. Experiments were conducted to improve our understanding of the metal bioavailability dynamics induced by sediment resuspension and subsequent redeposition (reequilibration). An isotopically modified bioassay, a novel stable isotope tracing technique, was used to measure metal bioavailability (assimilation rates) to clams within short time windows. Changes in metal partitioning were characterized by porewater analysis using in situ extraction and the diffusive gradients in thin-films technique. Results showed that sediment resuspension released metals into porewater, while reequilibration scavenged metals from the porewater. The assimilation rates of Ni, Cu, and Pb increased with the resuspension time, aligning with increasing porewater concentrations. Unexpectedly, during reequilibration, the metal assimilation rates did not decrease. The discrepancies between bioavailability to the clam and porewater extrapolations may be due to differing sustained conditions of metals in sediments. Overall, this study unveils the metal bioavailability dynamics in nonequilibrium sediments, which could not be accurately predicted relying solely on porewater analysis. Incorporating rapid bioassays to determine bioavailability offers a valuable tool for robust ecological risk assessment.

A Multidisciplinary Approach That Considers Occurrence, Geochemistry, Bioavailability, and Toxicity to Prioritize Critical Minerals for Environmental Research

Critical minerals (or critical elements) are minerals or elements that are essential to global security and development and have supply chains vulnerable to disruption. In general, knowledge of the environmental behavior and health effects of critical elements is needed to support the development of safe and environmentally responsible supplies. This knowledge includes identifying potential consequences of increased critical element production and use, alternative critical element sources such as mine wastes, and adverse effects of critical elements on ecosystem condition and organismal health. Here we identify significant data gaps in the understanding of critical elements in surficial and aquatic environments, and the need, given the large number of commodities (50) identified on the 2022 critical minerals list for the United States, for an approach to prioritize them for study of their environmental fate and effects. We propose a multidisciplinary approach for this prioritization, considering measures of occurrence, geochemistry, bioavailability, and toxicity. We describe relatively easy-to-obtain metrics for each of these topic areas and demonstrate the utility of this integrated prioritization approach using indium and zinc as examples. This approach facilitates prioritizing research with a focus on those critical elements that are most mobile in the environment, bioavailable, toxic, or simply lacking data in these categories.

A biodynamic model predicting copper and cadmium bioaccumulation in caddisflies: Linkages between field studies and laboratory exposures

Hydropsyche and Arctopsyche are filter-feeding caddisflies (Order: Trichoptera; Family: Hydropsychidae) that are commonly used to monitor metal exposures in rivers. While tissue residue concentrations provide important bioaccumulation data regarding metal bioavailability, they do not provide information regarding the mechanisms of uptake and loss, or exposure history. This study examined the physiological processes that control Cu and Cd uptake and loss using a biokinetic bioaccumulation model. Larvae of each taxon were experimentally exposed to either water or food enriched with stable isotopes (65Cu and 106Cd). Dissolved Cu uptake (ku) was similar between species (2.6-3.4 L-1g 1d-1), but Cd uptake was 3-fold higher in Hydropsyche than Arctopsyche (1.85 L-1g 1d-1 and 0.60 L-1g 1d-1, respectively). Cu and Cd efflux rates (ke) were relatively fast (0.14 d-1-0.24 d-1) in both species, and may explain, in part, their metal tolerance to mine-impacted rivers. Food ingestion rates (IR), assimilation efficiency (AE) of 65Cu and 106Cd from laboratory diets were also derived and used in a biodynamic model to quantify the relative contribution of dissolved and dietary exposure routes. Results from the biodynamic model were compared to tissue concentrations observed in a long-term field study and indicated that because dissolved Cu and Cd exposures accounted for less than 20% of body concentrations of either taxon, dietary exposure was the predominant metal pathway. An estimation of exposure history was determined using the model to predict steady state concentrations. Under constant exposure conditions (dissolved plus diet), steady state concentrations were reached in less than 30 days, an outcome largely influenced by rapid efflux (ke).

Enhancing Sediment Bioaccumulation Predictions: Isotopically Modified Bioassay and Biodynamic Modeling for Nickel Assessment

Quantifying metal bioaccumulation in a sedimentary environment is a valuable line of evidence when evaluating the ecological risks associated with metal-contaminated sediments. However, the precision of bioaccumulation predictions has been hindered by the challenges in accurately modeling metal influx processes. This study focuses on nickel bioaccumulation from sediment and introduces an innovative approach using the isotopically modified bioassay to directly measure nickel assimilation rates in sediment. Tested in sediments spiked with two distinct nickel concentrations, the measured Ni assimilation rates ranged from 35 to 78 ng g-1 h-1 in the Low-Ni treatment and from 96 to 320 ng g-1 h-1 in the High-Ni treatment. Integrating these rates into a biodynamic model yielded predictions of nickel bioaccumulation closely matching the measured results, demonstrating high accuracy with predictions within a factor of 3 for the Low-Ni treatment and within a factor of 1 for the High-Ni treatment. By eliminating the need to model metal uptake from various sources, this streamlined approach provides a reliable method for predicting nickel bioaccumulation in contaminated sediments. This advancement holds promise for linking bioaccumulation with metal toxicity risks in sedimentary environments, enhancing our understanding of metal-contaminated sediment risks and providing valuable insights to support informed decision-making in ecological risk assessment and management.

Isotopically Modified Bioassay Bridges the Bioavailability and Toxicity Risk Assessment of Metals in Bedded Sediments

The application of bioavailability-based risk assessment for the management of contaminated sediments requires new techniques to rapidly and accurately determine metal bioavailability. Here, we designed a multimetal isotopically modified bioassay to directly measure the bioavailability of different metals by tracing the change in their isotopic composition within organisms following sediment exposure. With a 24 h sediment exposure, the bioassay sensed significant bioavailability of nickel and lead within the sediment and determined that cadmium and copper exhibited low bioavailable concentrations and risk profiles. We further tested whether the metal bioavailability sensed by this new bioassay would predict the toxicity risk of metals by examining the relationship between metal bioavailability and metal toxicity to chironomid larvae emergence. A strong dose-toxicity relationship between nickel bioavailability (nickel assimilation rate) and toxicity (22 days emergence ratio) indicated exposure to bioavailable nickel in the sediment induced toxic effects to the chironomids. Overall, our study demonstrated that the isotopically modified bioassay successfully determined metal bioavailability in sediments within a relatively short period of exposure. Because of its speed of measurement, it may be used at the initial screening stage to rapidly diagnose the bioavailable contamination status of a site.

Effects and bioaccumulation of Cr(III), Cr(VI) and their mixture in the freshwater mussel Corbicula fluminea

Chromium has two main oxidation states, Cr(III) and Cr(VI), that can occur simultaneously in natural waters. Current consensus holds that Cr(VI) is of high ecotoxicological concern, but regards Cr(III) as poorly bioavailable and relatively non-toxic. In this work, the effects and bioaccumulation of Cr(III), Cr(VI) and their mixture were studied using the freshwater clam Corbicula fluminea as a model organism. Mixture exposures were carried out using solutions isotopically enriched in ⁵⁰Cr(III) or ⁵³Cr(VI), allowing to quantify the contribution of each redox form to total Cr accumulation in the clams. Following exposure to individual redox forms, Cr(III) accumulated preferentially in the digestive glands and Cr(VI) in the gills of C. fluminea. In mixture exposures, both redox forms accumulated mainly in the gills; the concentration of Cr(III) in the digestive glands being much lowered compared with individual exposures. Both oxidation states affected the expression of biomarkers related to energy reserves, cellular damage and mitochondrial functioning, as well as the expression of mRNA for detoxification genes. The observed effects differed between gills and digestive glands. The present study suggests that Cr(III) is a bioavailable and biologically active elemental species deserving more consideration by the ecotoxicological community.

Application of a Multi-Metal Stable-Isotope-Enriched Bioassay to Assess Changes to Metal Bioavailability in Suspended Sediments

The direct measurement of particulate contaminant bioavailability is a challenging aspect for the environmental risk assessment of contaminated sites. Here, we demonstrated a multi-metal stable-isotope-enriched bioassay to simultaneously measure the bioavailability of Cd, Cu, and Zn in naturally contaminated sediments following differing periods of resuspension treatment. Freshwater filter-feeding clams were pre-labeled with the isotopes ¹¹⁴Cd, ⁶⁵Cu, and ⁶⁸Zn to elevate isotope abundances in their tissues and then exposed to metal-contaminated suspended sediments. The assimilation of sediment-associated metals by clams would decrease the isotope ratios (Cd^114/111, Cu^65/63, and Zn^68/64) in tissues, providing a direct measurement of metal bioavailability. For the sediments tested here, the method revealed bioavailable cadmium and non-bioavailable copper in sediments but was inconclusive for zinc. With a longer resuspension time, the bioavailability of particulate cadmium increased, but that of copper was unaffected. Metal bioavailability predicted using traditional wet-chemical extraction methods was inconsistent with these findings. The study indicated that multi-metal stable-isotope-enriched bioassay provides a new tool for directly assessing metal bioavailability in sediments, and this method is amenable for use in in situ assessments.