Radiolabelled Cyclic Bisarylmercury: High Chemical and in†vivo Stability for Theranostics.

We show the synthesis of an in vivo stable mercury compound with functionality suitable for radiopharmaceuticals. The designed cyclic bisarylmercury was based on the water tolerance of organomercurials, higher bond dissociation energy of Hg-Ph to Hg-S, and the experimental evidence that acyclic structures suffer significant cleavage of one of the Hg-R bonds. The bispidine motif was chosen for its in vivo stability, chemical accessibility, and functionalization properties. Radionuclide production results in 197(m) HgCl2 (aq), so the desired mercury compound was formed via a water-tolerant organotin transmetallation. The Hg-bispidine compound showed high chemical stability in tests with an excess of sulfur-containing competitors and high in vivo stability, without any observable protein interaction by human serum assay, and good organ clearance demonstrated by biodistribution and SPECT studies in rats. In particular, no retention in the kidneys was observed, typical of unstable mercury compounds. The nat Hg analogue allowed full characterization by NMR and HRMS.

Meitner-Auger Electron Emitters for Targeted Radionuclide Therapy: Mercury-197m/g and Antimony-119

Targeted Radionuclide Therapies (TRTs) based on Auger emitting radionuclides have the potential to deliver extremely selective therapeutic payloads on the cellular level. However, to fully exploit this potential, suitable radionuclides need to be applied in combination with appropriate delivery systems. In this review, we summarize the state-of-the-art production, purification, chelation and applications of two promising candidates for Targeted Auger Therapy, namely antimony- 119 (119Sb) and mercury-197 (197Hg). Both radionuclides have great potential to become efficient tools for TRT. We also highlight our current progress on the production of both radionuclides at TRIUMF and the University of Wisconsin.

High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales (Globicephala melas).

Whales accumulate mercury (Hg), but do not seem to show immediate evidence of toxic effects. Analysis of different tissues (liver, kidney, muscle) and biofluids (blood, milk) from a pod of stranded long-finned pilot whales (Globicephala melas) showed accumulation of Hg as a function of age, with a significant decrease in the MeHg fraction. Isotopic analysis revealed remarkable differences between juvenile and adult whales. During the first period of life, Hg in the liver became isotopically lighter (δ202Hg decreased) with a strongly decreasing methylmercury (MeHg) fraction. We suggest this is due to preferential demethylation of MeHg with the lighter Hg isotopes and transport of MeHg to less sensitive organs, such as the muscles. Also changes in diet, with high MeHg intake in utero and during lactation, followed by increasing consumption of solid food contribute to this behavior. Interestingly, this trend in δ202Hg is reversed for livers of adult whales (increasing δ202Hg value), accompanied by a progressive decrease of δ202Hg in muscle at older ages. These total Hg (THg) isotopic trends suggest changes in the Hg metabolism of the long-finned pilot whales, development of (a) detoxification mechanism(s) (e.g., though the formation of HgSe particles), and Hg redistribution across the different organs.

Bioaccumulation of methylmercury in a marine copepod.

Methylmercury (MeHg) is known to biomagnify in marine food chains, resulting in higher concentrations in upper trophic level animals than their prey. To better understand how marine copepods, an important intermediate between phytoplankton and forage fish at the bottom of the food chain, assimilate and release MeHg, the authors performed a series of laboratory experiments using the gamma-emitting radiotracer 203 Hg2+ and Me203 Hg with the calanoid copepod Acartia tonsa. Assimilation efficiencies of Hg2+ and MeHg ranged from 25% to 31% and 58% to 79%, respectively, depending on algal diets. Assimilation efficiencies were positively related to the fraction of Hg in the cytoplasm of the algal cells that comprised their diet. Efflux rates of Hg2+ (0.29/d) and MeHg (0.21/d) following aqueous uptake were similar, but efflux rates following dietary uptake were significantly lower for MeHg (0.11-0.22/d) than Hg2+ (0.47-0.66/d). The calculated trophic transfer factors in copepods were >1 for MeHg and consistently low (≤0.2) for Hg2+ . The authors used the parameters measured to quantitatively model the relative importance of MeHg sources (water or diet) for copepods and to predict the overall MeHg concentrations in copepods in different marine environments. In general, MeHg uptake from the diet accounted for most of the body burden in copepods (>50%). For an algal diet with a MeHg dry weight bioconcentration factor ≥106 , >90% of a copepod's MeHg body burden can be shown to derive from the diet. The model-predicted MeHg concentrations in the copepods were comparable to independent measurements for copepods in coastal and open-ocean regions, implying that the measured parameters and model are applicable to natural waters. Environ Toxicol Chem 2017;36:1287-1293. © 2016 SETAC.

Metallic profile of whole blood and plasma in a series of 106 healthy volunteers.

In 2003, we simultaneously quantified 27 metals by inductively coupled plasma-mass spectrometry (ICP-MS) in the whole blood, plasma and urine of 100 healthy volunteers. We again determined the metallic profile in whole blood and plasma during 2012. ICP-MS validated multielementary method was performed for metals in whole blood and plasma. Whole blood vanadium and chromium were quantified using ICP-MS collision cell technology. The aims of the study were to compare and assess any changes in this profile, particularly due to the environment. Healthy male/female staff volunteers (n = 106) with no professional exposure to metals, or medication containing lithium, strontium; or food supplements with trace elements and vitamins and with no metal prosthesis were included. Tobacco consumption and the number of dental amalgams were recorded. Our results demonstrated a blood lead level that had drastically decreased, i.e. reduced by half, during this period (12.5 versus 26.3 µg/L, P < 0.0001). Known differences were observed between males and females for copper and zinc; cadmium and lead were higher in smokers. Median plasmatic mercury, a specific test for dental amalgam exposure, did not significantly increase (0.38 versus 0.28 µg/L, P = 0.11). The ICP-MS metallic profile is a very practical concept that is useful for clinical, forensic and environmental toxicology, including industrial hygiene monitoring.

Investigation of activation cross-sections of proton induced nuclear reactions on natTl up to 42MeV: review, new data and evaluation.

Cross-sections of proton induced nuclear reactions on natural thallium have been studied for investigation of the production of the medical important (201)Tl diagnostic radioisotope. The excitation functions of (204m)Pb, (203)Pb, (202m)Pb, (201)Pb, (200)Pb, (199)Pb, (202)Tl (direct, cumulative), (201)Tl (direct, cumulative), (200)Tl(direct), and (203)Hg were measured up to 42MeV proton energy by stacked foil technique and activation method. The experimental data were compared with the critically analyzed experimental data in the literature, with the IAEA recommended data and with the results of model calculations by using the ALICE-IPPE, EMPIRE-II and TALYS codes.

Mercury speciation in seafood using isotope dilution analysis: a review.

Mercury is a toxic compound that can contaminate humans through food and especially via fish consumption. Mercury's toxicity depends on the species, with methylmercury being the most hazardous form for humans. Hg speciation analysis has been and remains a widely studied subject because of the potential difficulty of preserving the initial distribution of mercury species in the analysed sample. Accordingly, many analytical methods have been developed and most of them incur significant loss and/or cross-species transformations during sample preparation. Therefore, to monitor and correct artefact formations, quantification by isotope dilution is increasingly used and provides significant added value for analytical quality assurance and quality control. This review presents and discusses the two different modes of application of isotope dilution analysis for elemental speciation (i.e. species-unspecific isotope dilution analysis and species-specific isotope dilution analysis) and the different quantification techniques (i.e. classical and multiple spike isotope dilution analyses). Isotope tracers are thus used at different stages of sample preparation to determine the extent of inter-species transformations and correct such analytical artefacts. Finally, a synthesis of the principal methods used for mercury speciation in seafood using isotope dilution analysis is presented.

Subcellular controls of mercury trophic transfer to a marine fish.

Different behaviors of inorganic mercury [Hg(II)] and methylmercury (MeHg) during trophic transfer along the marine food chain have been widely reported, but the mechanisms are not fully understood. The bioavailability of ingested mercury, quantified by assimilation efficiency (AE), was investigated in a marine fish, the grunt Terapon jarbua, based on mercury subcellular partitioning in prey and purified subcellular fractions of prey tissues. The subcellular distribution of Hg(II) differed substantially among prey types, with cellular debris being a major (49-57% in bivalves) or secondary (14-19% in other prey) binding pool. However, MeHg distribution varied little among prey types, with most MeHg (43-79%) in heat-stable protein (HSP) fraction. The greater AEs measured for MeHg (90-94%) than for Hg(II) (23-43%) confirmed the findings of previous studies. Bioavailability of each purified subcellular fraction rather than the proposed trophically available metal (TAM) fraction could better elucidate mercury assimilation difference. Hg(II) associated with insoluble fraction (e.g. cellular debris) was less bioavailable than that in soluble fraction (e.g. HSP). However, subcellular distribution was shown to be less important for MeHg, with each fraction having comparable MeHg bioavailability. Subcellular distribution in prey should be an important consideration in mercury trophic transfer studies.

Production of the mercury-197 through proton induced reaction on gold.

The excitation function of the proton exchange reactions on gold was investigated in the energy range from threshold up to 20MeV. We presented our experimental results upto 14MeV stacked-foil technique was used, the target was of high purity gold foils (99.99%.) Reactions cross-sections and excitation functions were studied. Both isomeric level cross sections of (197)Hg were measured and calculated theoretically using the DDHMS routine of the EMPIRE-II (v2.19) code. Isomeric cross-section ratios for the pair (197m,g)Hg were calculated and presented as a function of proton energy.

Biokinetics of Hg and Pb accumulation in the encapsulated egg of the common cuttlefish Sepia officinalis: radiotracer experiments.

Uptake and depuration kinetics of dissolved (203)Hg and (210)Pb were determined during the entire embryonic development of the eggs of the cuttlefish, Sepia officinalis (50d at 17 degrees C). (203)Hg and (210)Pb were accumulated continuously by the eggs all along the development time reaching load/concentration ratio (LCR) of 467+/-43 and 1301+/-126g, respectively. During the first month, most of the (203)Hg and (210)Pb remained associated with the eggshell indicating that the latter acted as an efficient shield against metal penetration. From this time onwards, (203)Hg accumulated in the embryo, indicating that it passed through the eggshell, whereas (210)Pb did not cross the chorion during the whole exposure time. It also demonstrated that translocation of Hg associated with the inner layers of the eggshell is a significant source of exposure for the embryo. This study highlighted that the maturing embryo could be subjected to the toxic effects of Hg in the coastal waters where the embryonic development is taking place.

Benthic methylmercury production in lacustrine ecosystems of Nahuel Huapi National Park, Patagonia, Argentina.

Seasonal trends of benthic methylmercury (methyl-Hg) production were examined in both littoral and open water sites of three lakes (Escondido, Moreno, and Morenito) in the North Andean Patagonia region of Argentina. Potentials of methyl-Hg production were measured by amending sediment samples with inorganic (197)Hg(II), incubating for either 24 and 32 h at room temperature, and subsequently assaying the radiolabelled organomercury produced. Seasonal variations of benthic methyl-Hg production were studied but no significant correlation was observed. Lake littoral sites exhibited up to two fold higher methyl-Hg production potentials in most cases. Sediment from lakes Moreno and Morenito generally exhibited much lower (up to 10 fold) methyl-Hg production potentials than those from Lake Escondido, possibly due to differences in particulate and dissolved organic matter quantity and quality, which is higher in Lake Escondido and primarily allochthonous, whereas in lakes Moreno and Morenito is primarily autochthonous. This study represents the first to directly examine benthic microbial Hg(II)-methylation in aquatic ecosystems of Patagonia.

Assimilation and subcellular partitioning of elements by grass shrimp collected along an impact gradient.

Chronic exposure to polluted field conditions can impact metal bioavailability in prey and may influence metal transfer to predators. The present study investigated the assimilation of Cd, Hg and organic carbon by grass shrimp Palaemonetes pugio, collected along an impact gradient within the New York/New Jersey Harbor Estuary. Adult shrimp were collected from five Staten Island, New York study sites, fed (109)Cd- or (203)Hg-labeled amphipods or (14)C-labeled meals and analyzed for assimilation efficiencies (AE). Subsamples of amphipods and shrimp were subjected to subcellular fractionation to isolate metal associated with a compartment presumed to contain trophically available metal (TAM) (metal associated with heat-stable proteins [HSP - e.g., metallothionein-like proteins], heat-denatured proteins [HDP - e.g., enzymes] and organelles [ORG]). TAM-(109)Cd% and TAM-(203)Hg% in radiolabeled amphipods were approximately 64% and approximately 73%, respectively. Gradients in AE-(109)Cd% ( approximately 54% to approximately 75%) and AE-(203)Hg% ( approximately 61% to approximately 78%) were observed for grass shrimp, with the highest values exhibited by shrimp collected from sites within the heavily polluted Arthur Kill complex. Population differences in AE-(14)C% were not observed. Assimilated (109)Cd% partitioned to the TAM compartment in grass shrimp varied between approximately 67% and approximately 75%. (109)Cd bound to HSP in shrimp varied between approximately 15% and approximately 47%, while (109)Cd associated with metal-sensitive HDP was approximately 17% to approximately 44%. Percentages of assimilated (109)Cd bound to ORG were constant at approximately 10%. Assimilated (203)Hg% associated with TAM in grass shrimp did not exhibit significant variation. Percentages of assimilated (203)Hg bound to HDP ( approximately 47%) and ORG ( approximately 11%) did not vary among populations and partitioning of (203)Hg to HSP was not observed. Using a simplified biokinetic model of metal accumulation from the diet, it is estimated that site-specific variability in Cd AE by shrimp and tissue Cd burdens in field-collected prey (polychaetes Nereis spp.) could potentially result in up to approximately 3.2-fold differences in the dose of Cd assimilated by shrimp from a meal in the field. The results of this study also suggest that chronic field exposure can impact mechanisms of metal transport across the gut epithelium that do not influence carbon assimilation. Differences in the assimilation and subcellular partitioning of metal may have important implications for metal toxicity in impacted shrimp populations.

Contaminated soils (II): in vitro dermal absorption of nickel (Ni-63) and mercury (Hg-203) in human skin.

Dermal absorption of heavy metal soil contaminants was tested in vitro with chloride salts of radioactive nickel (Ni-63) and mercury (Hg-203). Aqueous soil suspensions, spiked with either Ni-63 or Hg-203, were applied to fresh viable human breast skin tissue in Bronaugh diffusion cells perfused with Hanks HEPES buffered (pH 7.4) receptor containing 4% bovine serum albumin (BSA). Receptor fractions were collected every 6 h for 24 h when skin was soap washed. Tests were conducted concurrently in triplicate with and without soil for each skin specimen. Mean percent dermal absorption including the skin depot for Ni-63 was 1 and 22.8% with and without soil, respectively, while for Hg-203, values of 46.6 and 78.3% were obtained. Excluding the skin depot and considering only absorption in receptor, there was 0.5 and 1.8% absorption of Ni-63 with and without soil, respectively, and 1.5 and 1.4% for Hg-203. The potential bioavailability of the skin depot is discussed in relation to dermal exposure to these metals in contaminated soil.

Methylmercury production in the water column of an ultraoligotrophic lake of Northern Patagonia, Argentina.

Methyl-mercury (CH3Hg+) production was studied in freshwaters from lake Moreno, an ultraoligotrophic system belonging to Northern Patagonia. Hg2+ labelled with high specific activity 197Hg was spiked to water samples in concentrations of 10 ng l(-1), and incubated in laboratory for 3d time trends under different conditions. Experimental water was sampled daily to evaluate CH3(197)Hg+ production. Lake water used in the experiments was sampled just below the upper limit of the metalimnion ( approximately 30 m depth), where maximum values of chlorophyll a have been measured previously. Sampling was performed in late autumn, when the plankton fraction <50 microm exhibited mercury concentrations up to 260 microg g(-1) dry weight. The experiments analysed lake water filtered through 50, 20, and 0.2 microm (filter-sterilized) mesh nets. ASTM grade 1 water was also incubated for control. All the experiments were run in an environmental chamber under controlled temperature and light regime. High Hg2+ conversion to CH3Hg+, up to 50%, was measured in lake water, in a process stimulated by light. CH3Hg+ production was two-fold higher after 3d of incubation with illumination compared to total darkness. Sterile lake water showed conversions up to 30%, while the planktonic components seem to enhance the CH3Hg+ production. Overall, our results provide evidence that lake Moreno waters favour CH3Hg+ production in processes stimulated by light. Although biotic components certainly contribute to enhance mercury methylation, water chemistry plays a key role in this process. We hypothesize that dissolved organic matter, particularly its quality, could be decisive.

Validation of methodology for determination of the mercury methylation potential in sediments using radiotracers.

Experiments to determine the mercury methylation potential were performed on sediments from two locations on the river Idrijca (Slovenia), differing in ambient mercury concentrations. The tracer used was the radioactive isotope (197)Hg. The benefit of using this tracer is its high specific activity, which enables spikes as low as 0.02 ng Hg(2+) g(-1) of sample to be used. It was therefore possible to compare the efficiency of the methylation potential experiments over a range of spike concentrations from picogram to microgram levels. The first part of the work aimed to validate the experimental blanks and the second part consisted of several series of incubation experiments on two different river sediments using a range of tracer additions. The results showed high variability in the obtained methylation potentials. Increasing Hg(2+) additions gave a decrease in the percentage of the tracer methylated during incubation; in absolute terms, the spikes that spanned four orders of magnitude (0.019-190 pg g(-1) of sediment slurry) resulted in MeHg formation between 0.01 and 0.1 ng MeHg g(-1) in Podroteja and Kozarska Grapa. Higher spikes resulted in slightly elevated MeHg production (up to a maximum of 0.27 ng g(-1)). The values of methylation potential were similar in both sediments. The results imply that the experimental determination of mercury methylation potential strongly depends on the experimental setup itself and the amount of tracer added to the system under study. It is therefore recommended to use different concentrations of tracer and perform the experiments in several replicates. The amount of mercury available for methylation in nature is usually very small. Therefore, adding very low amounts of tracer in the methylation potential studies probably gives results that have a higher environmental relevance. It is also suggested to express the results obtained in absolute amounts of MeHg produced and not just as the percentage of the added tracer.

Effect of radioactive and non-radioactive mercury on wheat germination and the anti-toxic role of glutathione.

Studies to test the noxious effect of mercury ions on wheat germination and seedling growth showed that germination rate, shoot length, and fresh weights varied as a function of their concentration in the treatment solutions. At the same concentration, the radioactive mercury proved to be more harmful to the living seeds and seedlings. The detoxification action of glutathione for both radioactive and non-radioactive mercury was also followed. After a seven-day period of germination in the presence of the investigated compounds, the wheat plantlets were cut from the seeds, and their height, weight, and residual radioactivity were measured. The shoot length decreased from 8.1 (blank) to 4.6 cm (non-radioactive mercury) or even to 2.5 cm ((203)Hg), while glutathione had both an anti-toxic and an anti-radiotoxic effect (6.4 and 6.0 cm, respectively). The root weight of the lot decreased from 1.7 to 0.7 g and 0.4 g, respectively, while glutathione showed a healing action (1.5 and 1.7 g). The radioactive ions accumulated especially in roots (35.5 %), and less in shoots (11.2 %). Results were statistically validated.

Methodological considerations regarding the use of inorganic 197Hg(II) radiotracer to assess mercury methylation potential rates in lake sediment.

Methodological considerations on the determination of benthic methyl-mercury (CH(3)Hg) production potentials were investigated on lake sediment, using (197)Hg radiotracer. Three methods to arrest bacterial activity were compared: flash freezing, thermal sterilization, and gamma-irradiation. Flash freezing showed similar CH(3)Hg recoveries as thermal sterilization, which was both 50% higher than the recoveries obtained with gamma-ray irradiation. No additional radiolabel was recovered in kill-control samples after an additional 24 or 65 h of incubation, suggesting that all treatments were effective at arresting Hg(II)-methylating bacterial activity, and that the initial recoveries are likely due to non-methylated (197)Hg(II) carry-over in the organic extraction and/or [(197)Hg]CH(3)Hg produced via abiotic reactions. Two CH(3)Hg extraction methods from sediment were compared: (a) direct extraction into toluene after sediment leaching with CuSO(4) and HCl and (b) the same extraction with an additional back-extraction step to thiosulphate. Similar information was obtained with both methods, but the low efficiency observed and the extra work associated with the back-extraction procedure represent significant disadvantages, even tough the direct extraction involves higher Hg(II) carry over.

Accumulation of inorganic and methylmercury by freshwater phytoplankton in two contrasting water bodies.

Phytoplankton concentrate mercury from their aqueous surroundings and represent the primary entry point for Hg in aquatic food webs. We used 203Hg to compare the uptake of inorganic mercury, Hg(II), and methylmercury, MeHg, in four phytoplankton species (a diatom, a chlorophyte, a cryptophyte, and a cyanobacterium) in two waters containing different concentrations of dissolved organic carbon (DOC). At steady state, volume concentration factors (VCFs) for Hg(II) in the four species were similar and ranged from 0.5 to 5 x 10(4) for both water types, whereas VCFs for MeHg exceeded those for Hg(II) and ranged from 1.3 to 14.6 x 10(5). The VCFs for MeHg in the three eukaryotic cells in the high DOC water were 2-2.6 times greater than those in the low DOC water, but the VCFs for the prokaryote were similar in both waters. Higher cell surface area to volume ratios correlated with increased MeHg concentrations but not with Hg(II). In both water types, VCFs of Hg(II) were similar for living and heat-killed cells, but the VCFs of MeHg were 1.5-5.0 times greater in living cells, suggesting an active uptake component for MeHg. Hg(II) and MeHg were entirely bound to cell surfaces of the dead cells, whereas 59-64% of the MeHg and 9-16% of the Hg(II) in living cells entered the cytoplasm.

Novel methodology for the study of mercury methylation and reduction in sediments and water using 197Hg radiotracer.

Mercury tracers are powerful tools that can be used to study mercury transformations in environmental systems, particularly mercury methylation, demethylation and reduction in sediments and water. However, mercury transformation studies using tracers can be subject to error, especially when used to assess methylation potential. The organic mercury extracted can be as low as 0.01% of the endogenous labeled mercury, and artefacts and contamination present during methylmercury (MeHg) extraction processes can cause interference. Solvent extraction methods based on the use of either KBr/H2SO4 or HCl were evaluated in freshwater sediments using 197Hg radiotracer. Values obtained for the 197Hg tracer in the organic phase were up to 25-fold higher when HCl was used, which is due to the coextraction of 197Hg2+ into the organic phase during MeHg extraction. Evaluations of the production of MeHg gave similar results with both MeHg extraction procedures, but due to the higher Hg2+ contamination of the controls, the uncertainty in the determination was higher when HCl was used. The Hg2+ contamination of controls in the HCl extraction method showed a nonlinear correlation with the humic acid content of sediment pore water. Therefore, use of the KBr/H2SO4 method is recommended, since it is free from these interferences. 197Hg radiotracer (T1/2=2.673 d) has a production rate that is about 50 times higher than that of 203Hg (T1/2=46.595 d), the most frequently used mercury radiotracer. Hence it is possible to obtain a similar level of performance to 203Hg when it is used it in short-term experiments and produced by the irradiation of 196Hg with thermal neutrons, using mercury targets with the natural isotopic composition. However, if the 0.15% natural abundance of the 196Hg isotope is increased, the specific activity of the 197Hg tracer can be significantly improved. In the present work, 197Hg tracer was produced from mercury 51.58% enriched in the 196Hg isotope, and a 340-fold increase in specific activity with respect to natural mercury targets was obtained. When this high specific activity tracer is employed, mercury methylation and reduction experiments with minimum mercury additions are feasible. Tracer recovery in methylation experiments (associated with Me197Hg production from 197Hg2+ spike, but also with Hg2+ contamination and Me197Hg artefacts) with marine sediments was about 0.005% g-1 WS (WS: wet sediment) after 20 h incubation with mercury additions of 0.05 ng g-1 WS, which is far below natural mercury levels. In this case, the amount of Hg2+ reduced to Hg0 (expressed as the percent 197Hg0 recovered with respect to the 197Hg2+ added) varied from 0.13 to 1.6% g-1 WS. Me197Hg production from 197Hg2+ spike after 20 h of incubation of freshwater sediment ranged from 0.02 to 0.13% g-1 WS with mercury additions of 2.5 ng g-1 WS, which is also far below natural levels. 197Hg0 recoveries were low, 0.0058+/-0.0013% g-1 WS, but showed good reproducibility in five replicates. Me197Hg production from 197Hg2+ spiked in freshwater samples ranged from 0.1 to 0.3% over a period of three days with mercury additions of 10 ng L-1. A detection limit of 0.05% for Me197Hg production from 197Hg2+ spike was obtained in seawater in a 25 h incubation experiment with mercury additions of 12 ng L-1.

Metal-solid interactions controlling the bioavailability of mercury from sediments to clams and sipunculans.

The bioavailability of sedimentary Hg(II) and methylmercury (MeHg) was quantified by measuring the assimilation efficiency (AE) in the clam Ruditapes philippinarum and the extraction of the gut juices from the sipunculan Sipunculus nudus. Three factors (Hg concentration in sediment, Hg sediment contact time, and organic content of sediments) were modified to examine metal-solid interactions in controlling Hg bioavailability. The Hg AEs in the clams were strongly correlated with the extraction from the sipunculan gut juices for both Hg species. The bioavailability of both Hg(II) and MeHg generally increased with increased sediment Hg concentration but decreased with sedimentmetal contact time and increasing organic content (except that MeHg was not influenced by organic content). Hg(II) speciation in sediments, quantified by sequential chemical extraction (SCE), was dependent on geochemical conditions and greatly controlled the mobility and bioavailability of Hg(II) in sediments. Most bioavailable Hg(II) originated from the strongly complexed phase (e.g., Hg bound up in Fe/Mn oxide, amorphous organosulfur, or mineral lattice), whereas Hg bound with the organocomplexed phase (Hg humic and Hg2Cl2) was not bioavailable. Hg bound with the other geochemical phases (water soluble, HgO, HgSO4, and HgS) contributed very little to the bioavailable Hg due to their low partitionings. Further, the amount of bioavailable Hg was inversely related to the particle reactivity of Hg with the sediments. Detailed analyses of metal-solid interactions provide a better understanding of how Hg in sediments can predict Hg concentration and therefore bioavailability in benthic invertebrates.