Selenite reduction by mackinawite, magnetite and siderite: XAS characterization of nanosized redox products

Suboxic soils and sediments often contain the Fe(II)-bearing minerals mackinawite (FeS), siderite (FeCO3) or magnetite (FesO4), which should be able to reduce aqueous selenite, thereby forming solids of low solubility. While the reduction of selenate or selenite to Se(O) by green rust, pyrite and by Fe2+ sorbed to montmorillonite is a slow (weeks), kinetically limited redox reaction as demonstrated earlier, we show here that selenite is rapidly reduced within one day by nanoparticulate mackinawite and magnetite, while only one third of selenite is reduced by micrometer-sized siderite. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, red and gray elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2), was not observed. The local structures of the reaction products suggest formation of nanoscale clusters, which may be prone to colloid-facilitated transport, and may have a higher than expected solubility.     

Critical evaluation of the ability of sequential extraction procedures to quantify discrete forms of selenium in sediments and soils

Sequential extraction procedures (SEPs) have been widely used to characterize the mobility, bioavailibility, and potential toxicity of trace elements in soils and sediments. Although oft-criticized, these methods may perform best with redox-labile elements (As, Hg, Se) for which more discrete biogeochemical phases may arise from variations in oxidation number. We critically evaluated two published SEPs for Se for their specificity and precision by applying them to four discrete components in an inert silica matrix: soluble Se(VI) (selenate), Se(IV) (selenite) adsorbed onto goethite, elemental Se, and a metal selenide (FeSe; achavalite). These were extracted both individually and in a mixed model sediment. The more selective of the two procedures was modified to further improve its selectivity (SEP 2M). Both SEP 1 and SEP 2M quantitatively recovered soluble selenate but yielded incomplete recoveries of adsorbed selenite (64% and 81%, respectively). SEP 1 utilizes 0.1 M K2S2O8 to target "organically associated" Se, but this extractant also solubilized most of the elemental (64%) and iron selenide (91%) components of the model sediment. In SEP 2M, the Na2SO3 used in step III is effective in extracting elemental Se but also extracted 17% of the Se from the iron selenide, such that the elemental fraction would be overestimated should both forms coexist. Application of SEP 2M to eight wetland sediments further suggested that the Na2SO3 in step III extracts some organically associated Se, so a NaOH extraction was inserted beforehand to yield a further modification, SEP 2OH. Results using this five-step procedure suggested that the four-step SEP 2M could overestimate elemental Se by as much as 43% due to solubilization of organic Se. Although still imperfect in its selectivity, SEP 20H may be the most suitable procedure for routine, accurate fractionation of Se in soils and sediments. However, the strong oxidant (NaOCl) used in the final step cannot distinguish between refractory organic forms of Se and pyritic Se that might form under sulfur-reducing conditions.     

Factors affecting soluble selenium removal by a selenate-reducing bacterium Bacillus sp. SF-1

High concentrations of soluble selenium, selenate and selenite, have acute and chronic toxicity toward living things. With the aim of developing a biological process for selenium removal, the effects of a variety of parameters on the reduction of soluble selenium by a Bacillus sp. strain SF-1, which is capable of reductively transforming selenate into selenite and, subsequently, into nontoxic insoluble elemental selenium, were studied. The bacterial strain could effectively reduce 20 mM of selenate to selenite and 2 mM of selenite to elemental selenium in the presence of an appropriate carbon source and in the absence of oxygen. The reduction rate of selenate to selenite was much higher than that of selenite to elemental selenium, resulting in the transient accumulation of selenite during selenate reduction. The selenate reduction rate increased with increases in the selenate concentration up to 20 mM, while the rate of selenite reduction decreased sharply at selenite concentrations of more than 2 mM. The elemental selenium transformed from selenate via selenite was found both inside and outside the cells. Bacillus sp. SF-1 was able to utilize a variety of organic acids or sugars as a carbon source in selenate reduction. Although the copresence of sulfate did not inhibit selenate reduction, it was completely inhibited by some other oxyanions, including nitrate. A model sequencing batch system using the bacterial strain was developed and exhibited good performance in the treatment of wastewater containing high concentrations of selenate.     

Factors affecting soluble selenium removal by a selenate-reducing bacterium Bacillus sp. SF-1

High concentrations of soluble selenium, selenate and selenite, have acute and chronic toxicity toward living things. With the aim of developing a biological process for selenium removal, the effects of a variety of parameters on the reduction of soluble selenium by a Bacillus sp. strain SF-1, which is capable of reductively transforming selenate into selenite and, subsequently, into nontoxic insoluble elemental selenium, were studied. The bacterial strain could effectively reduce 20 mM of selenate to selenite and 2 mM of selenite to elemental selenium in the presence of an appropriate carbon source and in the absence of oxygen. The reduction rate of selenate to selenite was much higher than that of selenite to elemental selenium, resulting in the transient accumulation of selenite during selenate reduction. The selenate reduction rate increased with increases in the selenate concentration up to 20 mM, while the rate of selenite reduction decreased sharply at selenite concentrations of more than 2 mM. The elemental selenium transformed from selenate via selenite was found both inside and outside the cells. Bacillus sp. SF-1 was able to utilize a variety of organic acids or sugars as a carbon source in selenate reduction. Although the copresence of sulfate did not inhibit selenate reduction, it was completely inhibited by some other oxyanions, including nitrate. A model sequencing batch system using the bacterial strain was developed and exhibited good performance in the treatment of wastewater containing high concentrations of selenate.     

Release of reduced inorganic selenium species into waters by the green fresh water algae Chlorella vulgaris

The common green fresh water algae Chlorella vulgaris was exposed to starting concentrations of 10 μg/L selenium in the form of selenate, selenite, or selenocyanate (SeCN(-)) for nine days in 10% Bold's basal medium. Uptake of selenate was more pronounced than that of selenite, and there was very little uptake of selenocyanate. Upon uptake of selenate, significant quantities of selenite and selenocyanate were produced by the algae and released back into the growth medium; no selenocyanate was released after selenite uptake. Release of the reduced metabolites after selenate exposure appeared to coincide with increasing esterase activity in solution, indicating that cell death (lysis) was the primary emission pathway. This is the first observation of biotic formation of selenocyanate and its release into waters from a nonindustrial source. The potential environmental implications of this laboratory observation are discussed with respect to the fate of selenium in impacted aquatic systems, the ecotoxicology of selenium bioaccumulation, and the interpretation of environmental selenium speciation data generated, using methods incapable of positively identifying reduced inorganic selenium species, such as selenocyanate.     

Speciation of selenium in vitamin tablets using spectrofluorometry following cloud point extraction

Selenium is one of the trace and essential elements for good health but required only in a very narrow range. Hence, determination of selenium in trace level is important. In this work, cloud point extraction (CPE) with non-ionic surfactant Triton X-114 and the fluorometric ligand, 2,3-diaminonaphtalene (DAN) were used for extraction of trace amounts of organic and inorganic selenium species prior to their determination by spectrofluorometry. CPE parameters affecting complexation and phase separation were optimised. The limit of detection calculated by using nine replicate measurements of 0.020 mg/L Se solution after complexing with DAN and 10-fold CPE preconcentration was 2.1 μg/L. Accuracy of the method was checked using EnviroMat Waste Water, EU-L-2 as CRM and result was found to be in good agreement with the certified value. The suggested method can be used for selenium species of selenite, selenate, and total organic selenium at μg/L level.     

Metabolic pathway for selenium in the body: speciation by HPLC-ICP MS with enriched Se

Selenium (Se) is an ultramicro essential nutrient and both inorganic (selenite and selenate) and organic (selenocysteine and selenomethionine) forms of Se can be used as nutritional sources. Metabolic pathways for Se in the body were studied for selenite and selenate, with the use of enriched 82 Se, by speciation with separation by gel filtration HPLC and detection by element-specific mass spectrometry with ionization with inductively coupled argon plasma (HPLC-ICP MS). The concentrations of 82 Se in organs and body fluids and the distributions of their constituents depending on the dose and time after the intravenous administration of 82 Se-selenite and -selenate to rats were determined. Selenite was taken up by red blood cells within several minutes, reduced to selenide by glutathione, and then transported to the plasma, bound selectively to albumin and transferred to the liver. Contrary to selenite, intact selenate was either taken up directly by the liver or excreted into the urine. The 82 Se of selenite origin and that of selenate origin were detected in the forms of the two Se peak materials in the liver, A and B. The former one was methylated to the latter in vivo and in vitro . The latter one was identical with the major urinary metabolite and it was identified as Se-methyl- N -acetyl-selenohexosamine (selenosugar). The chemical species-specific metabolic pathway for Se was explained by the metabolic regulation through selenide as the assumed common intermediate for the inorganic and organic Se sources and as the checkpoint metabolite between utilization for the selenoprotein synthesis and methylation for the excretion of Se.     

Microbial selenate sorption and reduction in nutrient limited systems

In this study, batch sorption experiments and X-ray adsorption spectroscopy (XAS) were utilized to investigate selenate sorption onto Shewanella putrefaciens 200R. Selenate sorption was studied as a function of pH (ranging from 3 to 7), ionic strength (ranging from 0.1 to 0.001 M), and initial selenate concentration (ranging from 10 to 5000 microM) in the absence of external electron donors. The results show that the extent of selenate sorption is strongly dependent on pH and ionic strength, with maximum sorption occurring at low pH (pH = 3) and low ionic strength (0.001 M NaCl) conditions. The strong dependence of Se sorption with ionic strength suggests the formation of outersphere complexes with the cell wall functional groups. Langmuir isotherm plots yielded log Kads values from 2.74 to 3.02. Desorption experiments demonstrated thatthe binding of selenate onto S. putrefaciens was not completely reversible. XANES analysis of the cells after sorption experiments revealed the presence of elemental selenium, indicating that S. putrefaciens has a capacity to reduce Se(VI) to Se(0) in the absence of external electron donors. We conclude that Se sorption onto S. putrefaciens cell walls is the result of the combination of outer-sphere complexation and cell surface reduction. This sorption process leads to a complex reservoir of bound Se which is not entirely reversible.     

Antagonistic interaction of mercury and selenium in a marine fish is dependent on their chemical species

It is well-known that selenium (Se) shows protective effects against mercury (Hg) bioaccumulation and toxicity, but the underlying effects of Se chemical species, concentration, and administration method are poorly known. In this study, we conducted laboratory studies on a marine fish Terapon jurbua to explain why Hg accumulation is reduced in the presence of Se observed in field studies. When Se and Hg were administrated concurrently in the fish diets, different Se species including selenite, selenate, seleno-dl-cystine (SeCys), and seleno-dl-methionine (SeMet) affected Hg bioaccumulation differently. At high concentration in fish diet (20 μg g(-1) normally), selenate and SeCys significantly reduced the dietary Hg(II) assimilation efficiency (AE) from 38% to 26%. After the fish were pre-exposed to dietary selenite or SeMet (7 μg g(-1) normally) for 22 days with significantly elevated Se body concentrations, the Hg(II) AEs were pronouncedly reduced (from 41% to 15-26%), whereas the dissolved uptake rate constant and elimination rate constant were less affected. In contrast to Hg(II), all the MeHg biokinetic parameters remained relatively constant whether Se was administrated simultaneously with the fish diet or when the fish were pre-exposed to Se with elevated body concentrations. Basic biokinetic measurements thus revealed that Se had direct interaction with Hg(II) during dietary assimilation rather than with MeHg and that different Se species had variable effects on Hg assimilation.     

烟株对不同价态硒的吸收与分配

为了开发富硒烟叶生产技术和了解烟叶对硒的富集机理,采用水溶性Se（Ⅳ）（亚硒酸钠）和Se（Ⅵ）（硒酸钠）盆栽试验,探讨了烟株对硒的吸收和分配规律,以及不同价态和用量的硒对烟株生长发育的影响。结果表明,土壤施用适当浓度的两种价态硒（1～10 mg/kg）均有改善烟株形态性状和提高生物量的作用,高浓度硒（20～30 mg/kg）则有负面影响,且Se（Ⅵ）处理抑制效果大于 Se（Ⅳ）。相同浓度下,两种价态的硒均能有效提高烟株各部位硒含量,Se（Ⅵ）处理较 Se（Ⅳ）提高了3～30倍。土壤中的硒优先富集和分布于叶片中,其次是根系,茎秆最小。Monod方程拟合结果表明,Se（Ⅵ）处理烟株叶片、茎秆和根系最大硒含量分别是Se（Ⅳ）处理的1.83、15.81和20.98倍,烟株对Se（Ⅵ）的亲和能力显著大于Se（Ⅳ）,说明Se（Ⅵ）对烟株的生物有效性明显高于Se（Ⅳ）。     

转基因大豆对硒的富集作用和形态分布研究

目的研究转基因大豆对硒的富集作用以及富集前后转基因大豆中硒的形态分布和形态转化。方法 采用电感耦合等离子体质谱分析转基因大豆富集前后硒的总量,采用蛋白酶提取,高效液相色谱-电感耦合等离子体质谱(high performance liquid chromatography-inductively coupled plasma-mass spectrometry,HPLC-ICP-MS)联用技术分析转基因大豆中的硒酸盐(selenate,Se VI)、亚硒酸盐(selenite,Se IV)、硒代蛋氨酸(selenomethionine,Se Met)和硒代胱氨酸(selenocystine,Se Cys)等几种硒化合物。盆栽种植试验研究转基因大豆对硒的富集作用。通过对照试验,考察大豆植株不同部位总硒含量变化和形态分布的转化情况。结果转基因大豆对硒有较好的富集作用,吸收的亚硒酸盐在大豆植株中部分转化成了硒代蛋氨酸和硒代胱氨酸。结论 了解了转基因大豆的形态分布和转化情况,可以更好地评估转基因大豆的食用安全风险并进行生物利用开发。     

The effect of heat treatment on intrinsic and fortified selenium levels in cow's milk

The effects of two heat processing methods (pasteurisation and spray drying) routinely used in the processing of cow's milk and the production of infant formula powder on the selenium (Se) content of liquid milk, milk fortified with sodium selenite and sodium selenate were studied. Pasteurisation reduced intrinsic Se and selenate levels by 7.9% and 6.2% at p < 0.05 level and selenite levels by 7.0% at p > 0.05 level. Se losses following spray drying were 44.8% (p < 0.001), 11.4% (p < 0.01) and 10.0% (p < 0.01) for intrinsic selenium, selenite and selenate fortified milk, respectively. Total Se losses from unprocessed milk following processing (pasteurisation and spray drying) were 49.2% (p < 0.001), 17.6% (p < 0.001) and 15.6% (p < 0.001) for intrinsic selenium, selenite and selenate fortified milk, respectively.     

Bioavailability Assessment of Copper,Iron, Manganese,Molybdenum, Selenium,and Zinc from Selenium‐Enriched Lettuce

Cu, Fe, Mn, Mo, Selenium (Se), and Zn bioavailability from selenate‐ and selenite‐enriched lettuce plants was studied by in vitro gastrointestinal digestion followed by an assay with Caco‐2 cells. The plants were cultivated in the absence and presence of two concentrations (25 and 40 µmol/L of Se). After 28 days of cultivation, the plants were harvested, dried, and evaluated regarding the total concentration, bioaccessibility, and bioavailability of the analytes. The results showed that biofortification with selenate leads to higher Se absorption by the plant than biofortification with selenite. For the other nutrients, Mo showed high accumulation in the plants of selenate assays, and the presence of any Se species led to a reduction of the plant uptake of Cu and Fe. The accumulation of Zn and Mn was not strongly influenced by the presence of any Se species. The bioaccessibility values were approximately 71%, 10%, 52%, 84%, 71%, and 86% for Cu, Fe, Mn, Mo, Se, and Zn, respectively, and the contribution of the biofortified lettuce to the ingestion of these minerals is very small (except for Se and Mo). Due to the low concentrations of elements from digested plants, it was not possible to estimate the bioavailability for some elements, and for Mo and Zn, the values are below 6.9% and 3.4% of the total concentration, respectively. For Se, the bioavailability was greater for selenite‐enriched than selenate‐enriched plants (22% and 6.0%, respectively), because selenite is biotransformed by the plant to organic forms that are better assimilated by the cells.     

Characterization of Pseudomonas stutzeri NT-I capable of removing soluble selenium from the aqueous phase under aerobic conditions

Pseudomonas stutzeri strain NT-I was isolated from the drainage wastewater of a selenium refinery plant. This bacterium efficiently reduced selenate to elemental selenium without prolonged accumulation of selenite under aerobic conditions. Strain NT-I was able to reduce selenate completely at high concentrations (up to 10 mM) and selenite almost completely (up to 9 mM). In addition, higher concentrations of selenate and selenite were substantially reduced. Activity was observed under the following experimental conditions: 20–50°C, pH 7–9, and 0.05–20 g L^–1 NaCl for selenate reduction, and 20–50°C, pH 6–9, and 0.05–50 g L^–1 NaCl for selenite reduction. Under anaerobic conditions, selenate was reduced more rapidly, whereas selenite was not reduced at all. The high selenate- and selenite-reducing capability at high concentrations suggested that strain NT-I is suitable for the removal of selenium from high-strength industrial wastewater.     

Selenium speciation assessed by X-ray absorption spectroscopy of sequentially extracted anaerobic biofilms

Wet chemical methods such as sequential extraction procedures are commonly used to assess selenium fractionation in anoxic environments, allowing an estimation of the mobility and bioavailability of selenium. However, the interpretation can be biased by unselective extraction of targeted species and artifacts introduced during the extraction. Here, the selectivity of the single extraction steps to gain reliable selenium speciation information are scrutinized for the first time by direct, nondestructive X-ray absorption near edge structure (XANES) spectroscopy at the selenium K-edge. The sequential extraction procedures seriously overestimated the elemental selenium fraction, as major parts (58%) of the total selenium were present as metal selenides and organic selenium compounds, although extracted in the elemental fraction. Spectral fitting of the XANES spectra by the least-squares linear combinations utilizing a large set of model compounds, including previously neglected Se(-I) selenides, showed a novel degree of complexity in the speciation of selenium treating anaerobic biofilms, with up to 4 modeled selenium species contributing to the speciation, i.e., different elemental, organic, and metal-bound selenium species. Furthermore, a short exposure (10 min) to ambient air during the sequential extraction procedure induced the oxidation of organic selenium compounds, revealing the fragility of selenium speciation in anaerobic biofilms.     

石榴皮多糖硒酸酯制备工艺参数优化及其结构分析

以石榴皮多糖与亚硒酸钠为原料制备石榴皮多糖硒酸酯,以产物硒含量和收率为指标,原料配比、硝酸体积分数、反应温度和反应时间为单因素,通过单因素试验和Box-Behnken试验设计方法对工艺条件进行优化,并对产物结构进行分析。结果显示：石榴皮多糖硒酸酯最佳制备工艺条件为原料配比1∶1.0（g/g）、硝酸体积分数0.61%、反应温度77.0 ℃、反应时间11.3 h,在该工艺条件下,制得石榴皮多糖硒酸酯中硒含量为4.48 mg/g,产物收率为41.87%;紫外光谱、红外光谱和热重分析充分证实石榴皮多糖硒酸酯中含有Se＝O键、Se-O键和Se-C键。优化后的石榴皮多糖硒酸酯制备工艺条件实现了石榴皮多糖的硒化,为石榴皮资源的充分开发利用提供良好条件。     

Speciation of Se and DOC in soil solution and their relation to Se bioavailability

A 0.01 M CaCl(2) extraction is often used to asses the bioavailability of plant nutrients in soils. However, almost no correlation was found between selenium (Se) in the soil extraction and Se content in grass. The recently developed anion Donnan membrane technique was used to analyze chemical speciation of Se in the 0.01 M CaCl(2) extractions of grassland soils and fractionation of DOC (dissolved organic carbon). The results show that most of Se (67-86%) in the extractions (15 samples) are colloidal-sized Se. Only 13-34% of extractable Se are selenate, selenite and small organic Se (<1 nm). Colloidal Se is, most likely, Se bound to or incorporated in colloidal-sized organic matter. The dominant form of small Se compounds (selenate, selenite/small organic compounds) depends on soil. A total of 47-85% of DOC is colloidal-sized and 15-53% are small organic molecules (<1 nm). In combination with soluble S (sulfur) and/or P (phosphor), concentration of small DOC can explain most of the variability of Se content in grass. The results indicate that mineralization of organic Se is the most important factor that controls Se availability in soils. Competition with sulfate and phosphate needs to be taken into account. Further research is needed to verify if concentration of small DOC is a good indicator of mineralization of soil organic matter.     

Soil selenium treatments to ameliorate selenium deficiency in herbage

Sodium selenite, sodium selenate, and sodium selenate in a pill formulation were applied to three soils known to produce Se-deficient herbage ( < 100 μg Se kg^?1 dry matter (DM)). The ability of the salts, applied in spring 1985, to ameliorate Se deficiency was followed over 3 years by taking four harvests each year. Selenate treatment at 10 g Se ha^?1 and selenate prill treatment at 20 g Se ha^?1 produced herbage with Se levels (geometric means) of between 0.57-0.86 and 1.79-1992 mg kg^?1 DM respectively in the first spring harvest after treatment. Selenite was less potent and selenite at 100 g Se ha^?1 produced a response in herbage closely similar to that of selenate at 10 g Se ha^?1. Even at 300 g Se ha^?1 the selenite treatment produced herbage with only 1.00-1.36 mg Se kg^?1 DM at the first harvest. Application of selenate in the prill form at 60 g Se ha^?1 produced herbage potentially toxic to grazing animals with 4.81-4.94 mg Se kg^?1 DM. The addition of fertiliser N to Se-treated plots increased total Se uptake at the first harvest by a factor of 4 and had a small effect on Se concentration. The Se concentration levels in herbage from Se-treated plots declined exponentially (t_1/2 = 21-43 days). On one soil derived from Lower Old Red Sandstone and lava, selenite at 300 g Se ha^?1 gave herbage with Se contents significantly above background (P < 0.05) in all harvests over 3 years.     

Bioreduction of selenate using a hydrogen-based membrane biofilm reactor

A H2-based, denitrifying and sulfate-reducing membrane-biofilm reactor (MBfR) was shown to be effective for removing selenate (Se(VI)) from water or wastewater by reducing it to insoluble Se(0). When Se(VI) was first added to the MBfR, Se(VI) reduction--first to selenite (Se(IV)) and then mostly to Se(0)--took place immediately and then increased over three weeks, suggesting enrichment for dissimilatory selenium-reducing bacteria. Increasing the H2 pressure improved the Se(VI) reduction rate and total-Se removal, and lowering the influent Se(VI) concentration from 1000 to 260 microg Se/L increased the average Se removal to 94%, which corresponded to an effluent Se concentration of less than 12 microg Se/L, a value well below the standard of 50 microg Se/L. The fact that the effluent suspended solids contained reduced Se suggests that Se(0) was retained in the biofilm, which detached to form the effluent suspended solids. A series of short-term experiments elaborated on how decreased influent selenate loading and increased H2 pressure could systematically improve the reduction of Se(VI) and removal of total Se. Short-term experiments also demonstrated that selenate reduction improved with lower influent nitrate concentration, suggesting that H2 was more available for selenate reduction when the H2 demand for denitrification was smaller. Complete sulfate reduction, which occurred in parallel to nitrate reduction, dominated the electron-equivalent flux. Like selenate reduction, but unlike nitrate reduction, sulfate reduction was sensitive to H2 pressure and appeared to be inhibited by selenate. Finally, selenate reduction was relatively insensitive to pH in the range of 7.0 to 9.0. This research shows that the MBfR can be effective for removing Se(VI) in water or wastewater to below the 50 microg Se/L standard.     

Dietary selenium reduces retention of methyl mercury in freshwater fish

Adverse effects from organic mercury transported along aquatic food chains are health issues in humans and other top predators. Methyl mercury in organisms at the lower food chain levels is eliminated slowly, and laboratory studies have not clarified the role of selenium in the retention of methyl mercury in fish. Here, we investigated the effects of dietary selenium on the retention of organic and inorganic mercury in freshwater fish. Addition of selenite to the food augmented elimination of methyl mercury (but not inorganic mercury) from goldfish Carassius auratus in a dose dependent manner; selenite caused methyl mercury to be lost from the general body rather than from any specific organ. Seleno-cystine and seleno-methionine (but not selenate) likewise promoted elimination of methyl mercury from goldfish. The threshold for the augmenting effect of selenite on the elimination of methyl mercury in the zebra fish Danio rerio was 0.95 μg Se g(-1) food; higher concentrations reduced retention of methyl mercury in a dose dependent manner. Selenium concentrations in the food approaching natural background levels increase the elimination of methyl mercury from fish. Thus, selenium levels in a given aquatic food chain may affect mercury contamination along the food chain.