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Antagonistic Interaction of Mercury and Selenium in a Marine Fish Is Dependent on Their Chemical Species Fei Dang and Wen-Xiong Wang* Division of Life Science, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong ABSTRACT: 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.
’ INTRODUCTION Mercury contamination in fish poses a potential risk to higher trophic levels and thus has received wide attention. Methylmercury (MeHg), the predominant species in fish tissues, was more bioavailable and toxic than inorganic Hg [Hg(II)]. Since the 1960s,1 selenium (Se) has been shown as an effective agent to detoxify Hg and reduce Hg accumulaton in mammalian and fish. The protective effects of Se against Hg accumulation have been investigated widely in laboratory studies, but the results are often controversial. For example, elimination studies revealed that the presence of Se could increase2 or did not affect3 the Hg efflux rate. In the green mussel (Perna viridis), waterborne selenomethionine (SeMet) inhibited MeHg but enhanced Hg(II) dissolved uptake rate, whereas selenite did not affect the uptake of Hg.4 In minnows (Phoxinus phoxinus), the presence of selenate tended to increase the dissolved Hg(II) uptake.3 All these inconsistent results suggest that the modification of Se on Hg bioaccumulation is extremely complex and not well understood. Different chemical species, concentrations, and administration methods (e.g., whether administrated separately or in parallel) of Hg and Se could complicate the process of bioaccumulation. The protective role of Se on Hg, particularly the MeHg bioaccumulation, has been observed both in laboratory and field studies. Field studies showed a rather clear antagonism of Se r 2011 American Chemical Society
against Hg accumulation in aquatic organisms. Chen and Belzile5 reported significant inverse relationships between total Hg and Se in muscle tissues of perch (Perca flavescens) and walleye (Sander vitreus) with increasing distance from the Sudbury area in Canada, suggesting a strong antagonistic effect of Se on whole body Hg retention. Later, Belzile et al.6 also observed inverse but not significant correlation between MeHg and Se in tissues of zooplankton, mayflies (Stenonema femoratum), amphipods (Hyalella azteca), and young perch from the same lake. High concentration of Hg was found when the Se concentrations were low in whole body of freshwater fish in a survey across twelve western U.S. states.7 In the Sacramento splittail (Pogonichthys macrolepidotus) larvae, dietary SeMet decreased MeHg bioaccumulation, which was negatively related to the SeMet/MeHg ratio in the fish diet.8 An increase in Se concentration in Swedish lakes (40 nM Se) decreased the MeHg content in pike (Esox lucius), perch (Perca fluviatilis), and roach (Rutilus rutilus) over a 3-year period.9 A similar relationship was also observed between Hg and Se in largemouth bass (Micropterus salmoides).10 These results Received: November 3, 2010 Accepted: February 15, 2011 Revised: February 1, 2011 Published: March 02, 2011 3116
dx.doi.org/10.1021/es103705a | Environ. Sci. Technol. 2011, 45, 3116–3122
Environmental Science & Technology suggest a direct interaction between Se and MeHg during accumulation in fish.6,8,11,12 It has been hypothesized that Se could decrease MeHg assimilation5,6 or increase elimination,12 or both,8 to reduce MeHg bioaccumulation. Unfortunately, there were few experimental data available to support these hypotheses. The benefit of Se for mitigating MeHg bioaccumulation thus remains ambiguous. Indeed, it is difficult to distinguish the assimilation and elimination processes in the field conditions. Identification of the underlying mechanisms for such an interaction in complex environmental setting is therefore warranted. Traditional evaluations such as tissue-specific accumulation do not necessarily explain how MeHg and Se interact during bioaccumulation. Detailed examination of Hg biokinetics is therefore needed to elucidate the antagonistic effects during Hg and Se bioaccumulation. The aim of this study was therefore to explore the antagonistic effects of Hg and Se in a marine fish Terapon jurbua in the framework of biokinetic processes. We hypothesize that the interactions rely to a large extent on the chemical forms of the elements. Therefore, different Se forms at different concentrations and both Hg species were individually added to the fish diet. We considered both Hg(II) and MeHg given their differences in the biokinetics in fish.13,14 Dietary exposure was carefully considered in the present study since it is the dominant uptake route for Hg and Se bioaccumulation.15-17 In addition to directly test the interaction between Hg and Se during the dietary assimilation process, we also exposed the fish to different forms of Se to elevate its body burden in fish tissues and then measured the biokinetics of Hg in the fish.
’ MATERIALS AND METHODS Fish, Isotopes, and Fish Diet. Juvenile jarbua terapon Terapon jurbua (4-5 cm in length) were obtained from a fish farm in Hong Kong, acclimated in natural sand-filtered seawater at 20 °C and fed clean commercial fish diet at a daily rate of 3% body weight. Radioisotope 203Hg(II) (as HgCl2, t1/2 = 46.6 d, specific activity = 20.1 kBq μg-1 in 1 N HCl) was purchased from Eckert and Ziegler. Me203Hg was synthesized from 203Hg(II) following a well established method.18 The commercial fish diet (clean fish diet) was obtained from a commercial company in Xiamen, China. The measured metal concentrations were 15.3 ( 0.91 ng total Hg g-1, 2.73 ( 0.71 ng MeHg g-1, 1.02 ( 0.063 μg Se g-1, with the macronutrient being 42% crude protein, 3% fat, and 16% crude ash. Se level in fish diet was comparable to the low end of environmentally realistic values in zooplankton (1.02-6.07 μg g-1 dw19). We did not identify the Se chemical species in the fish diet, but the product information suggested that Se was present mainly as selenite. The uniform and spherical sized fish diet was 3 mm in diameter and could float on the water surface without sinking during 1 h feeding regime. Experimental Protocol. The present study was done, first, by examining the effects of dietary Se on Hg assimilation, when fish were fed diet supplemented with both stable Se (present at different chemical species and concentrations) and radioactive Hg for 1 h (see below for experiment one). In the mean time, other fish were pre-exposed to dietary selenite and selenomethionine for a period of 22 days. A control treatment with clean fish diet was also included. Thereafter, Se pre-exposed fish was used to assess the influences of enhanced Se body burden in fish on Hg assimilation after 1 h exposure to fish diet spiked only with
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radioactive Hg (experiment two), on Hg elimination rate constant (experiment three) or on Hg dissolved uptake rate constant during 6 h exposure to waterborne Hg (experiment four). In experiment three, Se pre-exposed fish were fed Hgsupplemented diet for another 8 days before 26 days depuration. Hg Dietary Assimilation from Fish Diet with Different Se Species and Concentrations. Commercial fish diet rather than natural prey was used to elucidate Se-Hg interaction during dietary assimilation. Clean fish diet was spiked with radioactive Hg in combination with different Se species (selenite, selenate, seleno-DL-cystine, and seleno-DL-methionine) and concentrations. Previous data on dietary Se toxicity thresholds for fish and wildlife differed among species and exposure time, ranging from 3 to 11 μg g-1 (dw) with the majority of threshold values being less than 7 μg g-1 (dw).20,21 However, dietary Se concentrations in the range of 1-20 μg g-1 were within the realm of environmental relevant invertebrates.17,19,22,23 Therefore, we used 5 and 20 μg g-1 as representative Se concentrations in the fish diet for 1 h exposure. Briefly, five grams of commercial fish diet was first incubated with 5 mL of different levels of freshly prepared solution of Se and radioactive 203Hg(II) or Me203Hg in MiliQ water for 4 h and then dried at room temperature for 48 h. The measured Se concentrations of selenite, selenate, selenomethionine, and selenocystine spiked fish diet were 6.3 ( 0.2 μg g-1, 6.1 ( 0.2 μg g-1, 5.4 ( 1.0 μg g-1, and 5.4 ( 0.5 μg g-1, respectively, compared to a target concentration of 5 μg g-1 for the low Se diet, and 18.3 ( 1.0 μg g-1, 16.7 ( 0.9 μg g-1, 17.3 ( 1.5 μg g-1, and 17.6 ( 1.9 μg g-1, respectively, for the high Se diet. The control fish diet was prepared in the same way but with the addition of only radioactive mercury in MiliQ water. The resulting radioactive Hg concentrations in the spiked fish diet were 246 ng Hg(II) g-1 or 143 ng MeHg g-1, irrespective of Se treatments. The Hg radiolabeled fish diet with different Se species and concentrations were fed to five individual fish in one polypropylene tank filled with 20 L seawater for 1 h (experiment one). After the pulse feeding, fish were placed in 500 mL of nonradioactive 0.22 μm filtered seawater for 1-2 min, measured for radioactivity, and depurated individually in 2 L natural sand-filtered seawater. At 12 h, 24 h, 36 h, and 48 h, the radioactivity remaining in the fish was monitored nondestructively, and the depuration medium was replaced. Our preliminary experiments showed that
