Bioavailability, Toxicity and Biotransformation of Selenium in Midge

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Bioavailability, Toxicity and Biotransformation of Selenium in Midge (Chironomus dilutus) Larvae Exposed via Water or Diet to Elemental Selenium Particles, Selenite, or Selenized Algae Mercedes Gallego-Gallegos,†,‡ Lorne E. Doig,† Justin J. Tse,‡ Ingrid J. Pickering,‡ and Karsten Liber*,† †

Toxicology Centre, University of Saskatchewan, 44 Campus Drive, Saskatoon, Saskatchewan S7N 5B3, Canada Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada

‡

S Supporting Information *

ABSTRACT: Elemental selenium (Se) is generally considered to be biologically inert due to its insolubility in water. It is a common form of Se in sediment near uranium mining and milling operations in northern Saskatchewan, Canada. Nanosized particles of many materials exhibit different properties compared with their bulk phases, in some cases posing health and ecological risks. Here we investigated the bioavailability and toxicity of Se nanoparticles (SeNPs) using 10-day waterborne and dietary exposures to larvae of Chironomus dilutus, a common benthic invertebrate. For comparison, larvae were also exposed to waterborne dissolved selenite and to dietary selenomethionine as selenized algae. Larval Se accumulation was evaluated using graphite furnace atomic absorption spectroscopy or inductively coupled plasma mass spectroscopy for total Se and X-ray absorption spectroscopy for Se chemical speciation. Exposure to nanoparticulate Se resulted in Se bioaccumulation, at high concentrations, inhibiting larval growth in both waterborne and dietary exposures; larvae predominantly accumulated selenomethionine-like species regardless of uptake route or form of Se tested. Despite the observed Se accumulation, our findings suggest there is little risk of direct SeNP toxicity to benthic invertebrates in Se-contaminated sediments in northern Saskatchewan. Nevertheless, elemental Se in sediments may be biologically available and may contribute directly or indirectly to the risk of Se toxicity to egg-laying vertebrates (fish and piscivorous birds) in Se-contaminated aquatic systems. It thus may be necessary to include elemental Se as a source of potential Se exposure in ecological risk assessments.

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INTRODUCTION Discharges from industrial activities such as metal mining and milling, coal mining, and petrochemical processing can result in selenium (Se) contamination in downstream aquatic ecosystems. Selenium is an essential micronutrient that can be both beneficial and toxic to fish within a narrow concentration range.1 Ecologically, reproductive failure and teratogenicity in egg-laying vertebrates is the main concern, as Se is transferred from female fish or waterfowl to their eggs.2,3 If the Se concentration in an egg exceeds toxicity thresholds, the developing embryo dies or is irreversibly deformed.4−6 Selenium toxicity therefore can impact the sustainability of fish and waterfowl populations, and the integrity of overall ecosystem structure and function. In northern Saskatchewan, Se naturally occurs in uranium ore. Mining and milling of uranium ore at the Key Lake operation (57°110′ N, 105°340′ W) has resulted in the release of Se (as selenate via treated effluent7) into the aquatic environment over 20 years. As a result, Se has been found at concentrations as high as 105, 88, and 17 μg Se/g d.w. in sediment, benthic invertebrates (chironomids), and fish (juvenile pike), respectively, downstream of this facility.7,8 To date, studies typically have focused on soluble Se species, both © XXXX American Chemical Society

inorganic (selenate and selenite) and organic (especially selenomethionine, SeMet).9−14 However, a significant portion of the Se in the sediments downstream of the Key Lake operations occurs as elemental Se, ranging from 17 to 81% of the total Se content (average of ∼50%).15 It is likely that this elemental Se is produced to some extent by microbial activity and that some fraction of it is in the form of nanometer-sized Se particles (SeNPs). Generally, the occurrence of red elemental selenium (Se0) in the environment has been attributed to the reduction of Se oxyanions through biotic or abiotic pathways. Studies investigating the microbial reduction of Se to elemental Se have found that production of red elemental selenium results in red colored SeNPs.16−18 Abiotic reduction, attributed to green rust (iron oxides),19 will also produce red nanometer-sized particles of elemental Se.20 Although elemental Se is generally considered to be biologically inert due to its insolubility in water,21,22 little is known Received: February 29, 2012 Revised: October 23, 2012 Accepted: November 15, 2012

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day. After 10 days of exposure (both waterborne and dietary exposures), all surviving larvae were retrieved, triple-rinsed in culture water, and gut-depurated for 24 h in clean beakers containing uncontaminated sand, water, and food. After depuration, the larvae were again retrieved and triple rinsed in ultrapure water and stored below −20 °C prior to either freeze-drying, weighing, and digestion for Se analysis (from three beakers), or grinding in liquid nitrogen and subsequent storage below −80 °C for synchrotron analysis (from remaining two beakers). Data were collected from all five beakers for survival. For comparison to the dietary and waterborne SeNP exposures, separate larvae were exposed to SeMet (as selenized algae) in additional dietary-Se treatments and to dissolved selenite in additional waterborne-Se treatments. For total Se analysis, one overlying water sample (5 mL) was collected daily from three replicates in the waterborne-Se treatments, with one set of samples collected before and after the water changes on days 3 and 6. Two samples (1 mL each) were collected daily from each replicate in the dietary-Se treatments (one from the top and one from the bottom of the overlying water column). Total Se was analyzed in water samples collected on days 1, 3, 5, 6, 7, 9, and 10 from two waterborne SeNP and dissolved selenite treatments; 15 and 100 μg Se/L. All other treatments were analyzed less intensively (at a minimum, Se concentrations were analyzed in the overlying water samples collected on day 0). All water samples were stored at −20 °C until analysis. Temperature and dissolved oxygen concentrations were measured daily in all replicates. Separate 30-mL samples from each test beaker were also collected before water changes and analyzed the same day for other routine water quality variables. General water quality during testing was similar among all dietary and waterborne Se treatments and their respective controls, and was as follows: dissolved oxygen, 7.58 ± 0.85 mg/ L; conductivity, 412 ± 13 μS/cm; mean pH, 7.8 (range 7.5− 8.1); alkalinity, 110 ± 8 mg/L as CaCO3; hardness, 145 ± 9 mg/L as CaCO3; ammonia, < 0.2 mg/L. Selenium Spiking Protocols. Water. Ultrasonication, stirring and vigorous shaking are methods typically used to suspend nanoparticles in liquid solutions,27 each of which has advantages and disadvantages.28 Vigorous shaking was used in this study because it is gentle compared with ultrasonication, which can produce reactive oxidizing species as a result of cavitation.29 Selenium nanoparticle treatments for the waterborne exposures (nominal concentrations of 5, 15, 50, 100, and 1000 μg Se/L) were made by diluting purified solutions of SeNPs with culture water. Once mixed, these solutions were promptly used in toxicity testing. To assess the bioavailability and toxicity of SeNPs relative to other common aqueous forms of Se, additional test organisms were exposed to selenite solutions. Selenite treatments were made by diluting stock solutions of sodium selenite with culture water to obtain nominal concentrations of 5, 15, 50, 100, and 1000 μg Se/L . Food. Dry fish food (1 g; Tropical Bio Flakes) was ground into a fine powder using an agate mortar and pestle and mixed with ultrapure water solutions of SeNPs (7 mL) to form homogeneous pastes having the desired Se concentrations. These pastes were then frozen (−20 °C) and freeze-dried under −92 °C and 20−42 mTorr vacuum (Dura-Dry, FTS Systems, Stone Ridge, NY, USA). Subsamples of the freezedried, SeNP-spiked fish food preparations were then

regarding the bioavailability and toxicity of nanosized Se particles. Selenium bioavailability and toxicity to invertebrates and fish has gained considerable attention in recent years; however, compared to more well-studied forms of environmental Se, such as selenate, selenite, and SeMet, elemental SeNPs have received little attention. Other authors23 studying the fate of colloidal-particulate elemental Se in aquatic system have concluded that SeNPs could be available for uptake by aquatic organisms through waterborne exposure or ingestion, or indirectly due to the oxidation of elemental Se into Se(IV), but little is currently known about the bioavailability and toxicity of SeNPs to benthic invertebrates that may be naturally in close proximity to such particles. As such, the first objective of this study was to evaluate the bioavailability and toxicity of SeNPs to larvae of the common benthic invertebrate Chironomus dilutus. Given the lack of information about the possible biotransformation of SeNPs once inside an organism, the second objective of this study was to evaluate the speciation of accumulated Se.

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MATERIALS AND METHODS Test Organism. Chironomus dilutus (formerly C. tentans), a nonbiting midge (chironomid), is widespread across North America and is commonly used in sediment toxicity assessment. Many chironomid species are deposit-feeders and are important in the transfer of contaminants from sediments to fish and aquatic birds.24 As sediment dwellers, native chironomids in northern Saskatchewan are likely exposed to elemental Se in Se-contaminated sediments. Although exposure most likely would be through ingestion of sediment (either bulk sediment or biofilm), aqueous SeNP exposure could occur in the surrounding pore water or overlying water as the result of sediment resuspension (possibly resulting from wave action or bioturbation). Selenium Nanoparticle Synthesis. Red elemental Se particles with an average size of 80−200 nm in diameter were synthesized following a slight modification of Zhang et al.25 using reduced glutathione and bovine serum albumin as detailed in the Supporting Information. The size range of particles tested herein (80−200 nm diameter) straddles the upper defined limit for “nanoparticles” (100 nm or less in at least one dimension). As such, we will refer to our test material as “nano”. Experimental Design. All bioaccumulation/toxicity tests were performed at the Toxicology Centre, University of Saskatchewan (Saskatoon, SK, Canada) in an environmental chamber having an ambient temperature of 25 ± 1 °C and a daily photoperiod of 16 h light:8 h dark. Test units consisted of 250-mL glass beakers having a layer of silica sand (∼90 g dry weight (d.w.), particle size range 250−425 μm) and 200 mL of overlying test water with continuous aeration. All experiments used C. dilutus larvae obtained from an in-house culture. Culturing protocols followed those outlined in ref 26. Ten 9− 10-day-old larvae were added to each beaker (5 replicate beakers per treatment) upon test initiation. Each test was staticrenewal with water changes on days 0, 3, and 6. These water changes were intended to reduce concentrations of metabolic wastes, maintain target concentrations of Se in the waterborneSe exposures, and to minimize the concentration of Se in the overlying water of the dietary-Se exposures. Chironomus dilutus larvae were fed 1 mL of fish food slurry (10 mg d.w., Tropical Bio Flakes Sera Pond, Heinsburg, Germany) per beaker per B

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Table 1. Summary of Selenium Exposure, Bioaccumulation, and Speciation Data for Chironomus dilutus Larvae Exposed to Various Forms of Selenium in 10-Day Waterborne and Dietary Tests selenium speciation (%)a

total selenium treatment control SeNPs

selenite

control SeNPs

selenized algae (SeMet)

water (μg Se/L)

food (μg Se/g d.w.)

0.40 ± 0.05 2.81 ± 0.12 8.09 ± 2.29 28.4 ± 1.10 60.2 ± 15.5 591 ± 1 3.56 ± 0.12 10.7 ± 2.90 35.0 ± 0.14 77.7 ± 10.8 762 ± 1.3

0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38

± ± ± ± ± ± ± ± ± ± ±

0.42 ± 0.01 1.24 ± 0.41 3.56 ± 1.19 10.7 ± 3.59 29.9 ± 10.2 109 ± 36.4 1 μg/g d.w.). Although the larvae in both 11 and 35 were not gut-purged prior to analysis, possibly skewing whole-body Se concentrations upward, the BAFs in the present study (see concentration factors, CFs, in Table 1) were similar to those of 11 and 35 and ranged from 0.133 to 0.560 in the SeNP treatments and from 0.521 to 0.754 in the selenized algae treatments, with the lowest BAFs occurring in the treatments having the highest concentrations of dietary Se. Dietary exposure appears to dominate Se bioaccumulation processes in higher organisms in situ (see review 36). Consumers, such as chironomids, ingest primary producers, microbes, and biofilm and are thereby exposed to and accumulate Se. The concentration of Se in primary producers and microbes at the base of a food web determines Se enrichment and trophic transfer in the associated aquatic food web.7,36 The investigation of Se accumulation in higher trophic levels has therefore come to focus mainly on organic Se species (e.g., selenomethionine) present in primary producers and biofilm. Nevertheless, C. dilutus larvae have been shown to accumulate Se from inorganic Se sources such as selenite (this study and 32). In the present study, C. dilutus larvae accumulated Se when exposed to elemental SeNPs, regardless of route of exposure. It is therefore possible that elemental Se (not just nanosized Se particles) can act as a source of Se that, once in an aquatic system, could supply bioavailable Se to benthic organisms and hence aquatic food chains. If this is the case, Se availability from elemental Se sources would likely be highly dependent upon redox conditions and biogeochemical processes both externally in the microenvironment of benthic organisms and internally in their digestive tracts. For example, elemental Se can oxidize to selenite under oxic conditions (such conditions often exist near the sediment−water interface), but certain bacteria (such as Bacillus megaterium) can dramatically increase the rate of oxidation.37 Toxicity of SeNPs to C. dilutus. Elevated concentrations of different chemical forms of Se have been shown to cause a variety of toxicological effects in fish,38−41 including the transfer of Se to eggs resulting in subsequent larval deformities (e.g., 42). Few studies have investigated the toxic effects of Se on invertebrates, possibly because invertebrates have largely been viewed as being relatively tolerant to Se accumulation (e.g., 43). Waterborne Exposure. There are no previous studies we are aware of that investigated the toxicity of waterborne SeNPs to chironomid larvae or other benthic invertebrates, but data are available for waterborne exposures involving other Se species (see 44 for a summary of Se toxicity to aquatic invertebrates).

Short-term acute toxicity to waterborne Se occurs at relatively high Se concentrations. For example, the 48-h LC50s for Chironomus decorus larvae exposed to selenate, selenite, and seleno-DL-methionine were 23.7, 48.2, and 194 mg Se/L, respectively.45 The 5-d LC50 for Chiromus riparius larvae exposed to waterborne selenite (no prior Se acclimation) was 4.1 mg Se/L.34 Within the range of SeNP and selenite concentrations tested in the present study (maximum concentrations of 591 and 762 μg Se/L, respectively), survival of C. dilutus larvae was unaffected (survival >80% in all groups) and only those larvae exposed to SeNPs in the highest Se treatment showed a statistically significant reduction in growth compared to the control (Figure 3a) (p < 0.001; 10-d NOEC = 60.2, LOEC = 591 μg Se/L; see Table 2 for toxicity thresholds). Waterborne SeNPs, measured as total Se in overlying water, appeared more toxic than selenite to C. dilutus larvae over a 10-d period (Table 2); however, given the uncertainties described above regarding Se speciation and route of exposure in the SeNP treatments, additional research is required to confirm this. Dietary Exposure. No previous studies were found investigating the toxicity of dietary SeNPs to benthic invertebrates. Few studies have evaluated the toxicity of dietary Se to invertebrates in general and fewer still have evaluated chironomids. Malchow et al.11 exposed Chironomus decorus larvae to dietary Se (seleniferous algae, Selenastrum capricornutum) and found that dietary Se concentrations ≥2.11 μg Se/g d.w. significantly reduced larval growth (whole-body Se concentrations ≥2.55 μg Se/g d.w.). Survival was unaffected (survival >80% in all groups) within the range of SeNP and organic Se (selenized algae) concentrations tested in the present study (up to 784 and 115 μg Se/g d.w., respectively) and only those larvae exposed to the highest SeNP treatment had a statistically significant reduction in growth compared to the control group (Figure 3b)(p < 0.001; NOEC = 219 μg Se/g d.w., LOEC = 784 μg Se/g d.w.; see Table 2 for toxicity thresholds). Exposure to dietary Se in the form of selenized algae did not reduce chironomid larval growth for those concentrations tested and appears to have had a stimulatory effect in the highest Se treatment. This stimulatory effect was likely due to differences in food quality between treatments, in that the highest dietary Se also has the highest Selenastrum content (approximately 5% of the food by dry weight). Critical Body Burdens. Benthic invertebrates are often considered important only as a vector for the trophic transfer of Se to higher-level consumers such as fish and birds. A review by DeBruyn and Chapman44 concluded that, based on existing laboratory data, the upper range of proposed Se dietary thresholds for fish and birds (i.e., invertebrate whole-body Se concentrations of 11 μg/g d.w.) would likely be associated with F

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dietary Se (Table 1). Regardless of Se form (SeNPs or selenized algae), speciation of bioaccumulated Se was similar (87 ± 5% R−Se−R, 10 ± 4% R−Se−Se−R) across the majority of treatments. The exception to this was the highest SeNP treatment, where elemental Se (Se0) was detected (14% of the total Se) instead of R−Se−Se−R. Similarly, Franz et al.32 found that Se accumulated in C. dilutus larvae exposed for 10 d to low concentrations of selenite and SeMet was present mainly as organic selenides (SeMet-like forms, 76−79%). Similar Se speciation was also described by Wiramanaden et al.15 and Tse et al.,46 who found predominantly R−Se−R and R−Se−Se−R species in chironomid larvae collected from a lake system downstream of the Key Lake uranium operation in northern Saskatchewan, Canada. Similar to dietary Se (although more variable), exposure to waterborne Se resulted in the accumulation of mainly R−Se−R and R−Se−Se−R species in the chironomid larvae. The exceptions to this were the highest SeNP and selenite treatments, where Se0 was also detected. The internal speciation of Se was more variable in the larvae from the waterborne SeNP exposure (70 ± 25% R−Se−R and 32 ± 16% R−Se−Se−R) compared to the larvae from the waterborne selenite exposure (67 ± 9% R−Se−R and 28 ± 9% R−Se−Se− R). The presence of Se0 in the larvae from only the highest SeNPs and selenite treatments appears to correspond to the decrease in R−Se−R species. It is tempting to speculate that the presence of Se0 in whole-body tissues is due to the internalization of intact SeNPs, but this would not explain the presence of Se0 in the selenite-exposed larvae. Elemental Se was also observed in the investigation of Franz et al.32 which used low, environmentally relevant forms of nonparticulate Se (selenite, SeMet). As such, elemental Se in chironomid larvae likely resulted from other biological processes, possibly associated with Se detoxification and depuration or gut microbial processes. Overall, the data suggest that the synthetic SeNPs used in the present study were not directly accumulated, but that the SeNPs served as a source of available Se, possibly in the larval gut or surrounding external environment. Environmental Implications. Synthetic SeNPs (80−200 nm in diameter) were not acutely toxic to C. dilutus larvae in 10-d studies at the concentrations tested in either waterborne (≤591 μg Se/L) or dietary (≤784 μg Se/g d.w.) exposures. The highest dietary concentration tested is more than 10 times the highest elemental Se content recorded in the sediments downstream of the Key Lake mining and milling operations in northern Saskatchewan (72.3 μg Se/g d.w.7,15). Even if all of the elemental Se in these sediments were present as nanoparticles and contained within an ingestible sediment fraction, it is unlikely that nanoparticulate Se poses a significant acute toxicity risk to invertebrates in these sediments. Similarly, the dietary IC25 for SeNPs was 177 μg Se/g d.w, a concentration well above the concentrations of elemental Se found in sediments downstream of the Key Lake operation. The present study found that elemental SeNPs can act as an available source of Se to benthic invertebrates such as chironomids. Regardless of the form of Se spiked into the test system (SeNPs, selenite, selenized algae), organic forms of Se (predominantly SeMet-like compounds) were the main compounds occurring in whole-body tissues after 10 days of exposure. Whether exposed to waterborne or dietary SeNPs, chironomids accumulated Se, even at the lowest test Se concentrations (2.81 ± 0.12 μg/L and 8.89 ± 0.21 μg/g d.w.,

Figure 3. Chironomid biomass (mg/larvae dry weight; mean ± SD) after 10-d exposure to (a) waterborne selenium (nanoparticulate selenium (SeNPs) or selenite) or (b) dietary selenium (SeNPs or selenized algae). Asterisks indicate a statistical difference (α = 0.05) between that treatment and the control group. Horizontal error bars in panel a are wider for the 15 and 100 μg Se/L treatments, which had detailed temporal sampling and analysis, compared to the remainder of the treatments which had a correction factor applied to the initial measured Se concentrations.

substantial toxic effects in some invertebrate prey species. Overall, they found that the internal concentrations associated with sublethal toxicity to invertebrates fell within the range of ∼1 to 30 μg Se/g d.w., with the results of Malchow et al.11 (≥2.55 μg Se/g d.w.) falling at the lower end of this range. In the present study, reduced larval growth was statistically significant only in the highest treatments of SeNPs, with wholebody Se IC25s of 51.1 and 77.1 μg Se/g d.w. for waterborne and dietary exposures, respectively; Table 2). Although not statistically significant from the controls, growth was reduced 28% in the highest selenite treatment. The concomitant wholebody Se concentration (50.6 ± 6.61 μg Se/g d.w.) was similar to the whole-body IC25s for waterborne and dietary SeNPs, suggesting similar toxicity based on internal Se concentrations. Regardless, the internal concentrations associated with sublethal toxic effects to invertebrates were higher in the present study than those concentrations (∼1−30 μg Se/g d.w.) summarized by DeBruyn and Chapman44 from other laboratory investigations. Se Speciation in C. dilutus Larvae. Accumulated Se occurred mainly as SeMet-like species (R−Se−R), and to a lesser extent as R−Se−Se−R, in gut-purged larvae exposed to G

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(4) Gillespie, R. B.; Baumann, P. C. Effects of high tissue concentrations of selenium on reproduction in bluegills. Trans. Am. Fish Soc. 1986, 115 (2), 208−213. (5) Lemly, A. D. Teratogenic effects of selenium in natural populations of freshwater fish. Ecotoxicol. Environ. Saf. 1993, 26, 181−204. (6) Ohlendorf, H. M. Ecotoxicology of selenium. In Handbook of Ecotoxicology, 2nd ed.; Hoffman, D. J., Rattner, B. A., Burton, G. A., Jr., Cairns, J., Eds.; Lewis: Boca Raton, FL, USA, 2003; pp 465−501. (7) Wiramanaden, C. I. E.; Forster, E. K.; Liber, K. Selenium distribution in a lake system receiving effluent from a metal mining and milling operation in northern Saskatchewan, Canada. Environ. Toxicol. Chem. 2010, 29 (3), 606−616, DOI: 10.1002/etc.63. (8) Muscatello, J. R.; Belknap, A. M.; Janz, D. M. Accumulation of selenium in aquatic systems downstream of a uranium mining operation in northern Saskatchewan, Canada. Environ. Pollut. 2008, 156, 387−393, DOI: 10.1016/j.envpol.2008.01.039. (9) Oram, L. L.; Strawn, D. G.; Morra, M. J.; Möller, G. Selenium Biogeochemical Cycling and Fluxes in the Hyporheic Zone of a Mining-Impacted Stream. Environ. Sci. Technol. 2010, 44 (11), 4176− 4183, DOI: 10.1021/es100149u. (10) Wu, L. Review of 15 years of research on ecotoxicology and remediation of land contaminated by agricultural drainage sediment rich in selenium. Ecotoxicol. Environ. Saf. 2004, 57 (3), 257−269, DOI: 10.1016/S0147-6513(03)00064-2. (11) Malchow, D. E.; Knight, A. W.; Maier, K. J. Bioaccumulation and toxicity of selenium in Chironomus decorus larvae fed a diet of seleniferous Selenastrum capricornutum. Arch. Environ. Contam. Toxicol. 1995, 29, 104−109. (12) Ingersoll, C. G.; Dwyer, F. J.; May, T. W. Toxicity of inorganic and organic selenium to Daphnia magna (cladocera) and Chironomus riparius (Diptera). Environ. Toxicol. Chem. 1990, 9, 1171−1181. (13) Hamilton, S. J. Review of selenium toxicity in the aquatic food chain. Sci. Total Environ. 2004, 326, 1−31, DOI: 10.1016/ j.scitotenv.2004.01.019. (14) Spallholz, J. E.; Hoffman, D. J. Selenium toxicity: Cause and effects in aquatic birds. Aquat. Toxicol. 2002, 57, 27−37. (15) Wiramanaden, C. I. E.; Liber, K.; Pickering, I. J. Selenium speciation in whole sediment using X-ray absorption spectroscopy and micro X-ray fluorescence imaging. Environ. Sci. Technol. 2010, 44, 5389−5394, DOI: 10.1021/es100822z. (16) Tomei, F. A.; Barton, L. L.; Lemanski, C. L.; Zocco, T. G.; Fink, N. H.; Sillerud, L. O. Transformation of selenate and selenite to elemental selenium by Desulfovibrio desulf uricans. J. Ind. Microbiol. 1995, 14, 329−336. (17) Kessi, J.; Ramuz, M.; Wehrli, E.; Spycher, M.; Bachofen, R. Reduction of Selenite and Detoxification of Elemental Selenium by the Phototrophic Bacterium Rhodospirillum rubrum. Appl. Environ. Microbiol. 1999, 65 (11), 4734−4740. (18) Lee, J.-H.; Han, J.; Choi, H.; Hur, H.-G. Effects of temperature and dissolved oxygen on Se(IV) removal and Se(0) precipitation by Shewanella sp. HN-41. Chemosphere 2007, 68, 1898−1905, DOI: 10.1016/j.chemosphere.2007.02.062. (19) Chen, Y.-W.; Truong, H-Y.T.; Belzile, N. Abiotic formation of elemental selenium and role of iron oxide surfaces. Chemosphere 2009, 74, 1079−1084, DOI: 10.1016/j.chemosphere.2008.10.043. (20) Charlet, L.; Scheinost, A. C.; Tournassat, C.; Greneche, J. M.; Géhin, A.; Fernández-Martínez, A.; Coudert, S.; Tisserand, D.; Brendle, J. Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay. Geochim. Cosmochim. Acta 2007, 71, 5731−5749, DOI: 10.1016/j.gca.2007.08.024. (21) Garbisu, C.; Ishiia, T.; Leighton, T.; Buchanan, B. B. Bacterial reduction of selenite to elemental selenium. Chem. Geol. 1996, 132, 199−204. (22) Masscheleyn, P. H.; Patrick, W. H., Jr. Biogeochemical processes affecting selenium cycling in wetlands. Environ. Toxicol. Chem. 1993, 12, 2235−2243.

respectively). The dietary concentrations for the three lowest SeNP treatments (8.89−77.2 μg Se/g d.w.) are comparable to the elemental Se concentrations within the sediments of Fox Lake (maximum 72.3 μg Se/g d.w.) and Unknown Lake (maximum 30.5 μg Se/g d.w.) downstream of the Key Lake operations.7,15 This suggests that elemental Se in the sediments downstream of the Key Lake operation could be an available source of Se to the associated benthic invertebrates and that elemental Se should be considered a potential source of Se exposure in ecological risk assessments. Overall, this study provides important insight into the potential impact of elemental selenium on aquatic ecosystems. Elemental Se is typically considered environmentally inert due to its insolubility in water, but in light of our findings elemental Se in sediments has the potential to act as a source of biologically available Se.

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ASSOCIATED CONTENT

S Supporting Information *

Additional text, figures, and table as mentioned in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (306) 966-7444; fax: (306) 931-1664; e-mail: karsten. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by an NSERC-Cameco CRD Grant (D. Janz PI; K. Liber and I. Pickering Co-PIs). I.J.P. is a Canada Research Chair. J.J.T. is a CIHR-THRUST Fellow. SSRL, a Directorate of SLAC National Accelerator Laboratory, is an Office of Science User Facility operated for the US DOE by Stanford University. The SSRL Structural Molecular Biology Program is supported by DOE OBER, and by NIH NIGMS (P41GM103393) and NCRR (P41RR001209). The Pickering−George group and Toxicology Centre members are thanked for their assistance.

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REFERENCES

(1) Young, T. F.; Finley, K.; Adams, W. J.; Besser, J.; Hopkins, W. D.; Jolley, D.; McNaughton, E.; Presser, T. S.; Shaw, D. P.; Unrine, J. What you need to know about selenium. In Ecological Assessment of Selenium in the Aquatic Environment; Chapman, P. M., Adams, W. J., Brooks, M. L., Delos, C. G., Luoma, S. N., Maher, W. A., Ohlendorf, H. M., Presser, T. S., Shaw, D. P., Eds.; SETAC Press: Pensacola, FL, USA, 2010; pp 7−45. (2) Muscatello, J. R.; Janz, D. M. Assessment of larval deformities and selenium accumulation in northern pike (Esox lucius) and white sucker (Catostomus commersoni) exposed to metal mining effluent. Environ. Toxicol. Chem. 2009, 28 (3), 609−618. (3) Janz, D. M.; DeForest, D. K.; Brooks, M. L.; Chapman, P. M.; Gilron, G.; Hoff, D.; Hopkins, W. A.; McIntyre, D. O.; Mebane, C. A.; Palace, V. P.; Skorupa, J. P.; Wayland, M. Selenium toxicity to aquatic organisms. In Ecological Assessment of Selenium in the Aquatic Environment; Chapman, P. M., Adams, W. J., Brooks, M. L., Delos, C. G., Luoma, S. N., Maher, W. A., Ohlendorf, H. M., Presser, T. S., Shaw, D. P., Eds.; SETAC Press: Pensacola, FL, USA, 2010; pp 141− 231. H

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dx.doi.org/10.1021/es300828r | Environ. Sci. Technol. XXXX, XXX, XXX−XXX