Bioaccumulation of Perfluorochemicals in Pacific Oyster under

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Environ. Sci. Technol. 2010, 44, 2695–2701

Bioaccumulation of Perfluorochemicals in Pacific Oyster under Different Salinity Gradients JUNHO JEON,† KURUNTHACHALAM KANNAN,‡ HAN KYU LIM,§ HYO BANG MOON,| J I N S U N G R A , † A N D S A N G D O N K I M * ,† Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheom-dan Gwagi-ro, Buk-gu, Gwangju 500-712, Korea, Wadsworth Center, New York State Department of Health, Empire State Plaza, P.O. Box 509, Albany, New York 12201-0509, National Fisheries Research and Development Institute (NFRDI), 408-1, Sirang-ri, Gijang-eup, Gijang-gun, Busan 619-705, Korea, and Department of Environmental Marine Sciences, College of Science and Technology, Hanyang University, Ansan 426-791, Korea

Received January 18, 2010. Revised manuscript received March 4, 2010. Accepted March 8, 2010.

Despite the reports of widespread occurrence of perfluorinated compounds (PFCs) in estuarine and coastal waters and open seas, little is known on the effect of salinity on bioaccumulation. In this study, effects of salinity on bioaccumulation of PFCs in Pacific oysters (Crassostrea gigas) were investigated. Furthermore, partitioning of PFCs between water and particles (oysters’ food) was examined at different salinities. The distribution coefficients (Kd; partitioning between water and particles) for selected PFCs, that is, PFOS, PFOA, PFDA, and PFUnDA, increased by 2.1- to 2.7-fold with the increase in water salinity from 10 to 34 psu, suggesting “salting-out” effect, and the salting constant (δ) was estimated to range from 0.80 to 1.11. The nonlinear regression analysis of bioaccumulation suggested increase in aqueous and dietary uptake rates (Kw and Kf), with the increase in salinity, which resulted in elevated bioaccumulation, although the depuration rates (Ke) also increased. The relative abundance of long carbon chain length PFCs (i.e., PFDA and PFUnDA) increased as salinity increased, while the proportion of PFOS and PFOA decreased, which is explained by the positive relationship between δ and carbon chain length. The contribution of diet to bioaccumulation in oysters ranged from 18 to 92%. Overall, salinity not only affected the chemistry of PFCs, but also the physiology of oysters, contributing to sorption and bioaccumulation of perfluorochemicals in oysters.

Introduction Environmental contamination by perfluorinated compounds (PFCs) has been increasing with the increase in their production and usage (1-5). Monitoring studies have * Corresponding author phone: +82-62-970-2445; fax: +82-62970-2434; e-mail: [email protected]. † Gwangju Institute of Science and Technology (GIST). ‡ New York State Department of Health. § National Fisheries Research and Development Institute (NFRDI). | Hanyang University. 10.1021/es100151r

 2010 American Chemical Society

Published on Web 03/15/2010

demonstrated that PFCs bioconcentrate in aquatic organisms from surrounding waters (6) and higher trophic level organisms accumulate great concentrations of certain PFCs (6-8). Bioconcentration factors (BCF) and bioaccumulation factors (BAF) of organic pollutants can be estimated from physicochemical properties such as octanol-water partitioning coefficients (Kow) (9). However, Kow is not an appropriate parameter for PFCs due to their water and oil repelling properties (10). Furthermore, accurate values for Kow of PFCs are still not available. A more accurate approach to assess bioaccumulation and bioconcentration of PFCs is to employ a mechanical mass balance model (11) from laboratory studies involving animal exposure models. In such models, uptake and depuration rates are quantified through a twophase (uptake and depuration) experiment. During the uptake process, two main routes contribute to accumulation: direct uptake from surrounding water (bioconcentration) and ingestion of contaminated food (dietary accumulation). Previous studies demonstrated that direct uptake of aqueous PFCs in rainbow trout surpassed the dietary uptake (12, 13) under freshwater conditions. However, the relative significance of dietary accumulation is dependent on a number of factors such as body size, lipid content, and trophic level of organisms (14, 15). Oysters are filter-feeding bivalves, and accumulate toxicants through the ingestion of contaminated particles. It is important to note that PFCs are ubiquitous contaminants in coastal and estuarine environments (16-20). In these areas, changes in salinity are typical features. The changes in salinity affect the physicochemical properties of compounds as well as the physiology of organisms, consequently leading to changes in contaminant accumulation and toxicity (21-24). The bioaccumulation potential of PFCs in salinity gradients is not well understood. The purpose of this study was to examine the effects of salinity on the distribution of PFCs on bioaccumulation process in oysters, by conducting a controlled uptake and depuration study. In addition, the relative importance of the dietary uptake of PFCs on bioaccumulation was evaluated. Partitioning of PFCs between water and particles (oysters’ food) was also examined.

Materials and Methods Standards and Reagents. Perfluorooctane sulfonic acid potassium salt (PFOS; 12C8F17SO3K; 99%) and perfluorooctanoic acid (PFOA; 12C7F15COOH; 90%) were purchased from Fluka Chemical Co. (Tokyo, Japan) and perfluorodecanoic acid (PFDA; 12C9F19COOH; 98%) and perfluoroundecanoic acid (PFUnDA; 12C10F21COOH; 95%) were from Sigma-Aldrich (Oakville, ON, Canada). The internal standard, sodium perfluoro-1-[1,2,3,4-13C4] octanesulfonate (MPFOS; 13C412C4F17SO3Na; 98%) and perfluoro-1-[1,2,3,413 C4] octanoic (MPFOA; 13C412C4HF15O2; 99%), were purchased from Wellington Laboratories (Ontario, Canada). The stock solutions of PFC mixture containing identical concentrations of PFOS, PFOA, PFDA, and PFUnDA, 3000 mg/L, were prepared in HPLC grade methanol and then stored at 4 °C. Uptake and Depuration Regime. The test organisms, Pacific oyster, Crassostrea gigas, were collected from a local oyster farm in Tongyoung, Korea. They were acclimatized to the salinities, 10, 17.5, 25, and 34 psu, at 9-12 °C, one month prior to the exposure test. The salinity of the test solutions was adjusted by mixing filtered seawater and groundwater, by checking with a salinity meter (Orion 3star, Thermo scientific, Williston, VT). For comparison of dietary acVOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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cumulation of PFCs with bioconcentration, oysters at each salinity level were exposed to an identical PFC concentration (10 µg/L of PFOS, PFOA, PFDA, and PFUnDA) in the presence and absence of food. The bioconcentration study was designed to estimate the initial uptake rate (Ku) in the absence of food, lasting for 7 days, whereas the bioaccumulation test (in the presence of food) lasted for 56 days; the initial 28 days for uptake and the latter 28 days for depuration. Test chambers containing 60 L of test solution were cleaned and refilled 3 times a week. Saline water cultured Chlorella ellipsoidea, used as the food, was supplied at every point of renewal of solution, and the food density was set at approximately 4 × 105 cells/L by adding 30 mL of liquid Chlorella to 60 L of the solution at nominal PFC concentrations (10 µg/L). Test media samples were taken on the following day to determine the aqueous concentration of PFCs. In order to minimize the discrepancy in the amount of PFC sorbed to organisms, each chamber accommodated the organisms with same weight; 50 ( 5 oysters weighing 1.0 kg in each chamber. Sampling of oysters was done on days 2, 5, 9, 14, 21, and 28 during the uptake phase and the depuration phase, while oysters from the bioconcentration test were sampled on days 2, 5, and 7 for initial uptake rate estimation. Four oysters were sampled from each chamber during every sampling period. Oyster tissue was removed from the shell and rinsed three times with filtered seawater, and then separately stored in zip lock bags. For controls, unexposed oysters were sampled to determine the background concentration of PFCs. The tissues were kept at -80 °C until analysis. Sorption of PFCs to Chlorella. Sorption kinetics of PFCs to Chlorella at various salinity levels was also explored to estimate average exposure concentration of PFCs in the food (Cf) of oysters and to identify the effects of salinity on PFC sorption. Four solutions of varying salinity levels (10, 17.5, 25, and 34 psu) containing 10 µg/L of PFC were prepared in triplicate, and Chlorella was injected in at 0.5 mL/L; the density matched the feeding conditions in oyster bioaccumulation experiments. Fifty milliliters of the solution was transferred to 50 mL polypropylene tubes and placed in culture chambers rotating at 100 rpm. The chambers were kept at 12 °C, on a photo cycle of 16 h-light and 8 h-dark. The concentrations of aqueous PFCs in the solution and blank (no chlorella) were determined at 0, 2, 4, 8, 12, 18, 24, and 48 h. Five milliliters of aqueous samples, filtered through 0.2 µm nylon filters, were concentrated before LC/MS/MS analysis. The fraction of PFC remaining in solution was calculated from the concentrations determined in blank, and was then computed based on the following equation for the estimation of coefficients. ft ) f0 + a · exp(-b·t)

(1)

where t is the time (hr); ft is the fraction remaining in solution at time t; f0 is the fraction at the equilibrium; a and b are the coefficients. For the solution used in the sorption experiment, the distribution coefficient (Kd) for PFCs between water and particulate matter (mostly composed of Chlorella) was estimated with the fitting parameter, f0, representing the fraction of PFCs in solution at equilibrium, assuming that all of the lost fraction was partitioned into particulate matter. The separation of particulate matter from solution was done with a 0.2 µm nylon filter, and the residue on the filter was weighed after dewatering. Quantification of PFCs. The analytical procedures for PFCs in oyster tissue are described by Taniyasu et al. (25) and involved LC/MS/MS analysis with minor modifications as follows: Frozen tissue was thawed and homogenized. One gram of the homogenized tissue was transferred to a 50 mL 2696

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polypropylene tube containing 25 mL of 0.01N KOH/ methanol. The mixture was shaken for 16 h at 200 rpm. After alkaline digestion, the mixture was centrifuged at 2500 rpm for 15 min. Then, 0.5 mL of supernatant from the mixture was transferred to a 50 mL polypropylene tube and the tube was diluted with 50 mL of DI water. After thorough shaking, the diluted sample was loaded at a rate of 2 drops/sec onto a preconditioned Oasis HLB cartridge (6 cm3, 200 mg, 30 µm). The cartridge was then dried with an air pump and eluted with 15 mL of methanol. The eluent contained in the 25 mL polypropylene tube was concentrated by nitrogen gas at 40 °C to near-dryness and reconstituted with 1.0 mL of methanol, containing 25 µg/L of the internal standard, MPFOS and MPFOA. Before injection into the instrument, the analyte was filtered through a 0.2 µm nylon filter. Concentration of PFCs was determined by Alliance 2695 (Waters) HPLC coupled to a Quattro Microtriple quadrupole mass spectrometer equipped with an electrospray source operating in negative ionization mode. A 10 µL aliquot of extract was injected onto a Keystone Betasil C18 column (2.1 mm i.d. × 50 mm length, 5 µm) with 2 mM ammonium acetate/water and methanol, as mobile phases, at a flow rate of 250 µL/min. BCF and BAF Estimation. A mechanistic mass balance model was applied, as described by Mackay and Fraser (11), for the calculation of BCF and BAF. The procedure for estimation of BCF and BAF is described in detail in the Supporting Information (SI). Briefly, the constant for depuration rate was estimated from the first-order decay model, C0)a × exp(-Ke × t), where a is a constant. The aqueous (Kw) and dietary uptake rate constants (Kf) were determined by utilizing curve fitting with the concentration data, according to the following equations: C0(t) ) (KwCw)/Ke[1 - e-Ket]

(2)

C0(t) ) (KwCw + KfCf)/Ke[1 - e-Ket]

(3)

where Cw ) the concentration of the chemical in water (µg/ L); Cf is the concentration of the chemical in food (µg/L); Kw is the aqueous uptake rate constant (L/kg/d); Kf is the dietary uptake rate constant (L/kg/d); Ke is the depuration rate constant (1/d). BCF and BAF were estimated as Kw /Ke and Kw/Ke + (Kf/ Ke) × (Cf/Cw), respectively, where (Kf /Ke) × (Cf/Cw) is defined as the dietary accumulation factor (DAF).

Results and Discussion Uptake and Depuration of PFCs in Oysters. Over the study, mortality of test organisms was not observed and the background concentration of PFCs in control oysters was determined to be below the detection limit, 1.0 ng/g wet weight. The results of the recovery tests with fortified oyster samples are reported in SI Table S1. Uptake and depuration kinetics of PFCs in oysters exposed to four different salinities are depicted in Figure 1. Concentrations in oysters increased with time, up to a maximum (PFOS 8.1 × 102 ∼ 1.6 × 103 ng/g, PFOA 8.6 × 101 ∼ 1.3 × 102 ng/g, PFDA 6.5 × 102 ∼ 1.7 × 103 ng/g, PFUnDA 6.7 × 103 ∼ 2.2 × 102 ng/g) on day 28, exponentially decreasing afterward. The time-dependent increase in concentration in oysters (C0) and aqueous concentrations (Cw) were computed using the eqs 2 and 3. Fitting parameters were then estimated by nonlinear regression (Table 1). Among the four PFCs tested, PFUnDA was the most bioaccumulative in oysters, whereas PFOA was the least accumulative. PFOA and PFDA were completely eliminated after 28 days of depuration while PFOS and PFUnDA still remained in the tissues of oysters. Compared to the kinetics study on PFCs in freshwater fish (13), lower accumulation and faster elimination rates were observed in oysters,

FIGURE 1. Uptake and depuration kinetics of PFCs in oyster, Crassostrea gigas, under four different salinities. The symbols indicate 10 psu (b), 17.5 psu (3), 25 psu (9), and 34 psu (- · · [ · · -.), respectively. resulting in a low bioaccumulation factor. Given that the environmental levels of PFCs are lower than 10 µg/L, it is not likely that bioaccumulation in oysters would occur owing to fast depuration kinetics. Sorption of PFCs to Chlorella. During the uptake phase, test solutions were taken after every renewal (n ) 6), along with food particles over a 24 h period, and PFCs were measured in aqueous solution. On the basis of aqueous concentrations and the fraction of PFCs remaining in the solution after 24 h (f24), the average PFC concentration in food was determined, as shown in Table 1. The fraction of PFCs in solution decreased in a time-dependent manner and there was an apparent salinity effect on the fraction at equilibrium (f0), which decreased with increasing salinity (SI Table S2; Figure S1). The distribution coefficient (Kd) for PFCs was determined with f0 and particulate matter content in solution (0.113 g/L). Kd increased between 2.1- and 2.7-fold as salinity increased from 10 to 34 psu (e.g., Kd for PFUnDA: 2.2 × 103 g/mL at 10 psu and 6.0 × 103 g/mL at 34 psu) (Figure 2). The results demonstrate the existence of “salting-

out” phenomenon of PFCs onto particulate matter, represented by chlorella, in this study. A previous report by Martin et al. (7), showed high concentration of PFCs in benthic organism, Diporeia suggesting that PFC-contaminated sediment could be a source of bioaccumulation in benthic organisms. Enhanced sorption of PFCs to particulate matter can increase the risks to benthic organisms and filter-feeding bivalves. The magnitude of increment in Kd was quantified using salting constant (δ), estimated by applying exponential fits over salinity, as described elsewhere (26). Kdsw ) Kd0·ekadsS

(4)

Where Kdsw and Kd0 represent the distribution coefficients in saline and pure water, respectively, S is salinity and kads is a constant for sorption salting constant (-kads ) 0.0352δ). Estimates of δ ranged from 0.86 to 1.11 L/mol (SI Table S3). The salting-out effect is more considerable for aliphatic organic compounds possessing long-chains like PFCs than for aromatic compounds having the same carbon number VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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10 17.5 25 34 10 17.5 25 34 10 17.5 25 34 10 17.5 25 34

salinity (psu) f24c 0.90 0.86 0.80 0.76 0.94 0.90 0.86 0.83 0.88 0.86 0.85 0.81 0.82 0.80 0.76 0.69

Cw ( SDb (µg/L)

7.2 ( 0.9 7.0 ( 0.8 6.1 ( 0.8 6.1 ( 0.6 7.1 ( 0.6 6.7 ( 0.5 6.8 ( 0.7 6.5 ( 1.1 7.2 ( 1.2 7.1 ( 1.1 6.8 ( 0.9 6.9 ( 1.0 7.3 ( 0.9 7.1 ( 1.2 7.0 ( 1.1 6.8 ( 1.1

1.5 ( 0.2 2.3 ( 0.3 3.1 ( 0.4 3.8 ( 0.4 1.0 ( 0.1 1.4 ( 0.1 2.3 ( 0.2 2.7 ( 0.4 2.0 ( 0.3 2.4 ( 0.4 2.5 ( 0.3 3.3 ( 0.5 3.2 ( 0.4 3.5 ( 0.6 4.4 ( 0.7 6.1 ( 1.0

Cf × 103 ( SD (µg/L) 0.8 ( 0.1 (0.95) 0.9 ( 0.2 (0.68) 1.3 ( 0.1 (0.91) 1.4 ( 0.1 (0.91) 0.07 ( 0.01 (0.55) 0.08 ( 0.00 (0.95) 0.11 ( 0.01 (0.93) 0.13 ( 0.01 (0.79) 0.5 ( 0.1 (0.71) 0.9 ( 0.1 (0.83) 1.2 ( 0.1 (0.82) 1.5 ( 0.2 (0.67) 7.0 ( 1.8 (0.81) 10.2 ( 0.9 (0.95) 14.0 ( 1.6 (0.93) 17.4 ( 2.3 (0.82)

Ceq ( SE (µg/g) 8.6 ( 0.7 (0.87) 6.6 ( 0.8 (0.82) 5.3 ( 1.2 (0.78) 13.4 ( 1.2 (0.80) 0.5 ( 0.1 (0.57) 0.7 ( 0.2 (0.78) 1.1 ( 0.2 (0.50) 1.1 ( 0.1 (0.63) 10.2 ( 2.2 (0.79) 18.6 ( 3.0 (0.67) 19.8 ( 2.5 (0.55) 34.4 ( 3.6 (0.71) 71.0 ( 20.1 (0.70) 132.5 ( 31.4 (0.72) 222.6 ( 20.4 (0.80) 376.9 ( 7.0 (0.99)

Kw ( SEd (L/kg/d)

Ke ( SE (1/d) 0.10 ( 0.02 (0.95) 0.12 ( 0.01 (0.97) 0.17 ( 0.04 (0.93) 0.42 ( 0.08 (0.96) 0.63 ( 0.14 (0.96) 0.24 ( 0.05 (0.94) 0.48 ( 0.14 (0.91) 0.53 ( 0.07 (0.98) 0.43 ( 0.09 (0.95) 0.20 ( 0.01 (0.99) 0.51 ( 0.10 (0.96) 0.87 ( 0.08 (0.99) 0.11 ( 0.02 (0.92) 0.14 ( 0.02 (0.98) 0.13 ( 0.02 (0.96) 0.26 ( 0.06 (0.93)

Kf × 10-3 ( SE (L/kg/d) 13.3 ( 2.9 24.3 ( 6.5 61.4 ( 20.1 130.2 ( 28.7 41.0 ( 16.0 9.4 ( 3.6 19.7 ( 6.7 22.4 ( 4.6 79.1 ( 26.3 16.2 ( 3.4 182.3 ( 46.8 318.3 ( 62.9 82.6 ( 36.1 136.4 ( 37.8 75.2 ( 16.7 327.8 ( 90.0

84.8 ( 15.0 57.1 ( 9.6 31.6 ( 9.9 32.3 ( 6.8 0.8 ( 0.3 3.0 ( 1.1 2.4 ( 0.8 2.1 ( 0.4 23.9 ( 7.1 94.4 ( 16.6 39.3 ( 9.2 39.4 ( 5.6 633.9 ( 221.6 946.8 ( 247.9 1652.7 ( 312.9 1433.1 ( 342.7

BCF ( SE (L/kg)

113.0 ( 31.4 127.1 ( 40.2 217.0 ( 98.3 230.2 ( 70.0 9.6 ( 0.3 11.4 ( 1.10 16.0 ( 7.6 19.4 ( 0.40 75.5 ( 33.7 122.5 ( 33.4 170.2 ( 59.3 212.8 ( 51.8 954.6 ( 534.7 1431.5 ( 545.7 2006.1 ( 586.1 2555.2 ( 930.1

BAF ( SE (L/kg)

a BCF and BAF were estimated by Kw/Ke and Kw/Ke + (Kf/Ke) × (Cf/Cw), respectively. The average aqueous and dietary concentrations (Cw and Cf) are provided with standard deviation ((SD) and estimates parameters are presented with standard error ((SE) and r2 value in parentheses. b During the uptake phase experiments, water samples (n ) 6) were taken on the following day of solution renewal (approximately 24 h after the renewal). c The fraction of PFC remaining in solution after 24 h in the sorption kinetic experiment. d Kw is the initial uptake rate estimated with data from bioconcentration test (no food regime). The parameter was estimated with eq 4.

PFUnDA

PFDA

PFOA

PFOS

chemicals

TABLE 1. Concentration (Cw and Cf) and Fitting Parameters ((Ceq, Kw, Kf, and Ke) Estimated by Curve Fittingsa

FIGURE 2. Distribution coefficients for PFCs sorption to Chlorella at different salinities.

(27). Furthermore, PFCs are easily dissociated in ambient water, forming strong ion pairs with cations (28). When such ion pairs form, the chemical hydrophobicity increases due to the neutralization of charged moiety. Hence, it is thought that the salting-out effect on long-chain acids can lead to changes in the ecotoxicological risks posed on biota in areas with varying salinities, such as estuaries. Salinity and Bioaccumulation. As shown in SI Figure S2, PFC accumulation increased by 2.4-fold as salinity increased from 10 to 34 psu. Interestingly, the relative composition of PFCs also changed with salinity; Proportion of long-chain PFCs, that is, PFDA and PFUnDA, increased at high salinity levels (88.2% at 10 psu to 91.6% at 34 psu) (SI Table S4). In other words, long-chain PFCs partitioned more into oyster tissues than PFOS and PFOA. This can be attributed to the relationship between chain length and salting constant (27), as discussed below. The plot of BAF over salinity (Figure 3) showed that BAF increased proportionally with salinity, even though no trend was observed for BCF; this implies that the increase in PFC accumulation with salinity is mainly due to the increase in dietary uptake. This may be due to altered physiology of oysters with changing salinity. Since BCF was calculated as the ratio of Kw to Ke, instead of Co to Cw, including different experimental conditions by which Kw and Ke were determined, care should be taken to assess the salting-out effects between water and oyster tissues. Nevertheless, the dietary accumulation factor demonstrated an increasing trend with the increase in salinity (SI Figure S3), often associated with the exponential increase of Kd. The contribution of diet to bioaccumulation ranged from 18 to 92%, which is relatively considerable when compared to the values reported in other studies (less than 1%,for example, BCF 4.0 and the dietary accumulation factor of 0.038 for PFOA, refer to 12, 13) demonstrating that dietary accumulation in freshwater trout is limited by the amount of food consumption (1.5% of body weight). The discrepancy in the significance of dietary accumulation between oysters and trout may be attributable to differences in feeding habitats. Our results demonstrate that Chlorella does not only act as a sorbent, but also as a carrier of PFCs, implying that in filter-feeders like oysters, the ingestion of contami-

FIGURE 3. Relative significance of bioconcentration factor (BCF) and dietary accumulation factor (DAF) to overall accumulation (bioaccumulation) of PFCs under variable salinities. The sum of BCF and DAF corresponds to BAF.

FIGURE 4. Linear relationship between carbon number and (a) Log Kd,avg, (b) Log BAFavg, and (3) Log δ. Kd,avg and BAFavg are the average values of Kd and BAF at each salinity level, respectively. The solid line indicates linear regression for perfluorocarboxylates. nated particulate matter is one of the major routes for the bioaccumulation of PFCs (7). According to the recommendations of the USEPA, chemical substances having BCF/BAF > 1000 are characterized as

“tendency to accumulate in organisms” (29), whereas, in Europe, bioaccumulative substances are defined with BCF/ BAF values greater than 2000 (30). On the basis of this criterion, only PFUnDA, at salinities higher than 17.5 psu, is VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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considered bioaccumulative. This supports relatively high concentrations of PFUnDA found in organisms collected from coastal and offshore waters of the western North Pacific Ocean (31), in spite of lower aqueous concentrations of this compound compared to PFOS, PFOA, and PFDA (31, 32). Fitted parameters denoted in Table 1 showed a general tendency to increase when salinity increases. Particularly, for the relationship between BAF and salinity, such increasing trends and discrepancy within PFCs were apparent, being quite similar to patterns for Kw. However, it was not clear whether effects of salinity on fitting parameters are associated with changes in chemical fugacity, or with changes in the biological state of oysters (or bioavailability) under varying degrees of salinity. This is because the results are consequences of the effects of salinity simultaneously imposed on the bioavailability of chemicals (21, 22, 24) as well as the physiology of organisms (23, 33, 34). In general, marine bivalves decrease their respiration and predation rates with decreasing salinity, inevitably accompanied by a decrease in the uptake of contaminants. The increase of uptake and depuration rate constants (Kw, Kf, and Ke), associated with increasing salinity, is strong evidence for physiological changes found in oysters, as revealed in this study. Based on the fact that biotransformation of PFCs is negligible (35), the faster depuration rate for PFCs at higher salinities is attributable to increases in the uptake volume of water associated with increased salinity. Oysters at lower salinity have high water content in tissue in order to regulate osmotic pressure and minimize water uptake. This biological response to salinity has consequently evoked changes in the bioaccumulation of PFCs. The effects of salinity on chemical activity were also observed during sorption kinetics experiment with chlorella. The salting-out effect may lead to an increase in the bioaccumulation of PFCs mediated by contaminated food. Overall, the increase of salinity induces not only increased uptake but also depuration of PFCs. However, the extent of the increase in uptake is greater than that in depuration, leading to the enhancement of bioaccumulation with increasing salinity. Relationship Between Carbon Number and Chemical Distribution. For perfluoroalkyl carboxylates, positive linear relationship between the number of carbon chain length and averaged distribution factors was observed within salinities (Kd,avg and KBAF,avg) and the salting constant (δ) in log scales (Figure 4). The relationship of Log BAFavg (slope in the linear regression: +0.676) was comparable to the previous findings (slope for Log BCF per carbon: +0.915) (13), though the test species were different. This can be explained by binding affinity to proteins, increase with increasing carbon chain (36). Yet, an increase in Kd in association with carbon chain length might be related to the surface chemistry of cell membranes, considering the great surface/volume ratio of Chlorella. The salting constant for PFCs was dependent on chain length, regardless of the functional group, comparable to the salting constants of several aliphatic compounds possessing 8-10 carbons, that is, octanal, nonanal, and decanal, ranging from 0.6 to 1 L/mol (27). This relatively high salting constant for long-chain PFCs contributed to significant changes in chemical fate as a result of salinity changes, the extent of variation being positively associated with carbon length. Thus, sorption and bioaccumulation of PFCs with long-chain length (i.e., PFUnDA) vary greatly in estuarine environments. Between carboxylate and sulfonate groups, PFOS showed greater affinity to Chlorella and oysters than PFOA, confirming that functional groups and carbon chain lengths affect sorption affinity (13, 37, 38). Ecotoxicological Implications. When contaminants enter the estuarine environment, they experience salinity gradients 2700

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by which the fugacity of the chemicals changes. Particularly, ionized organic compounds (e.g., PFCs forming ion pairs) are readily subjected to the effects of salt (39), changing the phase transfer of chemicals between aqueous and solid phases. In general, the partitioning of organic compounds to particulate matter increases as salinity increases, likely reducing water-mediated exposure to contaminants. However, benthic organisms, habitually in contact with particulate matter to collect nutrients, are at great risk due to increased chemical concentration in the particulate phase. As revealed in the present study, concentrations of PFCs in Chlorella at 34 psu are 1.7-2.7-fold greater than those at 10 psu (Table 1). When considering faster food uptake rates at higher salinity concentrations, the amount of PFCs accumulated in bivalve filter-feeders such as oysters could be considerably high. For that reason, changes in salinity should be taken into consideration for assessing ecotoxicological risks of PFCs in estuarine environment.

Acknowledgments The present work was supported by basic research project through a grant provided by the Gwangju Institute of Science & Technology in 2009.

Supporting Information Available Information on estimating BCF/BAF by mechanistic mass balance model, PFC analysis recovery test, estimation of fraction for PFCs in solution at equilibrium (f0), fitting parameters, accumulation of PFCs molecules under various salinities, relative composition (%) of accumulated PFCs, and sorption kinetics. This material is available free of charge via the Internet at http://pubs.acs.org.

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