Environ. Sci. Technol. 2008, 42, 6285–6290
Validation of Biokinetic Model of Metals in the Scallop Chlamys nobilis in Complex Field Environments KE PAN AND WEN-XIONG WANG* Department of Biology, The Hong Kong University of Science and Technology, Clearwater Bay, Kowloon, Hong Kong
Received March 6, 2008. Revised manuscript received May 26, 2008. Accepted May 30, 2008.
A transplantation experiment, using the scallop Chlamys nobilis as the model organism, was carried out to investigate variations in their bioaccumulation of Cd and Zn in different environmental conditions. The diffusive gradients in the thin films technique was employed to monitor in situ dissolved labile metal concentrations. The impact of food quality on assimilation efficiency and ingestion rate also was investigated in the laboratory. The results were combined in a biokinetic model explaining the metal concentrations accumulated in the scallops. The results confirm that scallops accumulate metals differently in different marine environments with comparable ambient metal concentrations. Food quality not only influences their assimilation of metals but also their clearance rates. The ingestion rate together with the growth rate was shown to have an effect on the bioaccumulation of metals. Bioaccumulation is dependent on both the ingestion rate (when food is the dominant metal uptake pathway) and the proportion of metal taken up from the water. Environmental conditions such as food availability and hydrology must be simultaneously considered in any attempt to study metal bioaccumulation in marine bivalves.
Introduction Marine bivalve mollusks can accumulate high concentrations of metals from the ambient environment, thus posing a potential seafood safety issue to the public (1). Many bivalve species such as mussels and oysters also have been employed in biomonitoring programs (2); thus, the accumulation mechanisms of these biomonitors for a particular metal should be known (3). Recently, the development of a biokinetic model that considers separately bioaccumulation from the dissolved phase and from food has allowed more accurate quantitative evaluation of metal uptake in aquatic animals (4, 5). Dynamic processes involved in metal uptake from the water and diet, as well as the subsequent loss, have been well-investigated in bivalves (6–8). Among the biokinetic parameters required in a biokinetic model (e.g., dissolved uptake, dietary assimilation, and efflux), the growth rate constant has received the least attention. Growth rate is incorporated into biokinetic models to calibrate the growth dilution of metals. It can be ignored when it is much smaller than the metal efflux but should be incorporated into any model treating rapidly growing small organisms (e.g., phytoplankton or bacteria (9)). For bivalves, growth also can produce a significant effect on metal * Corresponding author e-mail:
[email protected]. 10.1021/es800652u CCC: $40.75
Published on Web 07/09/2008
2008 American Chemical Society
concentrations in the soft tissues when it exceeds or is comparable to the efflux. However, the effect of growth on metal bioaccumulation is not easily evaluated. Growth actually reflects a balance between energy acquisition (feeding and absorption) and energy expenditure (metabolism and excretion). It is closely coupled with other physiological rates such as the ingestion rate, which is also a key parameter in determining metal concentrations in the animals. Meanwhile, the water current, food quality, and availability are all known to affect the growth of bivalves due to their direct influence on clearance and ingestion rates (10). Food quality is another important factor controlling the bioaccumulation of metals since it can affect dietary assimilation (11). Thus, food quality, ingestion rate, and growth constitute a complicated and interactive process in the bioaccumulation of metals, but such interaction remains rather speculative at present. The present study investigated the inter-relationship among food quality, ingestion rate, and growth in controlling the bioaccumulation of metals in bivalves. To achieve this objective, a transplantation experiment was carried out in Hong Kong waters under complex environmental conditions. The subtropical scallop Chlamys nobilis was selected as a model organism since this scallop exhibits a strong ability to take up metals from the environment (12). There has been widespread concern about the bioaccumulation of metals in scallops (12, 13). Scallops also have been used as sentinel organisms in environmental monitoring; thus, there is a need for a clear understanding of the mechanisms of metal accumulation to interpret biomonitoring data. Cadmium and zinc accumulation were specifically examined since these metals can be accumulated by scallops to high concentrations. Bioaccumulation of Cd and Zn in the scallops under various geochemical and hydrological conditions in the field was investigated, and the assimilation of metals and clearance rate related to food quality were measured in the laboratory. The recently developed diffusive gradient in a thin film (DGT) technique was employed to monitor the dissolved labile metal concentrations in situ (14). All these processes were then incorporated into a biokinetic model to predict likely metal concentrations and allometry in scallops.
Materials and Methods Transplantation. Transplantation experiments were conducted in Dapeng Bay and Clearwater Bay in eastern oceanic waters off Hong Kong from March to July 2007. Both sites are far from urban runoff and are considered relatively pristine. To obtain scallops with different growth rates, groups of scallops were transplanted to different locations with various hydrological and food conditions. Previous studies had shown that growth in scallops was affected greatly by factors such as temperature, current speed, water depth, food quality, and availability (10, 15). Taking advantage of different phytoplankton productivity and hydrological conditions in Hong Kong waters, the scallops were allocated to four different locations (see Supporting Information). Dapeng Bay is characterized by high total suspended particulate matter and high productivity of phytoplankton, while Clearwater Bay is the opposite (16). Stations I and II were in Dapeng Bay, whereas station III was in Clearwater Bay. Group A of scallops was immersed at a depth of 2-3 m at station I located 100 m offshore (water depth of 7 m). Another two groups (groups B and C) were maintained at station II (water depth of 12 m), which was farther offshore. At this station, group B was located in the surface water (depth of 2-3 m as for group A), while group C was located 1-2 m above the bottom VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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for the reason that bottom waters are generally less favorable for scallop growth than surface waters (17). Station III (group D) in Clearwater Bay was in a subtidal zone with a water depth of only 5 m. Water flow was turbulent at this station because of the tidal cycles, which was thought to affect the feeding rate of scallops and thus their growth. The scallops were suspended 0.5-1 m above the bottom at station III. Juvenile scallops C. nobilis of standard shell size (25 ( 5 mm) were obtained from an aquaculture farm. All scallops were developed from the same batch of larvae and grown in Dapeng Bay to ensure similarity in their life and environmental histories. To form each group, 200 individuals were placed in a 10 layer polyethylene mesh cage (30 cm × 30 cm × 120 cm). They were transplanted in March and allowed to adapt to local environmental conditions for 1 week. Random samples of 10 scallops were selected from each layer of each group at 15 day intervals for the first 2 months and later at monthly intervals. For group D, samples were taken monthly because of their slow growth. All samples were transported to the laboratory immediately and frozen at -80 °C prior to analysis. Field Sampling. At each sampling, an optical water quality monitor (YSI, model 6600) was deployed to measure the temperature, pH, oxygen, and salinity profile at each site. Water samples also were collected from each location using a 10 L polyethylene water sampler. Back in the laboratory, chlorophyll (Chl) a and total suspended particulate matter (SPM) concentrations were measured. The suspended matter was filtered out with a 47 mm GF/F filter (Whatman) in two replicates. Chl a was extracted with cold 90% acetone in the dark for 24 h and then determined spectroscopically (18). Trace metals in the SPM were measured by filtering water samples onto preweighted 47 mm × 1 µm polycarbonate filters (Osmonics). The filtrate was further passed through two blank filters used as controls. Isotonic ammonium formate was added to remove the salt. All the filters were then dried at 60 °C and weighed. The diffusive gradients in a thin film (DGT) technique was used to measure the dissolved labile metal concentration at each site. Each month, two piston-type DGT cells (DGT Research Ltd., Lancaster, U.K.) were hung outside each cage. The DGT unit contained a 0.45 µm polysulfone membrane as a protective outer layer, a 0.8 mm open pore gel as a diffusive layer, and a layer of Chelex 100 resin as a binding agent. The DGT devices were deployed for only 15 days each month to avoid heavy biofouling (but to have sufficient accumulation of metals). After collection, the DGT units were rinsed with deionized water and brought immediately back to the laboratory in ice-cooled black plastic bags. All samples were kept at 4 °C until analysis. Metal elution was carried out in 1 mL of 1 M HNO3 in an acid-cleaned 1.5 mL Eppendorf centrifuge tube for 24 h. All the equipment was cleaned with 0.1 M nitric acid and MilliQ water prior to use. Metal analysis was performed using a PerkinElmer (AAnalyst 800) graphite furnace atomic absorption spectroscope. The average metal concentration was calculated using the following equation: C)
M∆g DAt
(1)
where M is the mass of metal accumulated in the resin gel layer (µg), ∆g is the diffusive layer thickness of the diffusive gel (0.08 cm) plus the thickness of the filter membrane (0.014 cm), D is the diffusion coefficient of the metal given by DGT Research Ltd., A is the exposure area (cm2), and t is the development time (s). The diffusive boundary layer was assumed to be negligible considering the highly turbulent conditions. Laboratory Analysis of Metals in Scallops. The soft parts of the scallops were removed and then dried to constant weight at 80 °C. The average growth rate constant was 6286
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calculated by the slope of the regression between natural log of soft tissue dry weight and time. The Cd and Zn concentrations in the scallops and the SPM were quantified using conventional methods. Briefly, dried scallop tissue and SPM were digested with concentrated nitric acid (70%, Fisher Scientific) plus 1 mL of perchloride acid at 110 °C in an autoregulated heating block. Oyster tissue (1566a) and estuarine sediment (1646a) standards (National Institute of Standards and Technology, Gaithersburg, MD) were digested simultaneously, and the recovery was >95%. The method detection limit was 0.001 µg g-1 for Cd and 0.01 µg g-1 for Zn. Metal Assimilation Efficiency (AE) from Foods of Different Quality. To investigate the metal assimilation under different food conditions, a mass balance method was used. Three foods were used in this study: the diatom Thalassiosira nordenskioeldii, seston, and sediment. The diatoms were grown in f/2 medium with N, P, Si, vitamins, and metals but no Zn, Cu, or EDTA. The natural seston (1-50 µm) was collected from Clearwater Bay, Hong Kong. Seawater was first passed through a 50 µm mesh, followed by filtration using 1 µm polycarbonate filters and then resuspended in 500 mL of 0.22 µm filtered seawater. The filtration pressure was
