Development and Evaluation of a Database of Dietary

Critical Review pubs.acs.org/est

Development and Evaluation of a Database of Dietary Bioaccumulation Test Data for Organic Chemicals in Fish Jon A. Arnot*,†,‡ and Cristina L. Quinn†,§ †

ARC Arnot Research and Consulting Inc., 36 Sproat Avenue, Toronto, Ontario, Canada, M4M 1W4 Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada, M1C 1A4

‡

S Supporting Information *

ABSTRACT: Dietary bioaccumulation tests for fish have been conducted for about 40 years. Standardized test guidance has recently been developed. Test metrics of primary scientific and regulatory interest are the whole body depuration rate constant (kT), whole body growth corrected depuration rate constant (kTg), and corresponding chemical half-lives (t1/2 and t1/2g), dietary chemical absorption efficiency (AE), and biomagnification factor (BMF). A database of 3032 measurement end points for 477 discrete organic chemicals including 964 half-lives, 1199 AEs and 869 BMFs from 19 species (primarily trout and carp) was developed from the literature. Biological properties (e.g., organism weight, lipid content) and exposure conditions (e.g., temperature, feeding rate, dietary lipid content, exposure duration) are documented. Test chemicals range in molar mass from 120 to 1423 g·mol−1 with log octanol−water partition coefficients (KOW) ranging from 0.8 to 14.3; 50% of the database entries are for polychlorinated biphenyls. The measured end points are derived from various protocols and sources of variability are described. The data are evaluated and categorized using proposed data quality (confidence) criteria derived from the standardized test protocol providing initial guidance for data users. Half-lives range from 0.13 to 2600 days; however, approximately 54% have an identifiable source of uncertainty. The data suggest that chemicals absorbed from the gastrointestinal tract with a log KOW ≥ ∼5 and at least as high as ∼9 have biomagnification potential in fish. A mechanistic bioaccumulation model is compared to the measured data and used to illustrate the influence of growth and biotransformation rates on the BMF.

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Dietary bioaccumulation testing in fish has been conducted for about 40 years led by the pioneering experiments of Hamelink et al.,1 Zitko,33−36 Bruggeman, Hutzinger, Opperhuizen, and colleagues;4,37−40 Niimi, Oliver, and colleagues;41−46 Gobas, Mackay, and colleagues;5,7,47−49 and Muir, Fisk, and colleagues,50−57 with contributions from several others. In 1987 Niimi41 reviewed biological half-lives of chemicals in fish. In 2008 Barber58 reviewed several models for describing bioaccumulation from dietary exposure with a focus on chemical assimilation efficiency. In 2012 the Organization for Economic Co-operation and Development (OECD) dietary bioaccumulation testing guidance for fish was adopted in the revision of Test Guideline 305.59 The new guidance59 provides context for a systematic review of dietary bioaccumulation testing methods, literature, and data. The objectives here are to collect dietary bioaccumulation testing data, critically examine the existing data and literature against the guiding principles in the new OECD 305 guidelines,59 and provide recommendations for the selection

INTRODUCTION Chemical bioaccumulation in fish and food webs is of scientific1−17 and regulatory18−21 interest. Various criteria and metrics are used for bioaccumulation assessment including the octanol−water partition coefficient (KOW), bioconcentration factor (BCF), bioaccumulation factor (BAF), biomagnification factor (BMF), biota-sediment accumulation factor (BSAF), and the trophic magnification factor (TMF).22,23 Currently the majority of measured bioaccumulation data are fish BCFs determined in laboratory tests from chemical exposure in the surrounding water only.24 For hydrophobic chemicals, there are technical challenges obtaining reliable quality BCFs.24,25 In the environment, dietary exposures can be relatively greater than aqueous exposures for hydrophobic chemicals26,27 and there is a potential for biomagnification.28−31 For neutral organic chemicals, biomagnification occurs when there is an increase in chemical fugacity from the diet to the consumer and biomagnification is determined using fugacity ratios or lipid corrected BMFs (i.e., BMFL > 1 indicates biomagnification).22,23 Regulatory agencies can consider BMFs as part of a bioaccumulation assessment.32 Reliable testing data are desirable for bioaccumulation assessment, improved understanding of processes, and for the development and evaluation of models. © XXXX American Chemical Society

Received: December 22, 2014 Revised: March 23, 2015 Accepted: March 30, 2015

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DOI: 10.1021/es506251q Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Overview of the general OECD 305 dietary bioaccumulation test guidance methods and calculations59 including (A) the experimental stages of chemical exposure and depuration, initial data analysis to calculate (B) the depuration rate constant (kT) and (C) the growth rate constant (kG), followed by (D) the calculation of other metrics including the half-life (t1/2), the growth corrected depuration rate constant (kTg), the growth corrected half-life (t1/2g), the chemical concentration in fish at start of depuration (CF0), the chemical absorption efficiency (AE), the kinetic biomagnification factor (BMFK), and the growth corrected kinetic biomagnification factor (BMFKg) using measurements or estimates of the chemical concentration in diet (CD) and the dietary ingestion rate (I).

in which case kG is not included40 and k1 and kD have units μg· d−1. Dietary Bioaccumulation Test Methods and Metrics. Figure 1 summarizes the general OECD 305 dietary bioaccumulation test methods and metrics.59 Fish are first fed a measured quantity of intentionally contaminated food in the exposure (dosing) phase (Figure 1A). There is no chemical exposure from the water; k1CWD = 0 in eq 1. This is followed by a depuration phase in which the fish are no longer exposed to the test chemical. The CF measurements in the depuration phase are used to calculate kT (Figure 1B). Following test guidance59 CFs are log or ln transformed and plotted as a function of time and linear regression of the data is performed, the slope being −kT (Figure 1B). The growth rate can be determined throughout the testing period or in the depuration phase (Figure 1C) and used to calculate the growth corrected elimination rate constant kTg (Figure 1D). The corresponding half-lives are t1/2 and t1/2g, respectively. Alternatively, kTg and t1/2g can be calculated from XF.40,41,44,59,60 The parameters AE, BMFK (kinetic), and BMFKg (kinetic and growth corrected) are often subsequently calculated from kT and kTg using CD, I and fish concentrations at the beginning of the depuration phase (CF0).59 BMFs should be growth and lipid corrected.59 The mean lipid fractions in the fish (LF) and diet (LD) are used to calculate BMFL as BMF/(LF/LD). Growth and lipid corrected kinetic BMFs are BMFKLg (also see Supporting Information, SI1). Data Collection and Evaluation. Database Development. The BMF is applicable for biomagnification assessment.22,23 The BCF, BAF, and TMF can be estimated from kT,41,59,61 and t1/2 has been proposed for bioaccumulation assessment.62 Data were considered relevant63 if at least one of kT, t1/2, kTg, t1/2g, AE or BMF was clearly measured and documented from a dietary exposure experiment. Over 100 published studies were collected from the peer-reviewed literature and the public domain (finishing ca. September 2013). Only tests with clearly defined organic chemicals and species were considered. If reported, biological, and exposure conditions for which the OECD 305 provides guidance were

of data for models and bioaccumulation assessment. Theoretical concepts and terminology related to dietary bioaccumulation testing and data analysis are first presented. The new database of test information comprising several study parameters is described. Based on the OECD 305 guidelines,59 data quality criteria are developed and applied to evaluate and categorize existing data. Finally, some recommendations for future testing and guidance revision are provided.

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THEORY Chemical uptake and elimination in a fish can be expressed using a typical toxicokinetic (bioaccumulation) mass balance model as11,37 dC F/dt = k1C WD + kDC D − (k 2 + kE + kB + k G)C F = k1C WD + kDC D − k TC F

(1)

where dCF/dt is the net change in chemical concentration in the fish over time t (d), CF is the concentration in the fish (μg· kg−1), k1 is the chemical uptake rate constant from water (L(kg· d)−1), CWD is the dissolved chemical concentration in water (μg·L−1), kD is the chemical uptake rate constant from the diet (kg(kg·d)−1), and CD is the chemical concentration (μg·kg−1) in diet. kD is a product of the food ingestion rate (I; kg(kg·d)−1) and the chemical transfer efficiency from the diet (often referred to as the assimilation or absorption efficiency, α, AE or ED; unitless). The rate constants corresponding to potential decreasing chemical concentrations in the fish (d−1) are for elimination to water, fecal egestion, biotransformation and growth dilution as k2, kE, kB, and kG, respectively. The total chemical elimination rate constant is the sum of various individual loss processes as kT = k2 + kE + kB + kG. Following first-order kinetics the total biological half-life of the chemical in the fish is t1/2 = ln2/kT. Growth dilution is a pseudoelimination process because the chemical is not actually eliminated from the organism and dCF/dt is a function of changes in biomass. The chemical mass balance in eq 1 can also be expressed in terms of chemical quantity (body burden approach) in the fish (XF; μg) B


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Since kTg can be calculated from kT and kG (Figure 1D, eq S3), errors in kT or kG can propagate into kTg. When kT ≫ kG, errors in kTg correspond to errors in kT; however, when kT and kG are similar in value, errors in kG can influence kTg (see further discussion below). For chemicals with relatively slow elimination rate constants and high biomagnification potential (e.g., PCBs), errors in kT (or kTg) can lead to errors in BMFK (or BMFKg). If kT (or kTg) is overestimated (i.e., elimination is actually slower than estimated) then BMFK (or BMFKg) will be underestimated. Uncertainty in the AE and BMFK calculations is also a function of the uncertainty of the input parameters specific to the test. Uncertainty analysis could address uncertainty in bioaccumulation metrics if estimates of error for the parameters used in the calculations are available.68 The format for presenting the data review is as follows. First, the rationale for including the data assessment categories and assessment criteria is made by referring to the OECD guidance59 or by providing a brief explanatory statement. This is followed by some description of the parameters reported in the database and how the data are distributed into the data quality scores described below (e.g., summary statistics) and some discussion on the test category topic. Klimisch et al.63 suggested four classifications for assessing data reliability and these general principles are considered here. According to Klimisch et al.,63 data are classified as “reliable without restriction” if the study complies with nationally or internationally accepted guidelines or is generally conducted following these methods and key test information is described in sufficient detail. Data generated from studies that meet the basic scientific principles of the guidelines and include adequate documentation but lack key details or do not sufficiently comply with general guideline requirements are classified as “reliable with restrictions”. Data are classified as “not reliable” when key methodological considerations are clearly omitted or significantly differ from the guidelines or when key documentation or data evaluation reveals unacceptable performance of the test or data analysis. The database does not include any entries that only reported end points,63 and good laboratory practice (GLP)69 was not considered here because the vast majority of the data are not GLP certified.63 The overarching objectives of this data quality assessment are to develop some criteria, evaluate the existing literature, and identify data quality issues and significant deviations from current testing guidance. The data quality assignments are subject to error and misinterpretation due to incomplete and uncertain information and, by necessity, require professional judgment. While we maintain the general principles of the “Klimisch approach” we use the classifications (scores) High (H), Medium (M), and Low (L) confidence to provide relative screening indicators of perceived data quality and consistency with OECD guideline principles, that is, H > M > L. Some data screened L may actually be less uncertain than some data screened H or M, or vice versa. Data points screened as L may still be useful for certain purposes with due consideration of the issues identified in this review. Table 1 summarizes general requirements for test data to be categorized H and the database (Supporting Information) includes a table detailing the scoring methods. Chemical Information. The guiding principle for identifying data with high quality chemical information is that chemical specific analyses provide test results to a definable chemical structure. A commonly used representation for a chemical structure is a SMILES notation and is the criterion used here.

documented in the database. For example, the database includes fish mass and lipid content, feeding rate, growth rate, water temperature, diet (or dose) concentration, lipid and protein content of diet, and tissue analyzed. The database also includes chemical identification information (e.g., chemical name, chemical abstract service registration number (CAS#), and Simplified Molecular-Input Line-Entry System (SMILES)64 notation), and physical-chemical properties (e.g., molar mass, KOW obtained from65 with measured chemical properties selected preferentially over predictions, unless otherwise noted66,67). The database, including other test details and primary source references, is presented in the SI in a Microsoft Excel spreadsheet. The database contains 3032 measured test end points for 477 chemicals (based on CAS#). There are 964 half-lives, 1199 AEs and 869 BMFs (632 are BMFLg). There are 1512 study entries and some studies have more than one measurement end point from the same test for the same chemical. For example, there are 642 study entries with values for t1/2, AE and BMF. The data are measured from 19 fish species; the majority of entries are from trout species (56%) and common carp (19%). The chemicals range in molar mass from 120 to 1423 g·mol−1 (median = 326) and log KOW from 0.8 to 14.3 (median = 6.6). Fifty percent of the data entries are for polychlorinated biphenyls (PCBs), approximately 15% are other legacy organochlorine pollutants (e.g., hexachlorobenzene, dioxins, furans, mirex), 13% are hydrocarbons, and about 10% are brominated flame retardants, predominantly polybrominated diphenyl ethers (PBDEs). Approximately 10% of the chemicals in the database are ionogenic. Data Assessment Methods. The database includes critical review of the 3032 test end points following the data quality assessment criteria outlined in categories of test information below. Previous methods for assessing ecotoxicology63 and bioaccumulation24,25 data and the OECD guidelines for dietary bioaccumulation testing59 form the basis for developing the data quality assessment methods. The guidelines59 were developed from protocols implemented and refined over the past few decades. Because few studies to-date followed the guidelines, only some of the fundamental principles were considered. The methods evolved through the literature review. Categories of test information selected for review were based on OECD guidelines,59 the degree to which testing parameters were reported in the literature, some sensitivity analysis for guideline calculations, and professional judgment. This review cannot cover all aspects of the test and data analysis. A simple, generic sensitivity analysis (see Supporting Information SI-2 for details) was used to evaluate the significance of potential errors in typical AE and BMFK calculations (Supporting Information eqs S4 and S6) outlined in OECD guidance59 (Figure 1D). The analysis was for three hypothetical chemicals representing relatively slow, moderate and fast chemical elimination (kT); t1/2s are 100, 10, and 1 day, respectively. For all three chemicals, the AE and BMFK calculations are very sensitive to changes in CD and CF0. The AE calculation is sensitive to I but BMFK is not because I is in both the numerator and denominator. For relatively persistent chemicals (typically slow elimination), AE is relatively insensitive to changes in kT while BMFK is quite sensitive to changes in kT. For chemicals with relatively fast elimination, AE is sensitive to changes in kT and BMFK is insensitive to changes in kT. For chemicals with moderate elimination rates, AE and BMFK are moderately sensitive to changes in kT. C


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SMILES: Simplified Molecular-Input Line-Entry System; kT: depuration rate constant; kTg: growth corrected depuration rate constant; CD: chemical concentration in diet; I: dietary ingestion rate; bw: body weight; AE: chemical absorption efficiency; GIT: gastrointestinal tract; BMFSS: steady-state biomagnification factor; BMFKg: growth corrected kinetic biomagnification factor; BMFK: kinetic biomagnification factor.

measured and reported test temperature that is tolerable for the test species species documented and body size and lipid content measured and reported

Chemical purity and radiochemical purity were not considered and we assumed that all of the published analytical methods are appropriate. If only the total radioactive residues are quantified and the chemical is subject to biotransformation it is possible that metrics include quantification of the parent chemical and the metabolite thus confounding the value for the parent chemical.24,25,70 Parent chemical specific analyses are required for H classification. Radiolabeled chemicals were used for 3% of the study entries. Most tests using radiolabeled chemicals that were not subject to parent chemical specific analyses are considered L. For some chemicals (e.g., PCBs), biotransformation rates are generally very slow; therefore, tests using total radioactivity analysis for these chemicals are considered M if evidence suggested negligible biotransformation occurred. Chemical Mixtures. Test guidance59 states that more than one chemical can be tested in a single test provided certain criteria are fulfilled. The ring test for the OECD dietary guidelines71 included simultaneous exposures to five chemicals with log KOWs ranging from 4.5 to 6.1 (Supporting Information Table S2). Some tests have been conducted for a single chemical but most tests have been conducted for mixtures of chemicals and/or congeners, for example, PCBs, 51,54,72 PBDEs,73 and mixtures of congeners and isomers (e.g., chlorinated paraffins),52,53,55 and mixtures of stereoisomers (e.g., hexabromocyclodecane, HBCD).74 Some evidence suggests that multiple chemicals can be tested at the same time (coexposure) without substantial differences in the results16,17,71 but this is not always the case. Tests using chemical mixtures were scored H as long as there is no reported evidence of cometabolism, enzyme saturation, enzyme inhibition, or effects of bioformation or isomerization or toxicity on the results. Co-dosing PBDE congeners can result in potential misinterpretation of bioaccumulation parameters because of debromination.73,75−78 When fish are coexposed to certain mixtures of BDE congeners, the bioaccumulation parameters for a less brominated BDE congener can include quantification of the less brominated congener and debrominated metabolites from certain, more brominated, congeners. The issues of isomerization and chirality are not addressed in current guidance59 but, when applicable, require some consideration when testing because of isomer specific biotransformation rates and reported differences in data for racemic mixtures.57,79 When there is evidence of bioformation or bioisomerization the data are initially scored L. If methods are applied to address bioformation or bioisomerization, the data are scored M. An exception is that possible differences in PCB data as a result of chirality (8 congeners) are recognized,57 but not considered here. Biological Information. Test guidelines recommend starting the experiment with fish of similar age, body size and lipid content.59 Body size8,41 and lipid content24,37,80 can influence test metrics; therefore, it is important to measure and report these parameters. Study entries that include measurements for test organism mass and lipid contents are categorized H (1479/ 1512 for mass, 98%; 1332/1512 for lipid content, 88%). Reported initial fish mass range from 0.1 to 1000 g (median = 10 g) and final reported fish mass range from 3.8 g to 3500 g (median = 370 g). Mean “whole body” lipid contents range from 1.0% to 9.9% (median = 5.3%) and analyzed tissue lipid contents range from 0.43% (muscle) to 15.4% (not specified). Study entries that do not report mass or body size or lipid content are L (2% for mass; 12% for lipid content). In rare

a

growth correction documented for log KOW > 5; no identified source of error in the methods or calculations fed clean (uncontaminated) food while depurating in clean water in flow through systems with no detectable background contamination (exposure and depuration) no reported evidence of bioformation or isomerization, or toxicity that influence the results

approach to steady-state confirmed for BMFSS and high quality kTg (or kT) and AE for BMFKg (or BMFK)

data analysis

whole body (or whole body minus GIT or whole body minus GIT and liver) analyzed and r2 ≥ 0.7 for kT or kTg

experimental conditions

fed commercial feed (lipid content reported) for ≥7 day (exposure), I at 0.5−3% bw·d−1, CD measured for BMF and AE

chemical and biological information

chemical specific analysis for a definable structure (SMILES)

Table 1. General Criteria to Classify Data As High (H) Quality (See Text and Supporting Information for more Details for This Category and Others)a


D


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1000 to 1 000 000 ng·g−1).59 Chemical concentrations should be measured in the food to confirm the exposure doses and to reliably calculate BMF and AE (see above and Supporting Information SI-2). For the BMF and AE end points, studies had to clearly report that CD was measured to be categorized H. Food concentrations were measured for 91% of the study entries (H for BMF and AE end points). Food concentrations ranged from 0.04 to 5 000 000 ng·g−1 (median = 83 ng·g−1) on a dry weight (dw) basis. Measuring or reporting CD was not considered a data quality issue for half-life determination as long as the dose did not cause toxic effects and was not expected to exceed its solubility in the diet (see Supporting Information SI-3). Doses were assumed below levels that could cause saturation of biotransformation systems because current data for enzyme kinetics in fish are quite limited.92 Finalized guidance59 suggests a feeding rate (I) of 1−2% body weight per day (bw·d−1) to minimize rates of change in somatic or lipid tissues. Guidance for correcting the mean feeding rate for growth during the test is also available.59 Results from slightly different feeding rates conducted during the OECD ring trial were inconclusive of possible differences in I on the BMFL;71 however, some studies have shown the influence of different feeding rates on the BMF and AE.48 Reported daily (continuous) feeding rates range from 0.4 to 4.2% body weight (median = 1.5% bw·d−1). Tests with exposure durations >1 day and with I between 0.5 and 3% bw·d−1 are assumed H (n = 1154) and rates > or < this range are M (n = 259). Studies that only administered a single dose of the test chemical (or ≤1 day exposure) had feeding rates ranging from 0.5 to 12.4% body weight (median = 5.5%). If feeding rates were not reported or were reported to fluctuate, the study entries are L for BMF and AE calculations (n = 94). Since the calculations assume feeding rates are the same for all fish in the tank, it is necessary to closely monitor the fish during feeding (see above and Supporting Information SI-2).59 This assumption is not easily confirmed or testable and is an expected source of variability in the test results. Experimental Design. Flow through conditions are recommended and semistatic methods can also be used with due consideration.59 A critical assumption for high quality kT or kTg, and AE and BMF data is that the fish are not exposed to test chemical in water during the uptake phase (resulting in bioconcentration) and that they are not exposed to any test chemical during the depuration phase. One method to minimize exposure from the water during the depuration phase is to use flow through tanks and to ensure that all uneaten food is removed immediately after feeding. If the fish are in flow through tanks during the depuration phase the study entry is H (∼85%). If the text described a filtered system, or it was inferred to be a flow through or semistatic design the entries are M (∼10%). If the depuration phase used a static test design or when the depuration conditions could not be determined the data entries are L (∼5%). Background Contamination. Detectable levels of test chemical in control fish, depuration food or depuration water are reported for 44% of the study entries and the issue of background contamination is a source of uncertainty. There are practical limitations to ensuring clean diet and water during the depuration phase, largely determined by the chemical-specific detection limit. Background contamination in water, fish, and diet is particularly challenging for ubiquitous contaminants (e.g., PCBs). For persistent hydrophobic chemicals, even a small quantity of chemical in the water can contribute to the

cases (4/1512) lipid contents are measured and used in the calculations but not documented (scored M). Differences in fish species, age and sex may result in differences in chemical bioaccumulation.81,82 Guidance lists a variety of species and sizes recommended for testing59 but does not include specifications for age and sex. The sex of the fish is rarely reported and it is implied fish are not reproducing during a standard test. Species type, life-stage, and sex are not addressed here. Guidance recommends that the methods used for measuring lipid contents be documented59 because differences are recognized;83 however, potential differences in results due to differences in lipid analysis methods were not considered. In summary, study entries with high quality biological information include documented species with some reported measurement of body size and lipid content. Although not addressed in the present review due to a general lack of reporting, excessive mortality (e.g., > 10%) and reduced growth rates in dosed groups are other data quality parameters that could be considered in future studies. Water Conditions. Guidance provides a list of suggested temperature ranges for the listed test species and the water temperature must remain relatively constant (±2 °C) and the dissolved oxygen concentration must be ≥60% saturation. Differences in water temperature can result in different halflives for the same chemical in the same fish species41,84 due to biological factors such as changes in respiration85,86 and chemical factors such as changes in partitioning properties.87−89 Experimental temperatures ranged from 4 to 26 °C (median = 15 °C). Fish cannot survive or can be stressed outside of certain temperature ranges. While some tests were conducted at temperatures outside of the range of recommended temperatures for that species,59 no exposure temperatures were deemed unsuitable. Consistency in temperature throughout the test and dissolved oxygen concentration were not considered for data quality assessment; however, studies that did not report the exposure temperature are scored L (n = 40). Fifty-five percent of the study entries reported dissolved oxygen concentrations with values ranging from 7.2 to 10.9 mg·L−1 (median = 9.7 mg·L−1); all documented measurements were >7 mg·L−1 or >70% saturation. When reported, values for pH ranged from 7 to 8.5 (median = 7.1). Organic carbon content, ionic strength, and salinity may be important for some tests but were not considered here. Diet Type and Lipid Content. Guidelines59 recommend using dosed commercial pelleted fish food for consistency in the test and documentation of lipid content (and protein) in the food. Study entries using commercial fish food (e.g., pellets) are considered H (1232/1512) and nonstandardized diets (e.g., gelatin capsules, live feed) are M (278/1512). Studies that did not document the dosing matrix are L (n = 2). When reported, the lipid content of the diet ranged from 0.2 to 24% with a median value of 14%. The literature is somewhat conflicted as to whether or not dietary lipid content influences half-lives, BMFs and AE.29,30,48,49,90,91 Besides influencing growth rates, dietary lipid content may also affect half-lives as a result of enhanced or reduced fecal elimination and is required to calculate BMFL. If lipid content in the diet was reported, the study entry is scored H (70%), if not reported the entry is M (30%). Food Concentrations and Feeding Rate (Dose). The administered dose is a product of CD (assumed homogeneous throughout the supplied food) and feeding rate. Guidance for the chemical concentration in the diet is somewhat vague (i.e., E


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Figure 2. (A) An illustration of model predicted half-lives as a function of the octanol−water partition coefficient (KOW) and different methods for growth correction. (B) The distribution of measured half-lives in the database as a function of data quality scores (H: High; M: Medium; L: Low).

fish body burden due to high bioconcentration. If there is no detectable background contamination the data are H. If studies report evidence of exposure to the chemical during the depuration phase either in the diet or in the water the entry is initially classified L. For example, a dietary testing study involving 13 pesticides (ranging in log KOW 1.5−5)93 reported measured concentrations of the test chemical in the water during the uptake phase and during the depuration phase. Studies that made efforts to correct for background contamination are considered M because the results are still relatively uncertain. Cannibalism during the depuration phase has been reported40 and such data are scored L. When tests did not measure or report measurements for background levels the data are tentatively assumed H. Test Duration and Sampling Design. Guidance59 recommends at least 7−14 days of dietary exposure. Exposure durations reported in the literature range from 1 to 286 days (median = 21 days). Twenty-four percent of the study entries are 1 day exposures. Depuration durations reported in the database range from 0 to 300 days (median = 25 days). When possible, the ratio of the depuration time to the half-life was calculated (n = 943) and these ratios range from 0.035 to 250 (median = 1.92). Lower ratios may contribute error in half-life estimation and approximately 25% of the data have ratios < 1 (i.e., the depuration period was 4. Depuration sampling should include several time points (i.e., n ≥ 3). Guidelines recommend 5−10 fish sampled per time point;59 however, the number of fish sampled is not considered here. Tissue Analysis. Guidelines59 recommend using the whole body for chemical analysis but specific tissues can be analyzed with some considerations. Tests that analyze whole body (or carcass) are considered H. Tests that analyze “whole body minus the gastrointestinal tract (GIT)”, “whole body minus the GIT and liver”, or some variation (i.e., large majority of the body analyzed) are tentatively considered H. The implications of this classification assumption could be further evaluated. These “whole body” tests account for 94% of the study entries. If tissue specific data (muscle, liver, kidney, etc.) are used for analysis they should be interpreted carefully because of differences that may occur in internal distribution. If tissue lipid contents were reported for neutral organic chemicals so that the tissue data could be lipid corrected the data are scored M. For certain ionizable organics, such as perfluorinated chemicals, traditional lipid correction may not be appropriate94−97 and bioaccumulation data from tissue specific analyses

without lipid correction for these chemicals are currently considered M. Growth Rate Constants and Growth Correction. Fish growth rates are affected by biological factors (e.g., species, lifestage) and test conditions (e.g., temperature, feeding rates), and vary among individuals in test and environmental populations.28,98 Guidance recommends growth correction or the body burden approach59 to mitigate this variability so that chemicals can be evaluated in a consistent manner. Forty three percent of the total study entries (including AE and BMF studies that did not measure depuration) do not include a measured kG and 4% of the total entries report growth as “not significant”. Reported kG ranges from −0.0014 to 0.125 d−1 (median = 0.0126 d−1). The median growth rate (1.3% bw·d−1) and the median feeding rate (1.5% bw·d−1) highlight the relative efficiency of biomass production for fish in laboratory settings. A kinetic bioaccumulation model is parametrized with median values from the database to illustrate the influence of growth dilution on half-lives determined under typical laboratory conditions (see Supporting Information SI-4 for further details). Figure 2A shows different model simulations for half-lives (kB = 0 for all). The black line shows the t1/2s for chemicals when the model is parametrized using median database values, i.e., kG = 0.013 d−1 (no growth correction). The green line shows the expected relationship for t1/2gs using growth correction or the body burden approach, i.e., kG = 0 (growth correction). Differences between t1/2 and t1/2g become significant when log KOW > ∼4.5−5. At log KOW ∼6 the t1/2 is about 40 d and t1/2g is about 130 days (factor 3.3 difference). At log KOW ∼8 the t1/2 is about 50 days and t1/2g is about 1200 days (>factor 20 difference). When the t1/2s are not growth corrected (black line) the values are similar at log KOW ∼6 and ∼8 because kG is controlling kT (see Supporting Information Figure S1 and related discussion). Chemicals with log KOW > 5 require growth correction to be considered H. Chemicals with log KOW > 5 with no apparent growth correction are considered L. Guidance59 suggests using a linear least-squares correlation calculated from ln(fish weight) against time to calculate kG and a single kG is often applied for growth correction. Because individual fish grow at different rates in a test population there will be uncertainty in applying a single growth rate to the test population. Variability and uncertainty in k G can be considerable even in the same test, for example, coefficient of determination (r2) of 0.5 and 0.6 for growth rate model fits.57 There are different growth correction methods and models F


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through test systems at tolerable temperatures for the test species and there are no identified significant sources of error in the methods or calculations. Absorption Efficiency. Absorption efficiency data are summarized and evaluated separately depending on whether or not the exposure period met the OECD recommendation of at least 7 days.59 Shorter-term exposure duration tests deviate from standardized guidance, but AE can be calculated during the linear uptake phase between days 1 and 3 of exposure.59 A generally applicable screening criterion is that AEs > 1.0 (100%) are suspect indicating a source of error and are thus classified L. Plausible explanations for AE > 1.0 are the (bio)formation of the chemical in the organism (or perhaps the GIT) or additional unintentional exposure during the test or analytical errors. In some cases AE values >1.0 are reported for certain chemicals for which bioformation is not expected to occur102 and there is no obvious explanation. Measured doses (quantities for CD and I) are required for H classification for most AE data (76% of the entries). When the methods for calculating AE were not reported the entries are assumed L (3% of entries). Based on the sensitivity analysis described above, AEs calculated from L kT (or kTg) are tentatively classified M. Supporting Information Figure S3A shows 436 AEs (exposure durations 100%. Many of the AE data scored M do not use pelleted food and are inconsistent with guidance;59 however, these data can still provide useful information for chemical uptake from dietary exposures. For example, there is a cluster of high AEs (0.8−1.0) categorized M. The fish were fed live food (e.g., carp fed mayflies) at 5.5% bw for 1 day72 suggesting that either live food or higher feeding rates can result in relatively higher shortterm exposure AEs compared to other exposure regimes. Supporting Information Figure S3B shows 754 AEs from exposure durations ≥7 days ranging from 0 to 4.65 with a median of 0.45 (H = 225, M = 280, L = 287). The median, average and standard deviation for the H and M data are 0.42, 0.40, and 0.22 respectively. Exposure durations range from 8 to 247 days. Dietary types are also variable, but most (93%) are derived using some form of pelleted commercial fish food (following standardized guidelines). About 2.2% of the longerterm exposure AEs are >100% and categorized L. In one study, depuration data from a different experiment were used to derive AE and BMFs37 and are scored L. According to guidance,59 AE can be calculated using Supporting Information eq S4 and kT with wet weight (ww) concentrations and feeding rates (Figure 1) or at the linear uptake phase of exposure (e.g., tissues analyzed between days 1 and 3). Another method in the guidelines for calculating AE (and BMFK) and often reported in the literature is to use kTg, lipid corrected feeding rates (I), lipid corrected fish concentrations and iterative nonlinear regression.50−57 Examples of variability in AE for PCB 64 (ranging from 0.48 to 1.01) 5 0 , 5 1 , 7 2 and PBDE 99 (ranging from 0 to

reported in the literature and there is an apparent growth correction error in some studies50−57,99−101 (see Supporting Information SI-5 for further details). When simulating this error with the model using median values from the database (red line in Figure 2A) the half-life differences are 6, the halflife differences are >2-fold. For chemicals with log KOW > 6 documented with this apparent growth correction error, the kTg and t1/2g are tentatively screened L to indicate this potential source of uncertainty. For persistent hydrophobic chemicals, changes in lipid content during the test may contribute more uncertainty in the results than somatic growth; an issue not considered here because lipid growth is not commonly reported. Some studies for these types of chemicals used lipid corrected concentrations and feeding rates for the bioaccumulation metric calculations50−57 thus addressing the issue of changing lipid content. The use of lipid corrected concentrations may mitigate the potential growth correction error. Further data analysis may address these uncertainties. Depuration Rate Constants and Half-Lives. Figure 2B shows the distribution of the 964 t1/2 data (growth corrected and not growth corrected) as a function of the data quality evaluation (H = 231, M = 217, L = 516). Half-lives range from 0.13 to 2600 days (median = 41 days) corresponding to depuration rate constants ranging from 5.3 to 0.00027 d−1 (median = 0.017 d−1). Two negative t1/2s and 14 t1/2s reported as “> 1000 days” are scored L. Eighty six percent of the 964 t1/2s are reported on a growth corrected basis. The apparent growth correction error (see Supporting Information SI-5) occurs in approximately 45% of the t1/2g entries and corresponds with the high frequency count of chemicals with longer half-lives classified L (Figure 2B). Guidance59 includes methods for calculating the bioaccumulation metrics and it suggests the data be examined for outliers and that standard statistical procedures be conducted to examine and present the results; however, there are no recommendations for the reliability of kT, t1/2, kTg, or t1/2g calculations. If statistics from the test are available, the propagation of error (uncertainty analysis) in key bioaccumulation metrics could be calculated. Relative standard errors for half-lives range from 0.1% to 100% with an average of about 20%; however, standard errors have not been regularly reported (n = 382). The r2 associated with depuration or half-life data is reported more frequently (n = 754). In the depuration regression, r2 reflects the variance described by time. A high r2 for kT from multiple time points implies (but does not confirm) that depuration may follow first-order kinetics (often assumed) and is reasonably well described by time. A low r2 indicates (but does not confirm) that time may not be the primary factor and that non-first-order kinetics, or some source of variability or uncertainty may be confounding factors (e.g., background contamination, fluctuating internal distribution kinetics). Based on review of general relationships between relative standard errors and r2 and data availability, r2 was tentatively selected to identify possible data quality issues as r2 ≥ 0.7 (H = 46%); 0.5 ≤ r2 < 0.7 (M = 12%); r2 < 0.5 (L = 15%). Studies that do not report r2 are assumed M (27%). More rigorous evaluation methods should be considered if more data are available for analysis. In general, high quality kT, t1/2, kTg and t1/2g data are derived from experiments in which fish were regularly (daily) fed clean, commercial fish food while depurating in clean water in flow G


Critical Review


Figure 3. Measured and modeled biomagnification factors (BMFs) as a function of the octanol−water partition coefficient (KOW). (A) Measured BMFs categorized to be of High (H) and Medium (M) quality, (B) measured BMFs categorized to be Low quality (L), and (C) all measured BMFs for polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB). Modeled BMFs are lipid corrected based on representative (not study specific) conditions to illustrate (D) the influence of the biotransformation rate constant (kB,N; d−1) and growth rate constant (kG; d−1) on the BMFs. The legend for model simulations in (A) also applies to (B) and (C).

0.60)73,75,78,101,103 are described in Supporting Information SI-6 with additional discussion. Barber58 also reported substantial variability and uncertainty in measured AE estimates and models. The database includes chemicals with different biotransformation rates that likely contribute to variability in AE. Due to the variability in methods such as dosing (pellets, live food, gavage, etc.), exposure duration (1−247 days) and calculations, it is challenging to determine specific differences and controlling factors for AE from the overall data set. Test designs based on guidelines59 that focus on clearly defined manipulations in exposure (treatments) are best suited for improved understanding in AE to better ascertain the potential influence of diet type,72 dietary lipid content,49,72,90 feeding rate,48,49,71,72 chemical concentration,48,51,54 species,73,75,103 exposure duration, and chemical structure. BMF Data. Figures 3A (H = 186 and M = 131) and 3B (L = 552) show 869 BMFs (ww or lw) reported with or without growth correction. Thirty-nine percent of the BMFs in Figure 3A and B are for PCBs. Only PCB and HCB BMFs (most are BMFLg) are plotted in Figure 3C, regardless of their data quality score. Thirty BMFKLgs for HCB scored H range from 1.1 to 4.8 (median = 1.8) indicating a plausible range for test variability and a positive control chemical for BMF tests. The BMFs range from 2 × 10−6 to 89 with a median of 1.3. Twenty three values reported as “” are scored L (not shown). Approximately 63% of the BMFs are categorized L. Two hundred twenty BMFs are calculated as concentration ratios (i.e., CF/CD) and thus to be scored H require confirmation that steady-state was approached; however, 60% of the CF/CD BMFs either do not confirm steady-state or report that

steady-state was not approximated and are scored L. BMFK calculations that used M kT or kTg and M AE data are categorized M. BMFK calculations are scored L if the kT or kTg or AE data are considered L. Many of the BMFKs are scored L because of potential errors in kTg. For neutral organic chemicals, BMFLs can indicate an increase or decrease in chemical potential (or fugacity) between the diet and the consumer, i.e. BMF L > 1 or < 1 respectively.22,23,28 Approximately 80% are reported as BMFL or can be calculated using reported lipid contents. When only a ww BMF can be calculated from otherwise H quality test data, the BMF entry is considered M because lipid content in the fish and the food can influence the BMF and lipid correction is suggested.59 Commercial pelleted fish food is typically about 13−15% lipid and most laboratory fish are about 3−8% lipid, thus under these conditions BMFLs are often about 2−5 times higher than the ww BMFs. Some BMFs are scored L because the calculations did not follow standard methods or there are identifiable errors or uncertainties. The units and calculations for the BMF are not reported for the vast majority but they can often be inferred for BMFKs if the units for I are reported (Supporting Information eqs S6 and S7); BMF units are kg-lipid·kg-lipid−1 if I is lw-food· lw-fish−1·d−1.56 In some cases I is based on fish-ww and foodlw;73 therefore, these nonstandardized BMF units of kg-lw·kgww−1 are L. Maximum theoretical lipid and growth corrected BMFLgs in fish are about 8−1530,31,106 and higher measured values suggest a potential error. For example, BMFs > 20 and as high as 46 were reported for some PBDE congeners.73 These reported BMFs appear to be the result of bioformation (debromination) from coexposure to higher brominated H


Critical Review


log KOW 9.0; BMFKLg = 1.4). Binox M is ionogenic (acid pKa = 12); however, it exists almost entirely in the neutral form at environmental and physiological pH. The laboratory data suggest that chemicals with a log KOW ≥ ∼5 and at least as high as ∼9 have biomagnification potential in fish. This relationship is supported by the total data set for persistent chemicals in Figure 3C (measured and estimated log KOWs 5.0 to 9.1 with BMFLg > 1). There are also many chemicals within and beyond the log KOW range of 5−9 that have BMFs < 1. A kinetic bioaccumulation model (Supporting Information SI-4) was used to calculate BMFs at steady-state. The model is parametrized with median database values (i.e., not test specific conditions) and kB and kG are initially set to zero. The model predicted BMFLgs (black line Figure 3) are generally in good agreement with the H and M measured data with respect to log KOW ≥ ∼5 and BMFLg > 1. The model reveals that chemicals with log KOW < ∼5 do not biomagnify in fish from dietary exposures alone (under laboratory conditions) because chemical elimination rates to water (i.e., k2) are sufficiently fast to counter-act gastrointestinal biomagnification (Supporting Information SI-4). The maximum modeled BMFLg is ∼10 and is in good agreement with most measured maximum H or M BMFLgs. If kG and kB are zero and the chemical is absorbed from the GIT, the model suggests there is no decline in BMFLg at very high KOW (i.e., log KOW > 8). The model BMFLg predictions are supported by theoretical maximum BMFs for fish;30,31,106 however, the time to approach steady-state for such chemicals is long (years) and may not be approximated within the organism’s lifetime. There is uncertainty in the model and in the reliability of high values of KOW, and it may not be possible to reliably determine very slow growth rates. The model is used to show the influence of growth dilution on the BMFL (see also Supporting Information Figure S5). At median laboratory growth rates (kG = 0.013 d−1; kB = 0) chemicals with a range of log KOW from about 5.6−7.3 will have BMFs > 1 and the maximum BMFL is about 1.2. When kG is lowered 100 fold, that is, ∼0.00013 d−1, chemicals with a range of log KOW from about 5.1−9.8 will have BMFs > 1 and the maximum BMFL is about 8.4. Measured and modeled field BMFL and TMF data from aquatic food webs, although variable, are generally > 1 for PCBs and other persistent chemicals with log KOW ranging from about 5 to at least 8.7.22,109−112 Without growth correction, some chemicals that biomagnify in the environment may not have BMFLs > 1 in the test. This supports guidance59 recommendations for the growth correction of laboratory BMFs. The model calculations highlight the sensitivity of the BMF to kG and the importance of accurate growth correction methods for persistent hydrophobic chemicals. Model simulations are also provided to illustrate how changes in kB can reduce BMFs. Figure 3D shows that a kB,N (biotransformation rate constant normalized to a 10 g fish at 15 °C81) of ∼0.05 d−1 is required for laboratory BMFL < 1 independent of KOW and kG. This kB,N corresponds to a biotransformation half-life of about 14 days. When the BMFLg is not at a maximum value, kB does not have to be as fast for BMF < 1 (e.g., if AE is reduced or k2 plays a role). The kB,N of 0.05 d−1 is specific to the other parameters used in the simulation (e.g., lipid contents, body size, feeding rates, default AE, dietary exposure only); therefore, if these values change, the kB,N associated with the BMF threshold of 1 will also change.

congeners and other errors (e.g., nonstandardized BMF units73) and are scored L. Potential errors in reported BMFs can be identified through corroboration of the calculations using test data, if available. For example, if AE, I, lipid contents and kT (or kTg) are provided they can be used to recalculate BMFKL (or BMFKLg) using Supporting Information eq S6 (or Supporting Information eq S7). If the recalculated BMF does not equal or approximate the reported BMF this indicates a potential source of error. Some inconsistency in a recalculated BMF may exist because of differences in how the data were treated, for example, if the BMF is derived from lipid corrected fish and diet concentrations measured throughout the test rather than using the average values from the test. When data were available BMFs were recalculated and compared to the reported BMFs to flag discrepancies. We scored ≥ 50% difference between recalculated BMFs and reported BMFs as L and ≥ 20% and 1 have been measured for chemicals with log KOW 4.5−9.0. Only one of the 12 BMFsKLg for the log KOW 4.5 chemical (musk-xylene)16,71 is >1. The median BMFKLg for musk-xylene is 0.43 indicating some potential uncertainty in the musk-xylene BMF > 1. The second lowest KOW chemical with a measured BMFLg > 1 considered H or M is PCB 4 (log KOW 5.0; BMFLg = 4.6). The highest log KOW chemical with a BMFLg > 1 considered H or M is 4,4′methylenebis[2,6-bis(2-methyl-2-propanyl)phenol] (Binox M: I



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RECOMMENDATIONS The past 40 years of dietary testing data reflect significant research to improve bioaccumulation science. Data reporting and analyses have been quite variable and standardized test guidance59 should foster the development of more consistent data sets. Experimentalists and data users should review the testing guidance,59 related supporting studies,71 current theoretical models30,31,58,106 and consider the data quality issues raised herein. The database and proposed data quality assessment methods are expected to evolve. Guidance, critical analysis and GLP provide a means to address uncertainty but uncertainty cannot be eliminated, for example, OECD 305 ring test.71 Guidance includes criteria for “valid” experimental conditions but further guidance to address uncertainty in the test calculations would improve data interpretation and use. Uncertainty analysis could be considered as warranted, if the required data are documented. Authors and journals are encouraged to publish more details of the experiments as Supporting Information (archived online)113 to improve data interpretation and scientific understanding, to reduce animal testing and expense, and to reduce potential error in decision-making (e.g., peer-reviewed publications are often used for bioaccumulation assessment). The OECD 305 guidance59 provides a point of departure for the careful manipulation of specific (independent) test variables (e.g., feeding rates, food type, exposure duration) to gain further insight into underlying sources of variability in bioaccumulation mechanisms and data. Tests that seek improvements in mechanistic knowledge should use persistent chemicals (e.g., PCBs) to reduce the confounding issue of biotransformation in data interpretation. The OECD dietary ring test71 included a relatively narrow and low range of log KOW (i.e., from 4.5 to 6.1). Future evaluations of the guideline methods should include higher log KOW chemicals (i.e., > 8). For persistent chemicals, data analyses with body burdens (XF) should be compared to results obtained using growth correction methods. Reference chemicals could be useful for benchmarking test data17,114 and the benchmark chemical(s) should have similar KOW to the test chemical(s). Chemicals are suggested as a reference for AE calculations59 and a positive control should be considered for the BMF calculations (i.e., HCB BMFLgs < 1 indicate a source of error). Chemicals subject to bioformation from coexposed chemicals or isomers subject to bioisomerization should be tested individually to obtain more reliable information. Testing chemical classes that are poorly represented in the database could provide more insight into the biotransformation capacity of fish and expand the applicability domain of current predictive models.115−118 Additional species tested with the same chemical(s) would provide further insights into interspecies variability for biomagnification and biotransformation potential. Evaluated models can reduce unnecessary animal testing. In vitro119−122 and in silico115−118 kB estimates can be used to parametrize BMF models to screen and prioritize chemicals for BMF testing and to aid test design (e.g., hypothesize suitable depuration durations). Finally, further evaluations of the laboratory testing data and models with monitoring data123 could better ascertain the ecological relevance and effectiveness of the dietary testing methods and the data they provide for identifying and regulating bioaccumulative chemicals.

Critical Review

ASSOCIATED CONTENT

S Supporting Information *

Model description, additional results and discussion, and the evaluated database. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-416-462-0482; fax: 1-416-462-0482; e-mail: jon@ arnotresearch.com. Present Address §

Golder Associates Ltd., Vancouver, British Columbia, Canada

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding support from Environment Canada and ILSI-Health and Environmental Sciences (HESI). We thank HESI project monitoring team members (Mark Bonnell, Tom Parkerton, Lawrence Burkhard, Dan Merckel, Michelle Embry, Kent Woodburn, and Ken Drouillard) for comments on earlier drafts and Aaron Fisk and Andrea Buckman for providing datasets for analysis.

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REFERENCES

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