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What is Causing Biomagnification of Persistent Hydrophobic Organic Chemicals in the Aquatic Environment? Michiel T. O. Jonker* Institute for Risk Assessment Sciences, Utrecht University, P.O. Box 80177, 3508 TD, Utrecht, The Netherlands positive correlations between TMFs and hydrophobicity,2 since hydrophobicity and elimination rates are inversely related. Recent results by Van der Heijden and Jonker3 however suggest that there may be an alternative or additional reason for the observations that concentrations of persistent, nonmetabolizable HOCs increase with trophic level. Lipid-normalized organismwater accumulation factors (or necroconcentration factors (NCFs), being proxies for bioconcentration potential) for PCBs were found to vary considerably between different species (zooplankton, macroinvertebrates, fish), and generally increased with trophic level.3 It is true that in food web studies BAFs are also observed to increase with trophic level (increasing HOC concentrations in organisms living in an aquatic environment with the same aqueous HOC concentrations after all implies increasing BAFs), but such increases are attributed to nonequilibrium conditions, as explained above. In contrast, in ref 3 the NCFs were determined with organism homogenates and passive samplers, and represent true equilibrium values. These results suggest that not all lipids are equally capable of dissolving PCBs and question the implicit assumption in risk assessment that BCF values are independent of the organism. Interestingly, the differences in NCF values between different species reported in ref 3 (i.e., factors of 1 9) are similar to BMF and TMF values generally reported for PCBs. This may suggest that BMFs and TMFs simply result from different affinities of PCBs for different lipids, with the affinity increasing from zooplankton to fish, and that biomagnification is not necessarily a nonequilibrium process. Actually, at least for the lower chlorinated PCBs (e.g., PCBs 18 and 28) this seems not unlikely, as these compounds with a logKow of about 5.5 can be assumed to equilibrate rather rapidly in the environment; yet BMFs and TMFs of up to 5 have been reported. In Figure 1, TMF values recently reported by Walters et al.2 for the PCB congeners tested in ref 3, based on organism combinations similar to the ones studied in ref 3 are presented, along with values resulting after a correction for differences in lipid affinities (i.e., the literature TMFs divided by the NCFhigher organism/ NCFlower organism factors from ref 3). The Figure demonstrates that TMF values are reduced to values close(r) to 1 upon the correction and therefore may (partly) be explained by differences in equilibrium partitioning between water and lipids of different organisms. A very similar picture results when correcting other TMF or BMF literature data for PCBs (e.g., from refs 23 and 25 cited in ref 2; data not shown). Obviously, these results are by no
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ue to their hydrophobic character, many hydrophobic organic chemicals (HOCs), such as polychlorinated biphenyls (PCBs) accumulate in organisms. The most common ways of expressing the degree to which chemicals accumulate in individual species is by means of the bioconcentration factor (BCF) or the bioaccumulation factor (BAF).1 In addition to accumulation in individual species, PCBs and other persistent HOCs tend to “accumulate in food chains”, that is, concentrations in organisms often increase with trophic level.1 In order to quantify this behavior, the biomagnification factor (BMF) has been defined, which is given by the ratio of the chemical’s concentration in an organism and that in its food. Also, so-called trophic magnification factors (TMFs) or food web magnification factors (FWMFs) have been introduced, which widen the scope to the level of a food web by not just looking at one but at several predator prey combinations. TMFs and FWMFs therefore can be considered as the averaged BMF over different trophic levels. The exact cause of biomagnification is not yet fully understood, but the general perception is that the phenomenon results from slow elimination rates of HOCs.1 With an ongoing input of chemicals through contaminated food uptake, and an increased fugacity in the gastro-intestinal tract during food digestion, the elimination of the chemicals cannot keep up, leading to increasingly higher concentrations with increasing trophic position. In other words, biomagnification is considered being the result of nonequilibrium conditions. This hypothesis is supported by r 2011 American Chemical Society
Received: November 17, 2011 Accepted: November 21, 2011 Published: December 06, 2011 110
dx.doi.org/10.1021/es204161k | Environ. Sci. Technol. 2012, 46, 110–111
Environmental Science & Technology
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’ AUTHOR INFORMATION Corresponding Author
*Phone: +31 30 2535338; fax: +31 30 2535077; e-mail: m.t.o.jonker@ uu.nl.
’ REFERENCES (1) Arnot, J.A.; Gobas, F. A. P. C. A review of bioconcentration factor (BCF) and bioaccumulation factor (BAF) assessments for organic chemicals in fish. Environ. Rev. 2006, 14 (4), 257–297. (2) Walters, D. M.; Mills, M. A.; Cade, B. S.; Burkhard, L. P. Trophic magnification of PCBs and its relationship to the octanol-water partition coefficient. Environ. Sci. Technol. 2011, 45 (9), 3917–3924. (3) Van der Heijden, S. A.; Jonker, M. T. O. Intra- and interspecies variation in bioconcentration potential of polychlorinated biphenyls: Are all lipids equal? Environ. Sci. Technol. 2011, 45 (24), 10408–10414. (4) Gobas, F. A. P. C.; De Wolf, W.; Burkhard, L. P.; Verbruggen, E.; Plotzke, K. Revisiting bioaccumulation criteria for POPs and PBT assessments. Integr. Environ. Assess. Manage. 2009, 5 (4), 624–637.
Figure 1. TMF values for PCBs reported in ref 2, both before (black) and after (gray, white) correction for differences in lipid water partitioning between organisms. The correction was done by dividing the literature TMF values by the ratio of the necroconcentration factors of organisms being similar to those on which the TMF was based, that is, midge/zooplankton (gray bars) and stickleback/mussel (white bars). In formula: (TMF)/(NCFhigher organism/NCFlower organism).
means conclusive, since, among other things, (i) the organisms tested in ref 3 and those sampled in the field studies will not have been identical, (ii) warm-blooded animals from higher trophic positions (seals, birds) were not included in ref 3, and (iii) in some cases the corrections result in values
