IMRAD Analysis

Polychlorinated Biphenyls (PCBs) and Sex Hormone Concentrations in Ringed and Grey Seals: A Possible Link to Endocrine Disruption?

G. M. Troisi, S. J. Barton, O. Liori, M. Nyman in Archives of Environmental Contamination and Toxicology (2020) 78:513–524.

Introduction

The Baltic Sea is heavily polluted by adjacent industrial areas with persistent organic pollutants, such as polychlorinated biphenyls (PCBs) and organochlorine pesticides (like DDT). These pollutants are biomagnified in apex predators, marine mammals in this case, who store these contaminants in their blubber. The detrimental effects of POPs include endocrine disruption: this paper looks at the decline in the Baltic seal population in the 1980s, one of the populations most contaminated with POPs. In order to quantify the effects of POPs on the Baltic seals, Troisi et al. (2020) observe the correlation between PCB exposure and circulating sex hormone and resulting dose-response curves.

Methodology

Seal samples in this study came from surplus samples from a previous study, where seals from the Baltic were sampled under special permission from the Ministry of Forestry and Agriculture. The sample consists of adult seals, from both sexes, with an average age of 12.6 years. The seals were selected after mating season between 1997 and 2002. The unit of analysis used are blood samples obtained through blood centrifuging. This study uses a sample size of n=77 from ringed and grey seals, analyzed for PCBs and sex hormones using immunoassay methods. Hormones were quantified using enzyme-immunoabsorbent assay kits. ELISA kits were used to quantify PCB concentrations, and compared with data from a previous study.

Results and Discussion

This section goes through the PCB concentrations, hormone concentrations, and statistical tests used to quantify their conclusions. Their first finding: “The range of PCB concentrations found in ringed (range 0.88–46.87 µg/ml) and grey seals (range 0.30–87.06 µg/ml) corresponded with ranges reported by others for these seal populations.” They used ANOVA to see the impact of different variables on PCB concentrations, finding that PCB concentrations were higher in Baltic seals, specifically grey over ringed seals and that males had higher PCB concentrations than females. Thus, the seals were grouped by “Reference” and “Exposed” based on the levels of PCB found in their blood. Troisi et al. found that for all hormones, the mean concentrations in blood plasma were lower in “Exposed” seals than in “Reference” seals. They state that PCB exposure could lower circulating hormone concentrations in environmentally exposed seals, and could be deleterious on female pregnancy outcomes during gestation. Future studies should use a larger sample size to assert these conclusions. The mechanism by which this is possible is through PCBs ability to bind to sex hormone-binding globulin in blood, which displaces endogenous hormones, so that the hormones are unable to get to their target organs.

Conclusion

In conclusion, the findings of this study are not strong enough to prove cause and effect, but they contribute to the body of work that supports that these types of pollutants cause endocrine disruption in seals. The mode-of-action of how PCBs disturb hormone homeostasis is also unclear, but it likely has to do with hormone binding pathways. The international bans of POPs are not enough to completely eradicate the effects of POPs on seals and other marine apex predators. The impacts of POPs on seals should be contextualized in the ways humans have changed the environment, making it hostile to aquatic mammals.

Article Analysis

Is the topic of the paper somewhat original?

Yes, the topic, PCB exposure of Phoca hispida (ringed) and Halichoerus grypus (grey) seals and its linkage to endocrine disruption, is somewhat original. It builds on a body of work that examines the case study of PCB exposure of Baltic seals, but it is the first of its kind to study plasma sex hormone concentrations in ringed seals. The study tries to find a causal relationship between PCB exposure and endocrine disruption, which has not been done by other studies.

What was the aim of the study? What hypothesis did the researchers test? Are the conclusions reached important to you and others?

The purpose of the study is to observe how PCB exposure in Baltic seals is linked to sex hormone disruption, with the greater purpose of understanding how persistent organic pollutants affect marine apex predators. The hypothesis that the researchers tested was that seals exposed to PCBs had disrupted progesterone, testosterone, estradiol, estrone levels in comparison to seals that were not exposed to PCBs. Although the researchers were unable to conclude cause and effect, they were able to show how this study worked, the methods used, and their recommendations for future investigation.

Were enough data obtained to reach valid conclusions?

No, the data was limited by a small sample size due to the use of cryopreserved blood samples. The researchers didn’t feel the need to sacrifice more seals, which is reasonable considering that the study is being conducted because of population decline among this species in this area.

Were the outcome measures appropriate?

The outcome measures were appropriate. If you read other studies, you’d find that the Baltic seals faced a population decline not due to hunting or starving, among other possible effects, but due to difficulty reproducing. The outcome measures, which were the levels of sex hormones found in the blood, are appropriate in this case to understand if there is some sort of endocrine disruption preventing reproduction from happening.

Are the results consistent with those of other studies?

The results are consistent with those of other studies; Troisi et al. (2020) write that their findings “ contribute to the significant body of evidence that organohalogen pollutants cause endocrine disruption in seals, from the subcellular level (hormone receptor interactions and altered hormone biosynthesis and metabolism.” Other studies that take a look at this case study found that there is indeed reproductive disruption due to polychlorinated biphenyls in the Baltic Sea, exasperated by the fact that there is low exchange of water into this body of water, making it so that the pollutants here are relatively stagnant. This is explored in a paper I wrote for my class in environmental toxicology, which can be found here.

Have the authors discussed possible limitations of the study?

Besides the limitations due to sample size and using previous samples that were already collected as their unit of analysis, no other limitations of the study were discussed.

Who sponsored the study?

Kingston University sponsored this study. As it is the academic institution from which the researchers are from, there are no biased investments in this study.