Seals are swallowing a mouthful
The situation resembles an episode of CSI, except that in this case, the actors in white coats are investigating the poisoning of large marine mammals in the North Sea. Grey and harbour seals are the focus of attention here because, due to their position at the top of the food chain, they are more exposed than other animals to the harmful effects of many pollutants such as mercury or persistent organic pollutants that act as endocrine disruptors, attack the immune system and impair reproduction. But many questions still remain unanswered. Assessing the threats associated to these pollutants and establishing the extent to which they are responsible for various diseases is a particularly complex operation. For the last fifteen years or so, researchers at the University of Liege, in collaboration with many other universities in Belgium and abroad, have been attempting to get to the bottom of this global phenomenon. In order to help them achieve their goal, they have called upon expertise from disciplines as varied as veterinary medicine, oceanology, ecotoxicology, chemistry, cellular biology and the study of biomarkers as attested to by several recent studies (1).
Many organic and inorganic pollutants present in our oceans represent a major ecological challenge. Mercury and cadmium are often referred to as heavy metals but this is not strictly true from a chemical point of view. The term “trace elements” would be more accurate. Unfortunately, many other pollutants can accumulate in marine animals. Persistent organic pollutants are long molecules assembled around carbon chains and are usually the result of human activity. This is the case with certain pesticides or PCBs. These industrial compounds which were banned more than thirty years ago are so stable that they have remained present and active up to the present day. Finally, the pollutant that is most familiar to the wider public is mercury. “It exists in the natural state, explains Krishna Das, A senior research associate at the FNRS and a member of the Oceanography Laboratory of the University of Liege, “But human activity has resulted in an almost three-fold increase in atmospheric emissions as compared with the pre-industrial era. This raises an obvious question”. Mercury, under the influence of bacterial activity can become bonded to other compounds to form methylmercury. It is an organic molecule and, like many other pollutants, is very toxic. The entire trophic chain is affected by it in fact. In order to understand these levels of poisoning and quantify their consequences for the health and behaviour of animals, research that includes sea campaigns, laboratory studies, in vitro culture or analysis of biomarkers is necessary. The evolution of animal health is actually so dependent on a variety of factors with various causes that it is difficult to associate a given evolution to one phenomenon or another. The way animal life responds to its environment can only be understood in a global context. To understand this, multiple approaches are required.
Undesirable exposure to pollutants
This is the mammoth task that has been undertaken by Krishna Das and her collaborators for the last fifteen years, focussing on marine mammals like dolphins, humpback whales and more particularly, grey and harbour seals. The latter two species are at the top of the food chain in the North Sea. As predators, they are the most exposed to persistent organic pollutants such as polychlorobiphenyls or PCBs. These industrial pollutants are integrated and assimilated by all the species in the trophic chain and are only a small amount of them are eliminated by the kidneys and in fecal matter. Because they are lipophilic, they typically accumulate in adipose tissues. An efficient transfer of prey towards the tissues of predators has been observed, from the plankton to the seal. “We estimate that the concentrations of several pollutants are multiplied by around ten at each trophic level, a process called biomagnification”. This process does not fully explain their high concentration of pollutants. As seals are mammals they need to maintain a body temperature around 37°C and they must do so in an environment where there is a high level of heat dissipation. In order to compensate for this heat loss, they must eat large quantities of food, more than 6 % of their body weight per day (this can vary from species to species, from season to season, and in accordance with their growth stage and their physiological status). These mammals can weigh as much as 200 kg. They possess a thick layer of blubber which favours the accumulation of lipophilic pollutants. This is an unfortunate combination of factors from the seals’ point of views but it means that they make fascinating toxicological models.
(1) Xenobiotic and Immune-Relevant Molecular Biomarkers in Harbor Seals as Proxies for Pollutant Burden and Effects?