Mercury pollution in our seas could be more damaging to seals than was once thought, according to new research which highlights the effects of mercury on the animals' immune systems. The work, which was carried out by Belgian and German scientists and supported by a Marie Curie grant from the EU's Sixth Framework Programme (FP6), is published in the open access journal Environmental Health. Mercury is widespread in the environment; one of the main sources of the metal is the Earth's crust itself, but it is also pumped into the environment during the processing of raw ores and in medical and scientific waste. In the seas, it builds up in the bodies of marine creatures. Marine mammals are particularly prone to the accumulation of mercury in their bodies. One reason for this is their high position in the food chain; the levels of pollutants like mercury increase with every step up the food chain. Their relatively long lifespan is another factor, as is their high metabolism and high fat levels, which help to keep the animals warm in the chilly marine environment. High levels of pollutants such as mercury, other metals, pesticides and persistent organic pollutants can have a harmful effect on the immune system, hormone system, growth and reproduction of marine organisms. 'Mercury is known to bioaccumulate and to magnify in marine mammals, which is a cause of great concern in terms of their general health,' comments Dr Krishna Das of the University of Liège in Belgium, the lead author of the study. 'In particular, the immune system is known to be susceptible to long-term mercury exposure.' Dr Krishna and her colleagues analysed the levels of mercury found in the blood of harbour seals caught in the North Sea off the coast of Germany. They then studied the effects of these levels of mercury on immune system cells in the laboratory. They found high levels of mercury in the seals analysed, reflecting the seals' fish-based diet and the high levels of mercury contamination found in the North Sea, although there was wide variation between individuals. Furthermore, these levels of mercury seriously damaged white blood cells in the laboratory, with cell proliferation, DNA activity and other vital functions all affected. Some of the effects seen have also been documented in studies looking at other pollutants, raising concerns that these substances could have an additive effect. '[Mercury] could be an additional cofactor in the immunosuppressive pollutant cocktail generally described in the blood of seals and this therefore raises the possibility of additional additive effects in the marine mammal immune system,' the scientists conclude. The question of how this contamination affects seals clinically remains to be answered. Nevertheless, pollutants may already have contributed to the spread of disease in seal populations. In 1998 and 2002, phocine distemper virus killed thousands of harbour seals in northern Europe, and at the time many blamed pollutants for impairing the animals' ability to fight off the infection.