Thanks to industrialization, mercury levels in the atmosphere are at least three times higher than they were 150 years ago, and mercury levels in ocean surface waters are higher too. So it might seem reasonable to assume that fish accumulate more toxic mercury compounds nowadays than they did in the past. But they don't, according to a provocative new study of Pacific tuna.
Methylmercury is a potent neurotoxin that gets concentrated in big, long-lived carnivorous fish such as tuna and swordfish. To investigate whether levels are higher in fish than they used to be, Princeton geochemist François Morel and colleagues took measurements from 66 yellowfin tuna caught off Hawaii in 1998 and compared these to measurements taken from 71 yellowfins caught in 1971. To their surprise, the total mercury levels were similar, despite the fact that their modeling estimated a 9% to 26% increase in the concentration of inorganic mercury at the ocean surface over the same time period.
So why isn't the inorganic mercury getting converted into the organic forms that accumulate in fish? In freshwater lakes and wetlands, inorganic mercury is thought to be turned into methylmercury by sulfate-reducing anaerobic bacteria. In the ocean, such bacteria are most likely to live about a kilometer deep, so ocean biologists have assumed that this region is the source of methylmercury in the ocean. But if this region were the source, the 1998 tuna mercury concentrations would be higher than in 1971 because atmospheric mercury gets down this far, says Morel. So the toxin in the fish must come from elsewhere, perhaps hydrothermal vents, which are likely to have high concentrations of mercury and to host sulfate-reducers, he says.
"They have an interesting idea about the source of methylmercury in the oceans," says biogeochemist Cynthia Gilmour of the Academy of Natural Sciences' Estuarine Research Center in Leonard, Maryland. But she and others aren't convinced. Gilmour notes that the research seems to contradict earlier work showing that methylmercury levels have increased in seabirds, which feed on ocean fish.
Rebecca Renner, Science Magazine