Free Trial Issue : www. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Get smart. We also discharge mercury-laden industrial effluents directly into rivers or the ocean.
This is not just a scourge of modern life; Lamborg said a mercury mine in Slovenia has been dumping its wastewater into the Gulf of Trieste since Roman times.
Large predator fish such as tuna, for example, contain about 10 million times as much methylmercury as the water surrounding them. So where and how does the conversion of mercury to methylmercury take place? Lamborg said the process is probably biotic—done by living things.
Beyond that, our knowledge is sketchy. However, some species of bacteria do produce methylmercury, as a byproduct of their respiration. This has been observed in bacteria living in seafloor sediments along coasts and on continental shelves. It might also occur in deep-ocean sediments, but no one has looked there yet.
One common means is a chemical reaction called sulfate reduction, in which they use sulfate SO 4 2- in surrounding seawater for respiration and excrete sulfide S 2- into the water as a waste product. If seawater in porous spaces within the sediment also contains a lot of mercury, the stage is set for the production of methylmercury. The resulting compound, HgS, is small and uncharged—just right to be able to pass into microbial cells. Once inside, the mercury gets methylated.
Some of the methylmercury diffuses out of the sediments into the open water. There, it is taken up by phytoplankton to begin its journey up the food chain. But how much of the methylmercury made by bacteria in sediments finds its way into the water above?
Is that the only source of the methylmercury that turns fish toxic? Lamborg is skeptical of that idea. He thinks there has to be another source of methylmercury adding to the oceanic total. It occurs at midwater depths—from to 1, meters below the surface, depending on different locations in the ocean.
That rotting consumes oxygen. Lamborg is pursuing that hypothesis, but first he tested another possibility: whether methylmercury in the low-oxygen zone came from higher up in the water.
Scientists studying phytoplankton have found that 20 to 40 percent of the mercury inside them is methylated. Lamborg wondered: As the phytoplankton or zooplankton that eat them die, sink, and get degraded, does any of that methylmercury get released back into the water and accumulate in midwater depths? To find out, Lamborg collected tiny particles that were sinking through the water and tested them for the presence of mercury and methylmercury.
He caught the particles in sediment traps—polycarbonate tubes about 3 inches across and 2 feet long, that were suspended from a cable at 60 meters, meters, and meters below the surface. Before deploying the traps, Lamborg filled each one with particle-free seawater. Then he added extra-salty brine that was so dense that it formed a distinct layer at the bottom of the tube, which traps the particles.
He left the traps in place for four days, then hauled them up and ran the brine through flat, round filters a bit bigger across than a quarter. Lamborg collected sinking particles at several locations during a research cruise across the Atlantic from Brazil to the coast of Namibia in , and brought them back to his lab at WHOI for analysis.
To find out how much methylmercury fell into a trap, Lamborg converted all the mercury on the filter to elemental mercury. He then passed the sample over grains of sand that had been coated with gold. Then Lamborg heated the gold-mercury amalgam to vaporize the mercury.
You would squeeze some mercury in your pan and sluice it around, dump off the sediment, and then you would heat it up and burn off the mercury and leave the gold behind. It gets drawn into wiry Teflon tubes that take it to an atomic fluorescence spectrometer that determines how much mercury was in the sample.
Small fish that eat the zooplankton are therefore exposed to food-borne mercury that is predominantly in the methylated form. These fish are consumed by larger fish, and so on. Because the methylmercury is highly assimilated and lost extremely slowly from fish, there is a steady build-up of this form of mercury in aquatic food chains, such that long-lived fish at the top of the food chain are highly enriched in methylmercury.
Methylmercury therefore displays clear evidence of biomagnification, where its concentrations are higher in predator tissue than in prey tissue. This is almost unique among metals and is somewhat similar to biomagnification of organic pollutants like chlorinated hydrocarbons. Scientists have estimated that the amount of mercury in the atmosphere today is about two to three times what it was years ago. But maybe mercury that occurs in fish is a natural thing, and it may have been there all along.
The first step in exploring this assumption is to clarify the chemical nature of mercury in the environment. Mercury concentrations in the air are of little concern, but when mercury enters water, microorganisms transform it to a highly toxic form — methylmercury — that builds up in fish. Unfortunately, scientists are not yet able to measure methylmercury in ocean surface waters, so Morel and his coworkers approached the problem from a different angle.
They measured methylmercury levels in yellowfin tuna caught off the coast of Hawaii in and compared the numbers to a similar study from the same area in The researchers predicted that mercury in the surface waters should have increased by up to 26 percent during this time, according to a computer model.
The model took account of the change in atmospheric mercury, the sub-equatorial Pacific waters and the potential for mixing in the"thermocline" — a transition layer in the ocean where temperature changes rapidly.
The findings imply that the high levels of methylmercury in these fish are not coming from increased pollution, but from a natural source.
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