Mercury makes it deep into marine trenches

Mercury pollution, already known to be widespread, has reached previously unplumbed depths. Researchers sampled shrimp-like amphipods from the bottom of three deep-sea trenches and found high concentrations of mercury and methylmercury—higher even than many other aquatic environments (Environ. Sci. Technol. Lett. 2020, DOI: 10.1021/acs.estlett.0c00299).

The study “provides a very unique dataset,” says Celia Y. Chen, an aquatic ecologist and director of Dartmouth University’s Toxic Metals Superfund Research Program, who was not involved with the work. She highlights the difficulty of collecting samples from this “very understudied ecosystem.”

Marine trenches—long, V-shaped chasms slashed deep into the seafloor—are some of the most remote ocean habitats, home to bizarre and little-studied organisms. The ecosystems along the bottom of these trenches, which lie as far as 11 km below the surface, thrive on dead organisms and particulate matter funneled down from above.

Such sustenance can also deliver mercury, this study shows.

Most mercury pollution is launched into the atmosphere through fossil-fuel burning and mining, then redeposits around the globe. Methylmercury, formed in aquatic ecosystems when microbes take up and methylate deposited elemental mercury, is a potent neurotoxin that can bioaccumulate in marine food webs. Some apex predators can accumulate methylmercury in concentrations up to a million times that of seawater, says study coauthor Maodian Liu, now a postdoctoral fellow at Yale University.

To better understand the extent of mercury contamination and bioaccumulation in deep-sea ecosystems, the researchers, led by Xuejun Wang of Peking University and Yunping Xu of Shanghai Ocean University, sampled sediment and collected three species of amphipods from four locations 6,990–10,840 m below the surface of the ocean.

The team found that average methylmercury concentrations in amphipods from trenches were roughly three times as high as those of amphipods from freshwater environments and more than twice as high as those found in amphipods in coastal oceans. The average total mercury level in amphipods from marine trenches was more than seven times that of amphipods off the coast of New Jersey, one of the most industrialized and highly populated areas in North America.

Analyzing specific fatty acids, which can provide information on an organism’s diet, in the amphipods supported the researchers’ suspicions that one of the primary sources of mercury in deep-sea trenches is input of carrion from surface ocean environments.

The high amphipod methylmercury levels found in this study also help explain high concentrations previously reported in some deep-sea fish species that feed on the amphipods, says coauthor Wenjie Xiao. Such high contamination might be common throughout deep-sea food webs, he adds.

The study is “a fascinating and significant contribution,” says Lisa A. Levin, a biological oceanographer at Scripps Institution of Oceanography. “This paper furthers our understanding of the reach of this potent neurotoxin and forces the realization that what should be the most pristine of earth’s environments—the ultradeep ocean—is in fact highly vulnerable to contamination.”