Reviewing: Blum, J. D., Drazen, J. C., Johnson, M. W., Popp, B. N., Motta, L. C., & Jamieson, A. J. (2020). Mercury isotopes identify near-surface marine mercury in deep-sea trench biota. Proceedings of the National Academy of Sciences, 117(47), 29292-29298.
Mercury is a highly toxic element that can cause damages to our nervous and immune systems. While about a third of mercury in the ocean comes from the natural environment, such as volcanic activity and weathering and breaking down of rocks and soils, the remaining two-thirds comes from human activities, including fossil fuel burning and industrial wastewater runoff.
When mercury enters the ocean, it can change to different forms by a number of chemical and biological processes. For instance, microbes can convert mercury to a different form called methylmercury, or methylmercury can break down and return to mercury when exposed to sunlight. Methylmercury is a particularly poisonous form of mercury, as it can easily enter into animals’ cells (specifically through the cell membrane, a thin layer outside cells that acts as a barrier and protect what’s inside).
When mercury is converted to methylmercury, it will permeate through the cell membrane of tiny phytoplankton. The phytoplankton will continue to take up more mercury before they get eaten by larger plankton, small fish, and large fish. Methylmercury continues to accumulate in each organism – a process known as biomagnification – and, eventually, we end up eating fish that has very high amounts of methylmercury. Therefore, understanding the whereabouts of mercury in the ocean and understanding the amount of mercury in fish we eat is critical to public health.
Previously, mercury pollution in the ocean was thought to be mostly confined to the upper ocean. The average depth of the ocean is about 3600 meters (or about 12,000 feet), and scientists have estimated that about two thirds of human-produced mercury in the ocean are in the upper 1000 m. However, a new study found human-produced mercury in the Mariana and Kermadec Trenches – some of the deepest parts of the ocean, down to 11,000 m (or 36,000 ft).
The scientists collected amphipods (shrimp-like tiny creatures) and snailfish from the two trenches, and measured mercury isotopes in their tissues. Isotopes are atoms with the same number of protons but different number of neutrons. The scientists measured seven major mercury isotopes – all with 80 protons, but with different numbers of neutrons, ranging from 116 to 124.
As mercury in the ocean gets converted to different forms of mercury by microbes and sunlight, each conversion process leaves a unique signature on the isotopic composition of mercury, by changing the relative abundance of different mercury isotopes. So, by measuring the relative amounts of each isotope inside organisms that live in the trench as well as in the surrounding Pacific water and rainfall of the Pacific Ocean, the scientists were able to track back how mercury was getting into the deepest ocean.
Using the isotopic signatures, the scientists were able to fully reconstruct the fate of mercury – how it ended up in the tissues of amphipods and snailfish living in the deep-sea trenches. Mercury had originally entered the ocean from the atmosphere, then converted to methylmercury in the upper ocean (0-1000m) and was taken up by fish living in the upper ocean. When the fish died, the carcasses of the fish broke down into particles and sank to the bottom of the ocean. These methylmercury-containing particles were consumed by organisms living in the trenches, accumulating in their bodies. Methylmercury from upper ocean fish carcasses can account for about 60 to 90% of the methylmercury in the bodies of deep trench communities.
The results of this study show that human mercury pollution has penetrated into the deepest parts of the ocean. By understanding how mercury travels throughout the ocean and changes its chemical form, we can get better estimates of how long it takes for mercury in the ocean to move up the food chain and accumulate in fish that we consume, and better assess the risk of eating seafood.
I am a PhD student in chemical oceanography at University of Washington. I am studying how different forms of metals in the ocean are shaping microbial communities in the North Pacific Ocean. When not working, I like going for a walk, visiting farmers’ markets and playing keyboard.