#EBSstory On dolphins, mercury and climate change

How does climate change affect the prevalence of toxic mercury in natural habitats? And how do animals detoxify it in their systems?

Brett Poulin, an assistant professor at the University of California-Davis (US) that specializes in mercury, is collaborating with ESRF scientist Alain Manceau to find answers to these questions. Poulin studies the coastal wetlands of Florida, an ecosystem spanning the Atlantic Ocean and Gulf of Mexico that is particularly susceptible to climate change. His research evaluates how climate change affects the cycling of mercury, including changes in soil microbial communities, water, and fauna.

“Our field studies show that sea level rise increases the salinity of coastal wetlands, which results in changes in the microbial communities in the soils”, he explains. “This, in turn, can increase the production of methylmercury, the toxic form of mercury, which ends up in the soil and water, and eventually in fish and animals”. This methylmercury subsequently is exported to coastal waters and biomagnifies in animals, with the highest concentrations often observed in marine mammals. Understanding the internal detoxification processes of methylmercury in animals is essential for protecting wildlife, assessing the sensitivity of animals to mercury toxicity, and designing treatments against mercury poisoning.

Poulin is currently working with ESRF scientists Alain Manceau and Pieter Glatzel to analyze tissues from bottlenose dolphins that died in the Indian River Lagoon of Florida, and were preserved for science by researchers at the Hubbs SeaWorld Research Institute. Dolphins are well known for storing large quantities of mercury in their tissues: “We have long been puzzled at the enormous amount of mercury that dolphins can store”, says Poulin.

Dolphins, as well as other vertebrates, have developed mechanisms to detoxify methylmercury. They possess specific enzymes and biochemical pathways that break down methylmercury to non-toxic mercury-selenide, which is stored in tissues. However, scientists are still trying to understand why the extent of the detoxification mechanism differs between birds, mammals, and humans.

The samples Poulin and Manceau analyze come from the liver (the primary location where methylmercury is detoxified), kidneys, and muscle. They are analyzed on beamline ID26 at the ESRF by high energy-resolution fluorescence-detected X-ray absorption-near-edge structure and X-ray absorption spectroscopy (HERFD-XANES), which allows them to identify the different chemical forms of mercury in tissues and their interactions with other elements. “We combine high spectral resolution with a low detection limit and have designed a very stable and reproducible set-up”, says Pieter Glatzel, the scientist in charge of beamline ID26. The demethylation reactions fractionate the isotopes of mercury, which can be used to fingerprint the detoxification process identified by HERFD-XANES. The isotopes of mercury are measured concurrently by collaborators at the U.S. Geological Survey.

This research is the follow-up study of several scientific publications by this team where they discovered that petrels and other water birds detoxify methylmercury through a sequence of reactions involving reduced selenium in the form of a specific selenoprotein. “Detoxification of methylmercury by this selenoprotein has unwelcome implications, however, because it depletes the cells in essential selenium for the synthesis and activity of other selenoproteins that serve critical anti-oxidant functions in the brain and testis”, says Manceau. These breakthroughs were the result of several years of measurements and experimentation on ID26. ID26 was the first beamline globally to measure mercury by HERFD-XANES, advancing our understanding of the biochemistry and geochemistry of mercury in natural systems. “We now want to see whether mammals process methylmercury in the same way as birds”, says Poulin.

While the results from ID26 on the dolphin tissues will tell the researchers about the animal’s life history and the detoxification process, the broader goals of the work are to link climate change processes and other disturbances in coastal environments to contaminant uptake and toxicity. Mercury speciation and isotopic fractionation in wildlife provide, in this respect, powerful synergy.

Text and video by Montserrat Capellas Espuny

Top image: Bottlenose dolphins in the ocean.