Popular Science: Mysterious mercury-changing organisms under the Greenland Glacier are still eluding scientists
Greenland is a relatively remote, sparsely populated area where we would probably expect a low anthropogenic influence and a clean environment. However, the conclusions of research in this area, in which Czech scientists also participate, bring increasingly surprising conclusions. Previous research into the Greenland Ice Sheet has led scientists to an interesting finding regarding the composition of microbial communities with resistance to pesticides and heavy metals. These microorganisms were able to degrade pesticides and heavy metals, and their occurrence was independent of their distance from human activities. Another significant discovery was the release of methane, the source of which is probably microorganisms, known as methanogenic archaea and living under ice. Researchers have now measured unexpectedly high concentration of mercury in the meltwaters flowing from the Greenland Ice Sheet.
Mercury is highly toxic to living organisms. Mercury bioaccumulates in living organisms, especially in the kidneys, liver and spleen. It can also travel through the food chain to its top, i.e., to humans. Mercury poisoning in humans due to increased consumption of seafood is not exceptional at present. The cause of increased concentration of mercury in the environment is in most cases anthropogenic pollution, whether from industry, the burning of fossil fuels or from dental surgery wastewater. However, there are also natural sources such as volcanic emissions and the weathering of minerals containing mercury.
Studies on the amount of mercury in the glacier’s meltwaters are scarce but they suggest that some of the glaciers could be a significant source of mercury. The Greenland Ice Sheet, the second largest ice body after the Antarctic Ice Sheet, has not yet been studied in detail. Therefore, scientists have focused on mercury concentration in three rivers and three fjords in southwestern Greenland over the course of four years. They measured both inorganic mercury and organic methylmercury content. Methylmercury is formed by microorganisms and it is the form of mercury that is the greatest cause for concern. Then they sequenced metagenomic DNA to obtain an image of the genetic potential of the microorganisms for mercury metabolism.
The results were surprising, as the concentration of inorganic mercury in the meltwaters reached values one to two orders of magnitude greater than the concentration measured in the Arctic rivers so far. The authors note that such values are achieved, for example, in polluted rivers in Southeast Asia. The situation was similar for organic methylmercury whose concentration was also higher than concentrations measured in most pristine freshwaters. They were similar, for example, to those found in wetlands. Wetlands often produce more methylmercury than other aquatic habitats because the biogeochemical conditions (higher incidence of bacteria) common in wetlands facilitate the methylation of inorganic mercury to methylmercury, a more bioaccumulative and toxic form of mercury.
The concentration of dissolved mercury in the effluent varied depending on time and discharge, coinciding with changes in the DOC (dissolved organic carbon) and methane concentrations in the meltwater. This suggests that this phenomenon is likely to be a regular periodic flushing of isolated subglacial waters high in mercury concentration. Since mercury concentration in the Greenland Ice Sheet meltwaters far exceeded the levels found in the surface layers of snow or ice, it was possible to conclude that the culprit is most likely a geological source of mercury at the ice sheet bed. A subsequent DNA analysis showed that certain members of the microbial communities are able to reduce toxic Hg2+ into volatile Hg0. However, the presence of genes that would allow for methylation and the formation of methylmercury were not detected. Its origin has not yet been fully unraveled.
Unprecedentedly high concentrations of mercury in the meltwaters of the Greenland Ice Sheet are a potential cause for concern. We probably cannot trace the origins of the mercury origin to human activities or a secret military base, but rather to the bedrock geology. However, this does not mean that it is not a menace, especially in the context of global climate change and the increased rate of Greenland Ice Sheet mass loss. What will happen with toxic mercury and especially with the local environment in the future is a question. Further research is an important part of an already surprisingly mysterious puzzle.
Hawkings, J.R., Linhoff, B.S., Wadham, J.L., Stibal, M., Lamborg, C.H., Carling, G.T., Lamarche-Gagnon, G., Kohler, TJ., Ward, R., Hendry, K.R., Falteisek, L., Kellerman, A.M., Cameron, K.A, Hatton, J.E., Tingey, S., Holt, A.D., Vinšová, P., Hofer, S., Bulínová, M., Větrovský, T., Meire, L., Spencer, R.G.M. 2021. Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet. Nature Geoscience 14, 496–502. doi: 10.1038/s41561-021-00753-w