Trace elements are enriched in plants by natural processes, human activities or both. An analysis of mercury in Arctic tundra vegetation offers fresh insight into the uptake of trace metals from the atmosphere by plants. See Letter p.201
Mercury, a potentially toxic trace metal, accumulates in the Arctic as a result of human activities — an issue that has broad ecological and societal implications, particularly for indigenous communities that rely on local food sources. But the relative importance of natural versus industrial sources of mercury accumulation has been a subject of debate. On page 201, Obrist et al.1 provide unambiguous evidence that elevated mercury concentrations in Arctic plants reflect the uptake of gaseous elemental mercury (GEM) from the atmosphere. This insight is crucial to a better understanding of the mercury cycle, and suggests that the Arctic tundra is a major sink for mercury.
The dominant type of mercury in the atmosphere is GEM, a chemically stable form of the element2. Analysis3 of firn (an intermediate stage between snow and glacial ice that is formed from snow deposited in past seasons) from Greenland showed that atmospheric mercury concentrations increased rapidly after the Second World War, peaked around 1970, and have since declined. Global emissions of mercury to the atmosphere have been falling4 since 1990 — total emissions have dropped by 20%, and anthropogenic GEM has dropped by 30%.
Although the recent declines in mercury emissions to the atmosphere from human activities are encouraging, they are dwarfed by the two- to threefold decreases in releases of particulate-bound trace elements5 such as arsenic and chromium between the early 1980s and mid-1990s. The technologies used to remove these elements from the exhausts of smelters and coal-fired power stations are much less efficient at removing mercury5. Atmospheric lead emissions over the North Atlantic declined by a factor of five over the same interval6, largely because of the elimination of leaded petrol. Seen in this context, anthropogenic mercury emissions to the atmosphere are still cause for concern — especially because bacteria in oxygen-free sediments in lakes and wetlands convert inorganic mercury into methylmercury, a fat-soluble, organic neurotoxin that becomes concentrated in the food chain7.
Mercury transformations at Earth's surface are notoriously complex, involving simultaneous physical, chemical and biological processes. Many years of research were needed to unravel how bacteria convert ionic mercury to methylmercury; this was key to explaining the accumulation of mercury in aquatic organisms. A similar effort was needed to understand the rapid decrease in atmospheric mercury concentrations over the Arctic during polar sunrise (a phenomenon known as an atmospheric mercury depletion event or AMDE), and the concomitant accumulation of mercury in snow8. We now know that sunlight creates oxides of bromine (emitted from sea water), which convert mercury from its stable, gaseous form to chemically reactive ionic species that are scavenged by precipitation.
Complex reactions of this sort were initially thought to explain why atmospheric mercury is chemically trapped in the Arctic (Fig. 1), but it was subsequently found that sunlight also reduces ionic mercury in snow back to its volatile, gaseous form, which is then re-emitted to the air8. Moreover, AMDEs are restricted to coastal regions, where marine aerosols are abundant, and so could not explain the anomalous mercury accumulations observed in continental regions of the Arctic.
Enter Obrist et al., who took measurements of mercury concentrations and isotopic abundances in air, snow, vegetation and soils at the Toolik Field Station in the northern interior of Alaska, and at several other locations, over a period of two years. By using a state-of-the-art technique called multi-collector inductively coupled plasma mass spectrometry, combined with rigorous analytical protocols and procedures to optimize detection limits, accuracy and precision, the authors show that the relative abundance of mercury isotopes in vegetation matches that of GEM, but is significantly different from that of ionic mercury (Hg(II), the oxidized form of mercury, which is also found in air). They also show that Δ199Hg — a measure of the isotopic composition of mercury9 — for GEM overlaps with that of Arctic vegetation. These isotopic data reveal that GEM accounts for 90% of the mercury in plants, and that plant uptake of mercury is enhanced in summer.
Plant materials are continually added to soils, where they incompletely decompose and accumulate over time as humus. The mercury in Arctic plants will, therefore, accumulate in tundra soils. Thus, these soils, particularly those that are rich in organic matter, might represent a substantial global sink for this potentially toxic metal. Other Arctic regions will also need to be investigated to quantify the magnitude of this mercury reservoir.
Obrist and colleagues' findings raise key questions about the 'background' levels of mercury that occur in plants and soils. Enrichment of trace metals that result from biological processes can easily confound our ability to quantify contamination by human activities. For example, mercury concentrations in the most recently deposited sediment layers of Arctic lakes are greater than in older sediments. This is commonly attributed to anthropogenic mercury emissions. But some studies (see ref. 10, for example) suggest that the higher concentrations might be partly or entirely produced by an elevated rate of mercury scavenging by algae — caused by greater productivity of the algae as a result of the climate warming that started during the twentieth century.
The new work has implications for the accumulation of mercury in the two most important reservoirs of organic matter in the terrestrial biosphere: humus in soils and peat in waterlogged environments. The findings beg the question of how mercury uptake by vegetation affects the transport of this element by particulate and dissolved organic matter to northern rivers. And how will all of these processes be affected by climate change? Obrist and colleagues' work will inspire further studies of the biological cycle of mercury, which will help us to understand the impacts of human activities. The study also shows how careful analyses of the isotopic composition of mercury can help to unravel the complexities involved.Footnote 1
Obrist, D. et al. Nature 547, 201–204 (2017).
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