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University of Basel

Plants capture mercury from the air.

Text: Yvonne Vahlensieck

As a result of human activities, the amount of mercury in circulation is ever-increasing. To protect both health and the environment, it is important to understand the processes by which this pollutant spreads through soils, air and water – and in which plants appear to play a key role.

Harmful heavy metal: Measuring concentrations of mercury. (Photo: Frank Brüderli)
Harmful heavy metal: Measuring concentrations of mercury. (Photo: Frank Brüderli)

In the mid-1950s, the population of the coastal Japanese town of Minamata suddenly began developing unusual symptoms, including uncoordinated movements, paralysis and sight defects. Many thousands of people died of the mysterious disease. It wasn’t until a few years later that the cause was found to be mercury poisoning. After entering the sea via wastewater from a chemical plant, the heavy metal had accumulated in a toxic form in the fish that served as the principal source of food for those affected.

This environmental disaster drew attention to the dangers posed by mercury when it accumulates in the food chain. Small children along with pregnant and breastfeeding mothers are particularly at risk, as mercury can interfere with the early stages of nervous system development.

Releasing less mercury

The Minamata Convention on Mercury, which was approved in 2013, therefore aims to achieve a massive reduction in the release of mercury into the environment over the coming decades — for example, by calling for the introduction of alternative production methods in the chemical industry. After all, a substantial proportion of the mercury in the atmosphere — several thousand tonnes a year — is introduced by human activities, such as the production of plastics and cement, the operation of coal-fired power stations, and prospecting for gold.

According to a UN report, human activities cause over four times as much mercury to be released into the atmosphere as that due to natural events such as volcanic eruptions. “The big question now is whether the measures established by the Convention are effective,” says Dr. Martin Jiskra from the Department of Environmental Sciences of the University of Basel. According to Jiskra, it is essential that the measures reduce the mercury concentration not only in the air but also in the food chain. “But we can only make good predictions of this if we have good models of global mercury cycling.” The biogeochemist has researched the relevant processes for many years, thereby providing the basis for optimizing these models.

That is easier said than done, however, because mercury circulates between the air, land and water in a complex cycle, adopting various different forms in the process. When the heavy metal is first released by natural or human processes, it enters the atmosphere in its pure, elemental form. Some of the mercury then undergoes chemical reactions that convert it into a water-soluble form, which rain transports into the sea. Once there, microorganisms use it to produce what is known as methyl mercury — a biologically active, toxic form that progressively accumulates in fish and is responsible for the damage to health.

Vegetation as a mercury pump

As Jiskra’s research shows, however, one key component in this cycle has so far been neglected: the role of vegetation. In addition to CO2, plants also absorb pure mercury from the air via their stomata. Although the heavy metal has no biological function, the plants incorporate it into their leaves, which they then shed in fall. As the foliage decomposes, the mercury is returned to the soil and to surface waters.

“The plants therefore act as a sort of mercury pump,” says Jiskra. Around two-thirds of the mercury are removed from the atmosphere in this way, whereas only a third is removed in the water-soluble form by rainfall. “This deposition pathway was neglected in the past and completely changes the dynamics of current models.”

These new insights were only possible thanks to major advances in analytical methods in recent years. For example, mercury naturally occurs in different configurations, known as isotopes, which can be distinguished from one another based on their weight. “It’s like a fingerprint. As plants preferentially take up the lighter form, we can now track how much mercury is removed from the air by vegetation,” says Jiskra. To do this, the researcher analyses air samples collected at five measuring stations stretching from Finland to the Schauinsland near Freiburg, in the Black Forest.

As the mercury is only present at an extremely low concentration (about one billionth of a gram per cubic meter of air), the material for the analysis must be collected and concentrated from six cubic meters of air using activated carbon filters. Jiskra now wants to use this method to find out why the mercury concentration in our atmosphere is higher in winter than in summer: “The previous hypothesis was that power stations burned more coal for heating in winter, thereby releasing more mercury into the air.” However, initial measurements have shown that the seasonal variation is actually connected to the growing season: in summer, plants grow and therefore absorb more gas, reducing the proportion of light mercury isotopes in the air.

Climate change has consequences

Accurate models are, however, not only needed to verify the effectiveness of the Minamata Convention on Mercury. There is another motivation for Jiskra’s research: “It’s vital that we determine how the global mercury cycle is affected by climate change and changes in land use. This aspect is still largely overlooked.”

Vegetation also plays a significant role in Alaska, as Jiskra discovered while working as a postdoc there. Over the last few centuries, plants in the Arctic tundra have steadily absorbed mercury, which was then trapped in the ground by the permafrost. It has now become apparent that the frozen ground is thawing again and releasing large quantities of mercury, which then ends up in the sea. This has potentially serious consequences for the health of the regional population, for whom the Arctic Ocean provides the principal source of food.

In his latest project, Jiskra therefore wants to close more of the gaps in our knowledge of the mercury cycle: “In the past, measurements were primarily aimed the soil, whereas we now want to include all of the vegetation as well.” With this in mind, Jiskra and colleagues are adopting measuring techniques that were developed by climate researchers over recent years in order to study greenhouse gases, such as CO2 and methane.

A pilot system adapted to mercury has been set up in a meadow in the Canton of Zug and is already delivering promising results in real time. Several times a second, the device measures the direction and strength of the wind as well as the mercury concentration in the air.

In the future, the idea is to install the instrument securely above the treetops in order to provide information about how much mercury is absorbed by the forest. “We break the processes down into small pieces and then combine them into an overall picture, like a mosaic,” says Jiskra. “This allows us to understand the complex mercury cycle in its entirety, right through to the accumulation of mercury in fish.”

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