More and more experimental studies in the field and on computer models indicate that mercury could also be absorbed by plants. A study published on April 2nd in Nature geoscience manages to quantify the importance of the phenomenon.
For their research, researchers from the French research centre CNRS and the Paul Sabatier University of Toulouse in France had to mobilize the fifty or so stations that measure this toxic pollutant in forest, marine and urban environments. Mercury is indeed subject to global monitoring intended in particular to verify the international efforts to reduce emissions under the Minamata Convention.
By aggregating data from five forestry stations located in the northern hemisphere, Martin Jiskra and Jeroen Sonke of the Laboratory of Environmental Geology of Toulouse (CNRS / UPS / IRD) found a perfect correlation of the variations of mercury levels in the atmosphere with those of CO2. In the spring and summer, during growth, plants absorb CO2 and reduce its presence in the atmosphere, while in the fall, the cessation of photosynthesis causes a rise in the proportions of CO2. Mercury follows the same seasonality as CO2, so it must be concluded that this chemical element is also captured by the leaves.
“The plant stomata serve as a place for the exchange of all kinds of gases, some of which are trace gases such as mercury, and this assimilation has no influence on photosynthesis because the gaseous mercury is at levels three billion times lower than CO2,” says Jeroen Sonke.
When they die, the leaves release the absorbed mercury
To be clear, the researchers compared forest results with those recorded in marine and urban environments. Thus, they checked the measurements taken on the island of Amsterdam, in the South Pacific, an island that has the distinction of being 3000 kilometers from any land and is covered only with short vegetation.
The station managed by the French Polar Institute IPEV thus reveals seasonal variations in mercury and CO2 close to zero due to the absence of vegetation. As for urban stations, they have variations that have nothing to do with the seasons and everything with human activities. The researchers conclude that the vegetation acts as a biological pump for atmospheric mercury and plays a major role in the annual variation of this compound.
According to the most accurate estimates, the atmosphere of our planet contains 5000 tons of mercury. As this level is 20% lower during summer, the researchers were able to deduce that vegetation absorbs 1,000 tons of mercury each year.
Plants absorbing mercury might sound good at first but it’s not all innocent. “Finally the plants clean up the air, then pollute the soil, lakes, rivers and oceans,” details Jeroen Sonke.
In addition, once fallen in autumn, the leaves are degraded by microbes, which return some of the CO2, and mercury in the atmosphere “. The mercury cycle is thus much more complicated than previously thought, where it was thought that only rains and snowfall caused the pollutant to fall back to the ground. “This mechanism of mercury assimilation by plants is not correctly represented in our mathematical models of air quality,” continues Jeroen Sonke, “so we are working with modellers to better reproduce the seasonality of mercury.”
The team will now also try to determine what natural environments (tropical, temperate, boreal) are the most absorbers of mercury, thus improving models of forecasts to better take into account global warming in the cycle of this metal.