Climate change may boost mercury in ecosystems, food chain

Rising temperatures may cause localized increases in the amount of toxic mercury introduced into ecosystems, impacting wildlife and the food chain.

According to a recent study published in Science Advances, mercury in ecosystems has risen two- to five-fold since the dawn of the industrial age. Today, climate change may be causing an even greater problem for some areas as an increase in organic matter enters waterways.

"Increased temperature may regionally cause increases in precipitation and runoff, and if so, in export of organic matter from land to water," the study's author and environmental chemist Erik Björn said.

This relation is complex and projected to vary highly for different geographical regions, he added.

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Björn, and his fellow researchers, used the mesocosm research infrastructure at the Umeå Marine Science Centre in Sweden to construct different model ecosystems. This system consisted of 12 plastic tubes measuring at 75 centimeters in diameter and 5 meters high.

Each tube was filled with 2,000 liters of water and a sediment at the bottom. This allowed the researchers to control temperature, light conditions and the addition of nutrients and organic matter to form different types of model ecosystems.

"We combined this approach by the addition of isotopically enriched mercury tracers. We added three tracers to the sediment and two to the water phase to simulate different pools of mercury in the ecosystem," he said. "We then monitored throughout the experiment how methylmercury was formed and accumulated in biota for these different isotope tracers."

Because of the increases in natural organic matter from the soil, water becomes more brownish in color. This decreases the amount of sunlight that can penetrate, causing a decrease in photosynthetic production of the photoplankton.

Instead, bacteria that use part of the organic matter for their metabolism can then increase.

"This causes a change in the structure of the food web, from being dominated by phytoplankton to being dominated by bacteria," Björn said, adding that there are a larger number of steps in the bacteria-based food web compared to the phytoplankton-based food web.

Methylmercury is formed by certain types of microorganisms that use inorganic mercury located in soils, sediment and water. The inorganic mercury mainly originates from atmospheric depositions, both naturally and from emissions related to human activity.

A fraction of the toxic methylmercury will bind to sulfide and chloride, which can be then be available in the cellular uptake in bacteria and plankton, he added.

"Methylmercury has strong neurotoxic properties. Observed effects in wildlife like fish, birds and mammals, include behavioral, neurochemical, hormonal and reproductive changes," Björn said.

The impact of increased runoff on the aquatic ecosystem is due to increase precipitation among other things and is dependent on the watershed and water surface areas.

For example, in certain regions, temperature-driven increases in precipitation may be accompanied by increases in the evaporation of water from the soil and have only minor effects in runoff.

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"A larger watershed area to water surface area means a larger potential impact from land runoff, although several other factors also contribute," he added. "As a consequence, small boreal lakes and coastal regions are more impacted by runoff than the open oceans."

A further consequence of this is the establishment of a new type of “intermediate” predatory organisms like protozoa which feed on the bacteria.

"Methylmercury is enriched approximately by a factor of 10 in each step of the food web, and therefore the amount of methylmercury in zooplankton and higher predators increases in the bacteria-based food web," he added.

For predatory organisms, including humans, the main exposure route for methylmercury is through food consumption.

"In each step in the food chain, methylmercury gets concentrated approximately 10 times, which means that the concentrations increases throughout the food chain and is highest in top predators," Björn said.