Polychaete Worms Increase the Bioavailability of Mercury in Coastal Regions
Tom Sizmur, Rothamsted Research, Nelson O’Driscoll, Acadia University and João Canário, Instituto Superior Tecnico
Bay of Fundy mudflats at low tide
Worms that create burrows in coastal sediments can increase the bioavailability of mercury and methylmercury to other organisms. This is the result reported in a new ET&C Feature Article that presents a study carried out by researchers at Acadia University on the Bay of Fundy mudflats, home to the highest tidal amplitude on Earth. The worms engineer their environment by creating burrows, resulting in a network of microhabitats with altered sediment geochemistry. The sediment in these burrows contains about 50% more available mercury and methylmercury than sediment containing no worms.
Organisms that alter their environment to the benefit of the ecosystem they inhabit by, for example, increasing nutrient availability, are often referred to as "ecosystem engineers." Ecosystem engineers are enormously important keystone species in many ecosystems because they increase the diversity of habitats and increase the availability of resources to other organisms. Burrowing invertebrates, such as earthworms in terrestrial soils and polychaete worms in marine and freshwater sediments, can fundamentally change the chemical, biological and physical characteristics of the substrate that they inhabit. However, in contaminated environments these processes may accelerate the natural biogeochemical cycling of potentially toxic elements. This acceleration results in an elevated bioavailability and toxicity of contaminants to other organisms in the ecosystem.
Nereis diversicolor, the polychaete worm used in the experiments
The polychaete worms that live in the Bay of Fundy mudflats irrigate the sediments by pumping seawater through their burrows. Below the surface of the mudflats, where oxygen levels are depleted, the sediment has a distinctive black color because ferrous sulfides, produced by microbial sulfate reduction, precipitate as iron sulfide. The result of the fresh seawater pumped through the polychaete burrows is that the sediment surrounding the burrows become oxygenated. It is easy to spot a polychaete burrow because the oxidized sulfate in the burrow sediments causes it to be much lighter in color than the surrounding sediment. Because mercury binds very strongly to sulfides, mercury bioavailability is increased when sulfides are oxidized to sulfates.
Methylmercury is an organic mercury compound that is more toxic than inorganic mercury and biomagnifies up food webs in aquatic ecosystems. Methylmercury can reach toxic levels at the top of food webs, including in coastal seabirds that prey on worms and other invertebrates in mudflats. Therefore, understanding the factors that contribute to mercury methylation at the base of the coastal food web may help mitigate negative impacts on human populations. Inorganic mercury is converted to methylmercury in sediments primarily by sulfate reducing bacteria. By increasing the bioavailability of mercury to sulfate reducing bacteria and supplying the bacteria with food trapped from the water column, the polychaete worms increase the rate of mercury methylation in their burrow walls.
These findings have important implications for the way that contaminants are monitored in natural environments. When scientists take samples of soils or sediments to test the concentration of contaminants, the samples are usually homogenized to ensure that the average concentration is measured. The homogenization mixes up all the microhabitats that ecosystem engineers create in the environment, and all this “information” is lost. The burrows of polychaete worms contain many more microorganisms and invertebrates than the surrounding sediment, so they contribute more to the transfer of contaminants in the ecosystem. Furthermore, the burrows increase the surface area of the sediments by increasing the water–sediment interface, so they are important to consider when modelling the flux of contaminants from the sediment to the overlying water. When soil and sediment samples are homogenized before analysis, the bioavailability of mercury and other contaminants may be underestimated.
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