Coastal wetlands
are just one of dozens of environments where scientists found genes
that transform
mercury into the neurotoxin methylmercury.
(Photo courtesy of
Smithsonian Environmental Research Center/Grace Schwartz)
(October 10, 2015) Thawing
permafrost and contaminated sediment in marine coastal areas pose some of the
greatest risks for the production of highly toxic methylmercury, according to
findings published in the journal Science Advances.
The discovery of these newly identified locations for
methylmercury production builds on previous work in which scientists from the
Department of Energy’s Oak Ridge National Laboratory reported on two genes in
bacteria that convert inorganic mercury into the organic form. This variety,
called methylmercury, is far more dangerous to humans and the environment. Now,
scientists from ORNL and the Smithsonian Environmental Research Center have
found that these genes are present in microbes from many of the 3,500
environments they examined.
“We looked for genes that we know are involved in microbial
mercury methylation – from deep in the ocean to Arctic permafrost to the human
gut,” said corresponding author Dwayne Elias of ORNL’s Biosciences Division.
“Using new computational methods, we examined data from thousands of sites
around the globe where other scientists had sequenced every gene in that
environment.”
Elias noted that many of the metagenomes used in the study
were obtained from the Joint Genome Institute, and tools from the Integrated
Microbial Genomes System aided in the analysis.
While researchers confirmed the presence of the genes of
bacteria they suspected could methylate mercury, they found several new and
novel bacteria to add to the list. Researchers also looked at 1,500 human and
mammalian metagenomes and concluded that there is an extremely low risk of
microbial methylation of mercury within the human body. This essentially put to
rest concerns raised in their work published in Environmental Science and
Technology in 2013 in which they reported this possibility.
With the exception of the mammalian gut, the team found that
the mercury methylation genes are abundant in nearly every oxygen-free
environment, including rice paddies and marshes, aquatic sediments and certain
types of bioreactors. The genes are also abundant in invertebrate digestive
tracts and extreme environments. The genes were not typically found in aerated
habitats such as the open ocean.
Mercury is a global pollutant released to the atmosphere
through coal burning, artisanal (small-scale) mining, industrial uses and some
natural processes. Most of the harm comes from methylmercury bioaccumulation,
which is the buildup of the element in tissue that occurs when moving up the
food chain. Ocean fish are the primary source of methylmercury in human diets worldwide.