Newswise – A team led by the Oak Ridge National Laboratory of the Department of Energy and the University of Michigan has discovered that certain bacteria can steal a compound essential from other microbes to break down the toxic methane and methylmercury in the ‘environment.
The findings could inform strategies to manipulate these microorganisms to reduce emissions of methane, a potent greenhouse gas, and detoxify methylmercury, a potent neurotoxin that can build up in the food supply.
The study, published in The ISME review, have found that certain classes of methanotrophic or methane-consuming bacteria that were previously thought to be incapable of breaking down methylmercury can actually break it down in the environment. This activity is possible because microbes are equipped with cellular machinery to absorb and use a compound called methanobactin which is produced by other microbes.
Methanotrophs are widespread in nature. They live near the methane and air interfaces, such as the top layer of soils, river sediments, and wetlands where they can access oxygen while feeding on the methane that flows from anoxic or oxygen-poor environments located below.
These bacteria play a vital role in the carbon cycle, consuming substantial amounts of methane generated by other microbes called methanogens. This natural counterweight is important in limiting methane emissions, which are 25 times more powerful than carbon dioxide in warming the earth’s atmosphere.
A better understanding of how methane feeders work may indicate methods of using them, such as levers to control methane emissions. New knowledge can also better inform climate models that predict the future of the planet.
The researchers discovered these new methanotrophic behaviors while studying another global problem: mercury pollution. ORNL has a long history of mercury-related breakthroughs, including its 2013 discovery of Genoa that allow microbes to turn mercury into methylmercury toxin.
In 2017, a team led by ORNL was the first to to prove that some methanotrophs can break down methylmercury, a process called demethylation. Their latest findings build on this finding, showing that more methanotrophs than previously known can degrade methylmercury.
“As we gain new knowledge about methanotrophic activities, we may be able to more effectively manipulate these microbial communities to reduce methane emissions and improve the detoxification of mercury in the environment,” said Baohua Gu , member of ORNL and biogeochemist.
Producers and cheaters
Methanotrophs seek the simplest and fastest food supply, with a target of single-carbon compounds like methane and methylmercury, which have similar chemical structures. These microbes also need copper to fuel their metabolic processes. It is this need for copper that can limit methanotrophic activity, causing microbes to search the environment for sources of copper using many different methods.
Some methanotrophs use a surface protein to secure the copper. Others secrete a compound called methanobactin, or MB, which binds to copper in the environment and facilitates the acquisition of copper. Previous findings by the team had shown that only bacteria with the genetic and metabolic machinery to produce methyl bromide can break down methylmercury.
The researchers’ latest findings demonstrate that some methanotrophs that do not produce MB can detoxify methylmercury using the MB secreted by other methanotrophs. In other words, they steal it.
“They are effectively what we call cheaters,” said Jeremy Semrau, a University of Michigan microbiologist. “This has been seen before, when a microorganism produces something that benefits the community at large and others steal it. This allows some methanotrophs to meet their copper requirements.
The research team also showed that successful MB theft requires methanotrophs to have the gene, named mbnT, which enables the production of a specific protein called the TonB transporter. Aptly named, this protein displaces MB – and associated copper – in the microbe, allowing the breakdown of methylmercury and methane.
UM scientists have designed a strain of methanotrophs without mbnT gene, and the ORNL team analyzed the mercury in the samples. The deletion of mbnT The gene and transport protein of these microbes effectively deactivated their ability to take up MB or detoxify methylmercury.
This information could inform future avenues towards combating mercury pollution in the environment.
“I think it’s a wonderful strategy going forward where we could use methanotrophs to help clean up mercury contaminated sites, and that could actually happen, to some extent, naturally,” Semrau said. .
Another piece of the puzzle
Methanotrophs are common in the environment, but much remains to be learned about their activities. A team led by ORNL environmental scientist Scott Brooks collaborated with Semrau’s group at UM on the Discovery several new methanotrophs at East Fork Poplar Creek, a mercury-contaminated stream crossing the Oak Ridge Reservation and studied for decades.
Brooks and his team studied biofilms, which are complex communities of algae and bacteria that accumulate on rocks in streams as “green mud”. While the biofilms are about as thick as a few stacked credit cards, they are hot spots for processing mercury and nutrients.
The ORNL team had previously find that the oxygen-poor pockets of these biofilms harbor microbes that transform mercury into its most toxic form: methylmercury. Their recent discovery of methanotrophs in the oxygen-rich cavities of these same biofilms means that methylmercury degradation also occurs naturally in the stream.
“There are very steep chemical gradients and changes in concentration that occur over a very small distance,” said Brooks. This includes dissolved oxygen which “disappears in a few tenths of a millimeter”.
These tiny pockets of oxygen are enough for methanotrophs to thrive. Preliminary analysis showed that the microbial activity producing methylmercury exceeded the methanotrophic activity breaking down the toxin. With further study, scientists could potentially identify methods to tip the scales towards methylmercury degradation.
“It’s a beautiful marriage of two different research projects working in parallel,” said Brooks. “We’re seeing things that are consistent with each other and that help us confirm what’s going on with the mercury cycle in these complex microbial communities.”
The co-authors of the article titled “Evidence for methanobactin ‘theft’ and novel Chalaphore production in methanotrophs: impact on methanotrophic-mediated methylmercury degradation” included Xujun Liang, Lijie Zhang, Xia Lu and Baohua Gu from ORNL; Christina Kang-Yun and Jeremy Semrau of UM; Philip Dershwitz, Joshua Ledesma, Daly Pelger, and Alan DiSpirito of Iowa State University; Wenyu Gu of Stanford University; and Aloys Schepers, Andrew Flatley, Josef Lichtmannegger and Hans Zischka from the German Research Center for Environmental Health.
Co-authors of the article titled “Complete Genome Sequences of Two Gammaproteobacterial Methanotrophs Isolated from a Mercury-Contaminated Stream” included Scott Brooks and Baohua Gu of ORNL and Christina Kang-Yun, Jin Chang and Jeremy Semrau from UM.
This research was funded by the DOE Bureau of Science, the Bureau of Biological and Environmental Research, as part of the Biogeochemical transformations at critical interfaces Area of scientific interest.
UT-Battelle manages ORNL for the Office of Science of the Department of Energy, the largest support for basic research in the physical sciences in the United States. The Office of Science works to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.