Like you and me, microbes need some metals in their diet to stay healthy. The metals help the microbes to fully “digest” the food. After a good meal, the microbes, which gain energy by chemically reducing nitrates, release a harmless by-product: nitrogen, the gas that makes up 78% of Earth’s atmosphere.
But if a particular metal, copper, is not available, these microbes cannot complete the biochemical “digestion” process called denitrification. Instead of releasing nitrogen, they release the powerful greenhouse gas nitrous oxide.
Previous laboratory studies using pure cultures have shown that copper availability is important for denitrification. Now, research from the lab of Daniel Giammar, the Walter E. Browne Professor of Environmental Engineering at the McKelvey School of Engineering, and Jeffrey Catalano, Professor of Earth and Planetary Sciences in Arts & Sciences, both at Washington University in St. Louis, showed that there may not always be enough copper available for denitrification in the complex, dynamic aquatic environments inhabited by these microbes.
Their research was published in the journal on June 15 Geochimica and Cosmochimica Acta.
“Material in a beaker is not the same as material in the environment,” Giammar said. “A big part of our approach was to take real materials from real environmental systems and bring them into the lab and look at them in a controlled way.”
The results underscore the paramount role of copper in the release of nitrous oxide. “With normal background levels, these systems may not have enough metals to do the process,” said Neha Sharma, a graduate student in Giammar’s lab.
This is important because nitrous oxide is the third most potent greenhouse gas and 50% comes from microbes in aquatic ecosystems.
To better understand how copper affected the release of the gas in these systems, Sharma and Elaine Flynn, a senior scientist in Catalano’s lab, went to the source. Working with three US Department of Energy (DOE) laboratories – Oak Ridge and Argonne National Laboratories and the Savannah River Site – Sharma and Flynn collected microbes from wetlands and river beds. When they analyzed how much copper was in the systems, they found it wasn’t enough to complete denitrification.
“Then we wanted to see if adding copper manually affects the release of nitrous oxide,” Sharma said. It did. “All the nitrous oxide has been converted into other things,” but no harmful greenhouse gases.
This finding could point to new ways to curb atmospheric warming, Sharma said. “If we put a little bit of metal into the natural systems, it could reduce the release of N2O,” she said. It could also have a more immediate impact on researchers studying the climate.
“Currently, models that predict the release of gases from different systems do not take these factors into account,” Sharma said. “They know that factors like food availability or temperature can affect the release of greenhouse gases, but they don’t consider the effect of metals on that aspect of greenhouse gases.”
For humans to truly understand climate and make useful predictions, climate models must account for the full complexity of the real world in specific ecosystems.
Another study published in the journal in May ACS Earth and Space Chemistryanalyzed the behavior of four different metals from wetland soils on banks of the Savannah River Site and river sediments near Oak Ridge National Laboratory.
The research team, which included Sharma and Zixuan Wang, a graduate student in the lab of Zhen “Jason” He, a professor of energy, environmental and chemical engineering, wanted to know if the availability of the metals changed when the metals were underwater ( and there was little oxygen) compared to those exposed to air.
The team had reason to believe that the four metals – all of which are important for microbes’ biochemical reactions – might all work in a similar way. However, to their surprise, the metals behaved differently in similar situations.
“This means that the bioavailability of certain metals changes with the seasons,” Sharma said. “It just underscores the extreme complexity of natural systems.”
In order to grasp this complexity, a large number of specialists and partners are required.
“We’re environmental engineers and we’re always thinking, ‘Why is this important? What does that do for the climate? What can be done?’” Giammar said. “But we also worked with principal investigator Jeffrey Catalano,” which gave the work a strong geochemical perspective.
In addition to funding and access to watersheds from the DOE labs, this research also contributes to the DOE’s knowledge base.
It provides another piece of the “watershed function” puzzle, the study of biogeochemical functions or watersheds and their inhabitants. Meanwhile, other researchers in other fields are doing the same.
Collectively, knowledge can change the way people understand the relationship of the watershed to climate.
“If anything, we saw that the copper cap was a bigger deal than we thought it was,” Giammar said. “That’s why I think it’s important to address this environmental complexity.”
Relation: Sharma N, Flynn ED, Catalano JG, Giammar DE. The availability of copper determines the accumulation of nitrous oxide in wetland soils and river sediments. Geochim Cosmochim Acta. 2022;327:96-115. doi: 10.1016/j.gca.2022.04.019
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