Focus on the Environment
By Tom Garlinghouse
Hemmed in on one side by towering redwoods and on the other by the Pacific Ocean, the vibrant coastal city of Santa Cruz, some 70 miles south of San Francisco, is a great place for a young person to experience nature. That’s where Xinning Zhang, assistant professor of geosciences and the Princeton Environmental Institute, spent her childhood.
“Growing up there, you just can’t help thinking about and caring about the environment,” she said.
One of the environmental questions Zhang is exploring is why methane, a significant greenhouse gas, is increasing in the atmosphere. Although public focus has largely been on rising levels of carbon dioxide, methane is roughly 30 times more potent at trapping heat.
Most of us are aware that cows release methane, but wetlands are also a major source of the gas, formed when organic matter decomposes. The culprits are among the smallest living constituents of the soil — the microbes.
“The habitability of our planet really depends on microbes,” Zhang said. “Microbes control oxygen in the atmosphere; they were the first life forms on Earth. They are hugely important.”
Bogs host a community of microbes, including Archaea, which generate methane as a byproduct when they consume and digest components of organic matter. These “methanogens” thrive in oxygen-free environments like wet and boggy soils that typically occur in the wetlands’ deeper levels.
However, researchers have been surprised to find that quite a lot of methane comes from the oxygen-rich peat and soil near the surface. The conundrum of how these methane-producing microbes, for which oxygen is toxic, can thrive in oxygen-rich settings has been termed the wetland paradox.
To explore this phenomenon, Zhang and Jared Wilmoth, a postdoctoral research associate on Zhang’s team, analyzed peat samples collected from a bog in the Northeast U.S.
One set of samples was subjected to oxygen treatment followed by incubation under oxygen-free conditions. The other samples remained in an oxygen-free environment for the entire study period. Contrary to expectations, Zhang and her team found that the oxygen-treated peats produced a higher amount of methane than the peats maintained under continuously oxygen-free conditions.
To find out why the researchers sequenced the genomes of all microbes found in the peat samples. They found that oxygen stimulated another group of microbes to break down constituents of peat that tend to be toxic, such as tannins. As these microbes break down the compounds, they also lower the toxicity threatening the entire microbial community, including methane-producing Archaea. The result is the creation of far more methane than expected.
Zhang’s work, which is funded by the Princeton Environmental Institute’s Carbon Mitigation Initiative, suggests that oxygen variability is an important control on wetland methane emissions. Her team is also exploring how hydrology, timescales of oxygen variability, and different soil chemistries influence microbial methane production.