New study helps unravel the role of soil microbes in the global carbon cycle

When soil microbes eat plant matter, the digested food follows one of two pathways. Either the microbe uses the food to build its own body or it breathes its food in the form of carbon dioxide (CO).₂) in the air.

Now, a Northwestern University-led research team has, for the first time, tracked the path of a mixture of plant wastes as they move through the metabolism of bacteria to contribute to atmospheric CO.₂. Researchers found that microbes breathe three times more CO₂ from lignin carbons (unsweetened aromatic units) versus cellulose carbons (glucose sugar units), which add both structure and support to plant cell walls.

These results help untangle the role of microbes in the soil carbon cycle – information that could help improve predictions about how soil carbon will affect climate change.

The study, “Disproportionate carbon dioxide efflux in bacterial metabolic pathways for different organic substrates leads to variable contribution to carbon use efficiency,” was published June 11 in the journal Environmental science and technology.

“The reservoir of carbon stored in the soil is about 10 times greater than that in the atmosphere,” said Ludmilla Aristilde of Northwestern University, who led the study.

“What happens to this reservoir will have a huge impact on the planet. Because microbes can release this carbon and transform it into atmospheric CO.₂, there is enormous interest in understanding how they metabolize plant waste. As temperatures rise, more organic matter of different types will become available in the soil. This will affect the amount of CO₂ which is emitted by microbial activities.

An expert in organic matter dynamics in environmental processes, Aristilde is an associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering and a member of the Center for Synthetic Biology and the Paula M. Trienens Institute for Sustainability and energy. Caroll Mendonca, former doctoral student. candidate in Aristilde’s laboratory, is the first author of the article. The study includes collaborators from the University of Chicago.

“Not all courses are created equal”

The new study builds on ongoing work in Aristilde’s lab to understand how soil stores or releases carbon. Although previous researchers have typically tracked how broken down compounds in plant matter move individually through bacteria, Aristilde’s team instead used a mixture of these compounds to represent what bacteria are exposed to in the environment. ‘natural environment.

Then, to track how different plant derivatives moved through a bacteria’s metabolism, the researchers labeled individual carbon atoms with isotopic markers.

“Isotopic labeling allowed us to track the specific carbon atoms of each type of compound inside the cell,” Aristilde said. “By tracking carbon routes, we were able to capture their pathways in metabolism. This is important because not all pathways are created equal in terms of carbon dioxide production.”

The sugar carbons in cellulose, for example, took the glycolytic and pentose-phosphate pathways. These pathways lead to metabolic reactions that convert digested material into carbons to make DNA and proteins, which constitute the microbe’s own biomass. But the aromatic, non-sweet carbons of lignin took a different route: through the tricarboxylic acid cycle.

“The tricarboxylic acid cycle exists in all life forms,” Aristilde said. “It exists in plants, microbes, animals and humans. While this cycle also produces protein precursors, it contains several reactions that produce CO₂. Most of the CO₂ what is breathed through metabolism comes from this pathway. »

Expand results

After tracking the metabolic pathways, Aristilde and her team performed a quantitative analysis to determine the amount of CO₂ produced from different types of plant materials. After consuming a mixture of plant materials, microbes breathed in three times more CO₂ carbons derived from lignin compared to carbons derived from cellulose.

“Even if microbes consume these carbons at the same time, the amount of CO₂ generated from each type of carbon is disproportionate,” Aristilde said. “This is because carbon is processed via two different metabolic pathways.”

In initial experiments, Aristilde and her team used Pseudomonas putida, a common soil bacterium with a versatile metabolism. Curious to see if their findings applied to other bacteria, the researchers looked at data from previous experiments in the scientific literature. They found the same relationship they discovered between plant matter, metabolism and CO.₂ manifested in other soil bacteria.

“We propose a new metabolism-driven perspective to think about how different carbon structures accessible to soil microbes are processed,” Aristilde said. “This will be critical in helping us predict what will happen to soil carbon cycling with climate change.”