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New study helps disentangle role of microbes in the global carbon cycle

When soil microbes digest plant matter, the digested food follows one of two pathways.

A farmer sits in a drought-stricken rice field in Vietnam's southern Ben Tre province, which is plagued by intruding salt water
A farmer sits in a drought-stricken rice field in Vietnam's southern Ben Tre province, which is plagued by intruding salt water - Copyright AFP Nhac NGUYEN
A farmer sits in a drought-stricken rice field in Vietnam's southern Ben Tre province, which is plagued by intruding salt water - Copyright AFP Nhac NGUYEN

A microbiological study could improve predictions of how carbon in soil will affect climate change.

This is from a study that shows how soil bacteria respire more carbon dioxide after digesting non-sugar organic matter. This increases our understanding of how plant matter moves through bacteria’s metabolism.

Specifically, the Northwestern University findings show that microbes respire three times as much carbon dioxide from non-sugar carbons from lignin compared to sugar from cellulose. Although microbes consume both types of plant matter at the same time, each type enters a different metabolic pathway.

When soil microbes digest plant matter, the digested food follows one of two pathways. Either the microbe uses the food to build its own body, or it respires its meal as carbon dioxide into the atmosphere.

Now, a Northwestern University-led research team has, for the first time, tracked the pathways of a mixture of plant waste as it moves through bacteria’s metabolism to contribute to atmospheric carbon dioxide.

This was demonstrated by tagging individual carbon atoms with isotope labels and studying the bacterium Pseudomonas putida. This enabled the scientists to track carbon atoms specific to each compound type inside the cell and hence to capture their paths in the metabolism.

Microbes respire three times as much carbon dioxide from lignin carbons (non-sugar aromatic units) compared to cellulose carbons (glucose sugar units), which both add structure and support to plants’ cellular walls.

Sugar carbons in cellulose travelled through glycolytic and pentose-phosphate pathways. These pathways lead to metabolic reactions that convert digested matter into carbons to make DNA and proteins, which build the microbe’s own biomass. But aromatic, non-sugar carbons from lignin travelled a different route through the tricarboxylic acid cycle. This pathway contains several reactions that produce carbon dioxide. This makes the pathway a suitable target for minimisation and remediation.

Such findings help disentangle the role of microbes in soil carbon cycling. This is information that could help improve predictions of how carbon in soil will affect climate change.

Given that the carbon pool stored in soil is about 10 times the amount that’s in the atmosphere, what happens to this reservoir will have an enormous impact on the planet. As temperatures rise, more organic matter of different types will become available in soil. This will affect the amount of carbon dioxide that is emitted from microbial activities. Hence, activities that can stem the release of some of the carbon dioxide are of great importance.

The research has been published in the journal Environmental Science & Technology. The research is titled “Disproportionate carbon dioxide efflux in bacterial metabolic pathways for different organic substrates leads to variable contribution to carbon use efficiency.”

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Written By

Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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