Microbes help release CO₂ from eroding rocks

Rocks may seem chemically inert, but carbon and oxygen constantly move between the atmosphere and the land. A new study reveals insights into tiny players in this cycle. Microorganisms can release carbon dioxide from eroding rocks to the atmosphere more quickly than researchers expected, according to the study.

Scientists previously thought that eroding mountains served as carbon sinks, pulling CO₂ from the atmosphere by forming carbonate minerals. Jordon Hemingway of Harvard University and colleagues studied carbon release in the Central Mountain Range of Taiwan, which erodes quickly thanks to frequent typhoons and landslides.

They estimated that at least 67% of the organic carbon held in the bedrock had been released—presumably by oxidation to CO₂.

To determine whether microbes were responsible for this oxidation, the researchers looked at the ratio of carbon-13 to carbon-12 in fatty acids present in the soil. The team knew that only bacteria could produce these fatty acids. The carbon isotope ratio matched that of the organic carbon found in the rock, indicating that the microbes were the ones oxidizing the carbon. Hemingway’s team estimates that 6.1 to 18.6 metric tons of carbon per km² gets oxidized every year by microbes at these rapidly eroded sites (Science 2018, DOI: 10.1126/science.aao6463).

Hemingway points out that this microbial oxidation of carbon is important on the scale of millions of years. “In terms of anthropogenic climate change, the processes we looked at in our study are unlikely to have any measurable effect,” he says.

As a next step, Hemingway hopes to perform genomic analyses to determine which microbes are responsible for turning the carbon under our feet into CO₂ in the air.