Ancient rock as a source of food

Meltwater streams from the retreating glaciers could make more fossil organic material available for mining. © Torben Stichel

They are actually considered “difficult to digest” for microorganisms. But now a study shows that sediment bacteria can also use organic material from fossil rocks as a surprisingly effective source of carbon. The importance of this ability could increase as such material becomes increasingly accessible due to the retreat of glacial ice as part of climate change. The researchers say that mining could in turn represent another source of fossil greenhouse gases.

It is a well-known fact that plants – and especially tiny algae in the sea – absorb large amounts of the greenhouse gas carbon dioxide. Because they use it as part of photosynthesis to produce carbon-rich compounds. The resulting biomass then serves as a food source for other living beings. In the end, microorganisms break down these organic compounds and release carbon dioxide again in the process. Part of the biomass can, however, also escape decomposition due to certain environmental conditions and be bound in certain rocks in the long term. This is most evident in coal, but other materials, such as shale, can also contain significant amounts of fossil organic compounds.

This so-called petrogenic material can then also get into marine sediments as a result of erosion processes – melting glaciers, for example, can be particularly responsible for this process. So far, however, it has been assumed that it is simply deposited there again. For microbes, the ancient carbon-containing substances are considered "difficult to digest" because certain conversion processes have made them difficult to access for their metabolism. In principle, it was known that some bacteria can still crack fossil material. However, it is unclear to what extent this is happening.

On the trail of carbon sources

An international research team led by Manual Ruben from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven has now investigated this question. They examined sediment cores from the Hornsund - the southernmost large fjord of Svalbard. The researchers report that the glaciers that flow out there have drastically retreated in the course of global warming in recent decades. In the process, more material with fossil organic components presumably found its way into the sediments with the meltwater.

In order to get clues as to which carbon sources the bacteria use from the sediment cores, the scientists examined organic biomarkers that come from their cell membranes. The focus was on the carbon signature of these substances. According to the principle "you are what you eat", conclusions about the food of the microbes are possible. The results of radiocarbon analyzes show how old their carbon sources are: The researchers explain that the extent to which the microbes also metabolize ancient organic material is reflected in signatures of the levels of the carbon isotope C14.

If necessary, also fossil feed

As the team reports, their results showed that if available, the microbial communities in the sediment prefer fresh organic material, which comes from dead seaweed, for example. But the analyzes also show that if necessary, they can draw heavily on the fossil supply brought in by the melting glaciers. The Hornsund's sediment microbes can even get up to 50 percent of their carbon from these rather persistent organic compounds, the results show. Apparently they have the corresponding metabolic abilities. The scientists conclude that this is proof of the important role of petrous organic carbon as a food source for sediment microbes.

But what significance could the findings have? It is clear that the decomposition of fossil carbon compounds releases greenhouse gases - similar to the combustion processes used by humans. As the researchers emphasize, there are also many other areas in the far north of the earth with conditions such as those found in the Hornsund studied. In addition, the strong retreat of the glaciers could ensure an increasing supply of petrous organic carbon. How large the resulting greenhouse gas inflows could be cannot be clarified on the basis of the information so far, emphasize Ruben and his colleagues. In conclusion, they therefore write: “Given the size of the global petrous reservoir, there is now a need for further quantitative research on this topic.”

Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Nature Geosciences, doi: 10.1038/s41561-023-01198-z

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