Soil microbes: Drought resistance is key

Outdoor experiment

In this outdoor soil experiment in Styria, drought was simulated in combination with future climate conditions. © Markus Herndl/ HBLFA Raumberg-Gumpenstein

As climate change occurs, we will experience severe droughts more and more frequently. But they not only affect crops and water levels, but also the microorganisms in the soil. Researchers have now found that 90 percent of them have extremely difficult times with drought. Only four to nine percent of soil bacteria continue to grow even in drought, including the genus Streptomyces. Bacteria like them are likely to be the future - with as yet unknown consequences.

Whether forest fires in Greece or a half-dry Lake Garda in Italy: extreme droughts like this will become more and more common due to climate change. However, it not only leads to fires, crop failures and water rationing, but also affects the microorganisms that live in the drying soil. This is a problem because soil microbes, with tens of thousands of different species, contribute around a quarter of global biological diversity. They also perform numerous important ecosystem functions by maintaining soil fertility, helping plants grow, and regulating carbon storage.

Simulations in Styria

Although soil-dwelling microorganisms are so important for the environment, how they respond to increasing drought was virtually unknown. Since bacterial growth is best measured in aqueous solutions, dry soils always fell through the cracks. But researchers led by Dennis Metze from the University of Vienna have now managed to close this knowledge gap for the first time. To do this, the team first took soil samples from a special field experiment in Styria. There, scientists simulate various current and future climate conditions, for example by artificially heating grassland and treating it with CO2. Some of the bacteria contained in the soil samples were previously led to believe that they lived in a climate three degrees hotter and significantly drier than was actually the case.

In order to find out what influence dryness has on the microorganisms, Metze and his colleagues added isotopically labeled water vapor to the samples they obtained. When bacteria thrived and continued to divide in dry conditions, they incorporated the oxygen from the water vapor into their DNA. By specifically looking for this marker in the DNA, the researchers were able to identify which types of bacteria continued to grow despite the drought and which had stopped growing.

Drought resistance important for survival

The result: “Most bacteria stopped growing as the drought increased,” reports Metze. More than 90 percent of the microbes stopped dividing or even died under dry conditions. “However, this did not apply equally to all microorganism groups,” continued Metze. Under the environmental conditions of a current drought, at least four percent of the bacterial communities continued to grow. They didn't seem to mind the dryness. If the bacteria were exposed to the dry climate conditions of the future for six years, the proportion of drought-resistant communities increased to nine percent. Their populations had probably prevailed against less well-adapted species and increased significantly, according to the researchers.

Specialized species of actinobacteria, particularly the genus Streptomyces, turned out to be particularly drought-resistant, as Metze and his team were able to observe in the laboratory. Since bacteria in this genus grow in threadlike form, they could “bridge” the separated pores in dry soils and thus continue to thrive. This is extremely practical because Streptomyces are known to help plants survive drought better. In general, however, it is still unclear what consequences the future shift in microbial communities towards drought-resistant species will have.

Source: University of Vienna; Specialist article: Nature Communications, doi: 10.1038/s41467-023-41524-y

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