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The story behind that earthy smell in spring

The earthy smell in spring when the fields are ploughed and the garden soil in flower beds is dug over has a previously unknown purpose. Researchers from Lund University in Sweden, the Swedish University of Agricultural Sciences and other institutions have examined the soil smell and were able to show that it is linked to intricate interplay between millimetre-long insect-like organisms that has existed on Earth for 400 million years - and soil bacteria that are prolific producers of antibiotics.
Two insect-like organisms. Photo.
Springtails. Photo: Wikimedia Commons.

The earthy smell comes from geosmin, a substance produced by soil bacteria of the Streptomyces genus. Up to now, no one has known if the scent serves a purpose for the bacteria, something that researchers from Sweden, the UK and Hungary have now investigated.

The study shows that the earthy smell, geosmin, attracts springtails (Collembola), primitive insect-like organisms that are found on all continents. Many of them live in the soil and subsist on microorganisms, such as microscopic fungi and bacteria.

In the study, the researchers show that the Streptomyces bacteria generate geosmin only when they produce spores. When the springtails are attracted to the bacteria, they help to spread the spores, which become attached to the springtails and are also spread via excretion.

The geosmin smell is thus the link in the interplay between the springtails and the soil bacteria. For the springtails, the smell is the way to food, and for the soil bacteria it is a way to spread their spores over a distance that would not otherwise be possible.

“Both the springtails and the bacteria benefit from the earthy smell. It could be said that the smell fulfils an important ecological function”, says Klas Flärdh, a microbiologist at Lund University who led the study.

“Streptomycetes are also of great benefit for humans as these soil bacteria are one of our most important sources of antibiotics. You could call the streptomycetes superproducers of antibiotics. They are among nature’s best chemists and produce large amounts of substances that we use as antibiotics and other types of medicine.”

The researchers carried out studies in the field and the laboratory. By baiting traps with Streptomyces bacteria, they found that springtails were attracted to them. Using electrodes attached to the springtails’ antennae, they then showed that the springtails sense the geosmin smell with their antennae.

The researchers also examined the behaviour by placing the springtails in Y-shaped tubes where they could choose one of two paths. One path smelled of soil, the other did not.

“The experiment showed that it is the earthy smell that attracts them. Knowledge about such ecological interactions in the soil is important for sustainable agriculture”, says Paul Becher, associate professor at Swedish University of Agricultural Sciences and one of the researchers behind the study.

Publication: "Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal" - nature.com

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