It rises when digging in the garden and we don’t like it in food: everyone knows the typical smell of earth. In particular, the soil bacteria from the group of streptomycetes known as suppliers of antibiotics exude the responsible substances. But what for? A study now shows that the smell is, as it were, a call for carriers: the soil microbes give it away to attract tiny animals that ensure their spread.
Streptomycetes are microbes with a special significance for humans: representatives of these bacteria were used to obtain the antibiotic streptomycin in 1943, which could put many dangerous pathogens in their place. Streptomycetes are harmless bacteria that form fungal networks in the soil. They break down organic material and are therefore of great importance for the material cycle and soil fertility. However, hardly anyone knows that they are also special for another reason: everyone has already noticed them with their noses – they cause the typical smell of earth.
Why the smell?
Two volatile substances are known to be responsible for this: geosmin and 2-methylisoborneol (2-MIB). Studies have shown that the production of these two substances is widespread among the various representatives of the streptomycetes. Geosmin and 2-MIB therefore seem to have an important function – but which ones? The researchers led by Paul Becher from the Swedish University of Agricultural Sciences in Alnarp have now suspected that they are attractants that target the “noses” of the springtails (Collembolen). These are millimeter-sized arthropods that feed on rotting plant material and on the microbes that grow on this substrate.
As part of the study, the researchers initially conducted preference tests with springtails of the species Folsomia candida by. As they report, the test animals in Y-shaped test vessels always choose the branch from which the fragrance of Streptomycete material flowed. The researchers showed that geosmin and 2-MIB form the attractants by using streptomycete mutants, which both substances can no longer produce. These versions left the tiny experimental animals on the left and opted for the “normal”, apparently seductively scented Streptomycete samples. The experiments also clarified that springtails value streptomycete material as a source of food: they greedily took in the sample material.
Subsequent electrophysiological studies on the antennae of the springtails provided further evidence of the importance of geosmin and 2-MIB as attractants: When the two fragrances were in the air, the researchers found characteristic reactions in the antennae of the animals. According to the results, they have a very fine nose for these substances, reports Becher and his colleagues.
Springtails transport spores
Next, the researchers devoted themselves to studying the genetic makeup of geosmin and 2-MIB in the streptomycetes. They were able to show that the activity of these genes goes hand in hand with the formation of spores in the microbes. That means: they exude the fragrance when they form these resilient forms of persistence that serve their own distribution. In keeping with this, the scientists were able to show that springtails carry the spores on their bodies and also release them unharmed via their excretions after they have eaten streptomycete material.
Becher and his colleagues thus come to the conclusion: What we perceive as the typical earthy smell is an attractant that Streptomycetes use to get “carpooling opportunities”. Springtails are extremely good for this role because they get around a lot. As their name suggests, they can carry themselves far through the air with a small jump fork on the rear and are also carried away by the wind. This allows them to open up new habitats – and the same applies to their microscopic passengers – the streptomycetes.
Source: Nature, doi: 10.1038 / s41564-020-0697-x