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Soil gets its characteristic earthy smell from certain chemicals produced primarily by soil-dwelling bacteria called Streptomyces. But until now, we didn’t know why these bacteria produce these odours, and what role they play in the soil ecosystem.
To find out more, Paul Becher at the Swedish University of Agricultural Sciences and his colleagues set up field traps in woodland containing colonies of Streptomyces. They thought that the smell may act as a signal to other organisms that they are poisonous, because some bacteria like Streptomyces can be toxic. “They’re a kind of warning signal,” says Becher.
Instead, the smell – which comes from gases, including geosmin and 2-methylisoborneol (2-MIB), released by Streptomyces – seems to attract invertebrates that help the bacteria disperse their spores.
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Becher and his team found that springtails – tiny cousins of insects – that feed on Streptomyces were drawn to the traps containing the bacterial colonies, but they weren’t drawn to control traps that didn’t contain Streptomyces. By comparison, insects and arachnids weren’t attracted to the traps.
To see whether the springtails were being lured by the chemicals, the researchers attached electrodes to the springtails’ antennae in the lab, then exposed these springtails to geosmin and 2-MIB.
The researchers detected a spike of electrical activity in the springtails’ brain, but they didn’t show any electrical responses to other test compounds, which suggests the animals were responding to the chemicals.
Becher and his team found that, while in the lab, Streptomyces make more geosmin and 2-MIB when they form spores. When the springtails approach and eat the bacteria, the spores either stick to the springtails’ bodies or are contained in their faecal pellets and dispersed through the soil.
Springtails are probably unaffected by Streptomyces toxins because they live underground where they are already exposed to other bacteria, so have evolved detoxifying machinery, says Becher.
“For me, [this] makes so much sense,” says Marie Elliot at McMaster University in Canada. “Streptomyces don’t have any direct way of moving these spores to different locations. Attracting creatures like springtails would provide a great way.”
Journal reference: Nature Microbiology, DOI: 10.1038/s41564-020-0697-x
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