Soil smell is bacterial lure

The delightful smell of soil is caused by small molecules like geosmin produced by the bacteria living there. In fact, every species of Streptomyces bacteria makes geosmin—but it’s not clear why they go to the trouble. To work this out, biologist Klas Flärdh at Lund University teamed up with chemical ecologist Paul G. Becher from the Swedish University of Agricultural Sciences. Becher’s team went into the Swedish woods with traps and found that tiny invertebrates called springtails, or collembolans, that live in the soil were lured into the traps containing Streptomyces coelicolor. Back at the lab, the team strapped electrodes onto the antennae of springtails and measured how their electrical activity correlated with the presence of odor samples from S. coelicolor, which they analyzed with gas chromatography. Peaks in the GC relating to geosmin and a similarly earthy terpene called 2-methylisoborneol corresponded to antennae activity. Then, the researchers looked at when in its life cycle the bacterium makes geosmin. Streptomyces species produce most of their geosmin when they’re making spores (Nat. Microbiol.2020, DOI: 10.1038/s41564-020-0697-x). Springtails attracted by geosmin eat some of the bacteria but also spread the spores that are stuck to their bodies or are contained in their feces.