The smell of rain sounds like something that lives in the heart rather than the nose. It is real, though — and it is not one single scent. Petrichor, the earthy aroma released when rain reaches dry ground, includes compounds associated with soil and plant oils. One of its most recognizable notes is geosmin, an extraordinarily potent microbial terpenoid. Ozone can contribute a sharper smell around thunderstorms, but that is a separate chemical phenomenon.
Geosmin takes its name from the Greek for earth (“geo”) and odor (“osme”), and the earth connection is literal. The same compound helps give beets their soil-sweet depth, puts the muddy note in freshwater fish, and, together with 2-methylisoborneol, causes many of the earthy and musty taste-and-odor complaints involving drinking water. Perfumers prize it; brewers, water utilities, and fish farmers spend real money getting rid of it.
An Earthy Discovery
Geosmin was first isolated in 1965 by Nancy N. Gerber and Hubert A. Lechevalier at Rutgers University, from Streptomyces, a genus of soil bacteria.[1] That origin explains the rain connection: raindrops striking porous ground can generate tiny aerosols that carry material — including scent compounds — up from the soil surface.[2] The rain, in other words, is mostly the messenger. Decades later, chemists traced the production to a single bifunctional bacterial enzyme. Geosmin is classified as a degraded sesquiterpenoid: the enzyme begins with the 15-carbon precursor farnesyl diphosphate, cyclizes it, and then performs a fragmentation and rearrangement that produces the 12-carbon bicyclic alcohol.[3] Unlike the familiar plant terpenes associated with cannabis, geosmin is best known as a microbial terpenoid.
Why Streptomyces Makes It
Bacteria rarely spend energy without a reason, and the exact job geosmin does for every organism that makes it is still an open question. For Streptomyces, though, a 2020 study identified one answer: springtails, tiny soil arthropods that sense geosmin — along with 2-methylisoborneol — with their antennae and follow it to the colony. The genes for geosmin production are switched on by the same regulators that control sporulation, so the scent broadcasts precisely when there are spores to carry.[4] The springtails come to feast, and the spores travel onward in and on their bodies. At least in this Streptomyces–springtail system, the bacteria appear to pay for transportation with their own biomass. It is the same bargain plants strike when they hire insects with scent — nerolidol calls in predatory mites, and farnesene warns aphids off.
One Molecule, Three Verdicts
Humans are strikingly good at detecting geosmin — the nose picks it up at less than five parts per trillion, a few nanograms in a liter of water.[5] Whatever tuned us so finely, we are not the only ones listening, and different species hear entirely different messages in the same molecule.
Fruit flies treat it as a fire alarm. A 2012 study found that Drosophila devotes a dedicated olfactory channel — one receptor, Or56a, wired to a single glomerulus — to geosmin and nothing else, and the signal overrides even the pull of vinegar. The flies will not feed or lay eggs where geosmin lingers, with good reason: females tricked into laying eggs on a Streptomyces mutant that could not make geosmin lost their larvae to the microbes. For the fly, geosmin means dangerous microbes.[6]
Female Aedes aegypti mosquitoes read the same signal and reach the opposite verdict. To them, geosmin smells like microbe-rich water, exactly what their larvae eat, so it draws them to egg-laying sites. The researchers found a practical twist: beetroot peel extract, rich in the earthy stuff, can stand in for costly purified geosmin as a cheap lure for mosquito traps and surveillance.[7]
One microbial odor, three verdicts: dinner bell for the springtail, danger sign for the fly, nursery sign for the mosquito — and, for us, the smell of rain. Nature rarely wastes a good molecule. For the family of molecules geosmin is cut from, start with our terpene education hub.
References
[1] Gerber NN, Lechevalier HA. Geosmin, an earthy-smelling substance isolated from actinomycetes. Appl Microbiol. 1965;13(6):935-938. https://pubmed.ncbi.nlm.nih.gov/5866039/ [2] Joung YS, Buie CR. Aerosol generation by raindrop impact on soil. Nat Commun. 2015;6:6083. doi:10.1038/ncomms7083 [3] Jiang J, He X, Cane DE. Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme. Nat Chem Biol. 2007;3(11):711-715. [4] Becher PG, Verschut V, Bibb MJ, et al. Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal. Nat Microbiol. 2020;5(6):821-829. doi:10.1038/s41564-020-0697-x [5] Garbeva P, Avalos M, Ulanova D, van Wezel GP, Dickschat JS. Volatile sensation: the chemical ecology of the earthy odorant geosmin. Environ Microbiol. 2023;25(9):1565-1574. doi:10.1111/1462-2920.16381 [6] Stensmyr MC, Dweck HKM, Farhan A, et al. A conserved dedicated olfactory circuit for detecting harmful microbes in Drosophila. Cell. 2012;151(6):1345-1357. doi:10.1016/j.cell.2012.09.046 [7] Melo N, Wolff GH, Costa-da-Silva AL, et al. Geosmin attracts Aedes aegypti mosquitoes to oviposition sites. Curr Biol. 2020;30(1):127-134.e5. doi:10.1016/j.cub.2019.11.002Last updated October 2026 · Originally published July 8, 2022.
