The earthy scent that rises after a rain shower, known as petrichor, is a familiar experience. The primary molecule responsible for this smell is geosmin, a name derived from the Greek words for “earth” and “smell”. This organic compound is not a product of the soil or rain itself but is produced by specific types of bacteria living within the soil.
The Source of the Earthy Scent
The primary producers of geosmin are soil-dwelling bacteria from the genus Streptomyces. These microbes are abundant in soil ecosystems worldwide, where they function as decomposers, breaking down dead organic matter and releasing nutrients. Streptomyces are also a significant source of antibiotics used in modern medicine.
The production of geosmin serves a specific ecological purpose for the bacteria. Streptomyces bacteria produce geosmin only when they are forming spores for reproduction. The earthy scent acts as a chemical signal, attracting tiny, insect-like organisms called springtails (Collembola) that live in the soil and consume the bacteria.
Attracted by the scent, springtails consume the Streptomyces bacteria. They have developed a resistance to the antibiotics that the bacteria produce, which can be harmful to other soil organisms like nematodes. As the springtails move and feed, they help disperse the bacterial spores, which stick to their bodies and are also spread through their fecal matter. This symbiotic relationship allows the bacteria to spread over greater distances. While other microorganisms like cyanobacteria also produce geosmin, Streptomyces are the main source of the earthy aroma.
From Soil to Scent
The release of geosmin from the soil into the air creates the phenomenon of petrichor. When rain falls on dry, porous soils, it traps small pockets of air. These bubbles, containing volatile compounds like geosmin, travel upwards through the falling raindrop and burst upon reaching the surface.
This bursting action releases a fine mist of particles, known as aerosols, into the atmosphere. These aerosols act as carriers for the geosmin molecules, lifting them from the ground to be transported by the wind. This is why the earthy smell is often most potent at the beginning of a rain shower after a dry spell.
Humans are exceptionally sensitive to the smell of geosmin, capable of detecting it at concentrations as low as one hundred parts per trillion. This sensitivity is greater than a shark’s ability to detect blood in the water. Some scientists theorize this heightened sense may be an evolutionary trait, linking the smell of rain with the life-sustaining arrival of water, signaling the end of a drought and the promise of fresh plant growth.
Geosmin’s Effect on Taste
Geosmin’s influence extends beyond smell, impacting the flavor of water and certain foods. When geosmin-producing bacteria are present in reservoirs or other water sources, they can impart a “musty” or “dirty” taste to drinking water. This is a frequent issue for municipal water treatment facilities, which work to remove the compound. Though the taste can be unpleasant, geosmin itself is not harmful to human health.
Geosmin can also accumulate in the tissues of aquatic organisms, causing an “off-flavor” in fish. Species like catfish and trout can absorb the compound, resulting in a muddy taste that makes them unpalatable. This can have economic consequences for the aquaculture industry.
Conversely, the presence of geosmin is a defining characteristic of some vegetables. The distinct earthy flavor of beets, for instance, is directly attributable to this compound. It can also be found in other produce like carrots and mushrooms, where it contributes to their natural flavor profile. In these instances, the earthy taste is often considered a desirable trait, unlike its effect on water or fish.