Why do we smell earthy odor after first rain?

After the first rain, we smell very unique and pleasant odor; everybody likes it and no scent manufacturing company has ever been able to synthesise it. This earthy odor is known as Petrichor (in Greek) or as Geosmin (not Jasmine) in science. What could be the reason or what exists in soil that produces such characteristic odor? Many poems have been written on Petrichor, for a while leave these concepts behind and let's know the actual reason. You will be astonished to such unusual reason! The scientific explanation has been discussed here which is responsible for the production of earthy smell.

Gram positive Eubacteria, Streptomyces and Actinomyces which also represent normal flora of the soil are the causative agents of Petrichor/Geosmin odor. They are filamentous bacteria, have fungus like appearance and are spore formers. The well known species that produce earth odor are Streptomyces coelicolor, S. albicans, S. griseus, S. lividans, S. avermitilis, S. plicatosporus and Actinomyces israelii. They are saprophytic group of bacteria but some species S. somaliensis, S. sudanensis are pathogenic to humans and cause mycetoma; S. scabies and S. caviscabies are plant pathogenic. They possess large genome as compared to other Eubacteria and have potential to metabolize aromatic compounds, amino acids, sugars or alcohols and also are the producers of antibiotics like griseofulvin, streptomycin, actinomycin, nystatin, neomycin, amphotericin and bleomycin. In addition to the production of secondary metabolites like antibiotics, they have immense biotechnological potential. Both Actinomyces and Streptomyces belong to group Actinobacteria and the family Actinomycetes.

Actinomyces and Streptomyces are adapted to dry, desiccation or damp weather conditions. During such stress, they readily form spores which are stress resistant and survive desiccation or extreme heat. The Geosmin is chemically Dimethyl-9-decalol. It is contained in spore coat of these soil bacteria. When rain drops hit the ground, the soil containing spores are kicked up into the air (Imagine the spore as a ball tossed into the air!). The spores are microscopic, circular and lightweight than soil particles. They remain suspended as a soil-water aerosol. When we inhale the aerosol so formed, we smell of Geosmin present in the spores. This exactly works like our room fresheners!

The one query still remains to explain. Why is not Petrichor last long in whole rainy season, and why only after first rain? The cause is that these bacteria do not form spores in moist soil which is formed after the rain. They revert back to their filamentous vegetative form. They are present as spores in the soil only before the start of rain; during this time the weather is already dry or warm and damp. So no spores in wet conditions and therefore no Geosmin and not that peculiar Petrichor! The most important thing is that these bacteria are found in all types of soils and all over the world and Petrichor is universal. So this great odor is available for every Homo sapiens to smell, moreover Actino and Streptomyces are not discriminating!

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