June was, as I am sure everyone remembers, a very dry month here in Western Pennsylvania, and the weeks of heat and drought continued for many of us until the middle of July. Our lawns stopped growing and turned brown, and our gardens began to fade and wither. Some trees began to lose leaves, and a great deal of the partially formed fruit on the apple and pear trees tumbled down to the ground. On July 18, though, it rained, and then it continued to rain off and on for the rest of the month (hard on bike riding but much appreciated by all of our ecosystems!).
On the afternoon of the first rain, Deborah and I stepped out on our deck and enjoyed the incredible scent of rain on dry soil. This aroma is called “petrichor”, and it comes from a complex array of organic chemicals that have been secreted by plants and soil bacteria into the dry soil. The rain stirs up and aerosolizes these chemicals and generates the aesthetically satisfying atmospheric. It is a smell of a cleansing power, a smell of renewal and change. It is a smell of life.
Petrichor was first defined in the scientific literature by two Australian researchers, I. J. Bear and R. G. Thomas. They later demonstrated that as these complex chemicals build up in dry soils they act to inhibit the germination of seeds. When the rains come and the petrichor chemicals are released from the soil this chemical check on seed germination is released. The aroma we smell is the signal for long suppressed plants to begin the growth phase of their life cycles.
Another chemical that plays a role in the “after the rain” scent is geosmin. Geosmin (also known as dimethyl-9-decalol) is the familiar “earth” smell that we perceive during spring soil thaws or after plowing or tilling. Geosmin is synthesized by several types of soil bacteria, but it is especially the product of a genus of actinobacteria called Streptomyces. Streptomyces are very important in breaking down complex and resistant chemicals in the soil and play vital roles in biological decomposition and in the recycling of nutrients in our ecosystems. Streptomyces also synthesize an incredible array of diverse chemicals that collectively are referred to as “secondary metabolites.” Secondary metabolites often play complex roles in the chemical ecology of an ecosystem, and it is thought that some of the Streptomyces secondary metabolites are extremely important mediators in symbiotic relationships among soil organisms. Some of these secondary metabolites act as chemical defense agents against other bacteria, and a number of these Streptomyces “antibiotics” (including streptomycin, actinomycin, and neomycin) were isolated and identified by Selman Waksman and his students in the 1940’s. These discoveries led to Waksman’s Nobel Prize in 1952.
Geosmin is concentrated in the spore covers of the Streptomyces bacteria. Human olfactory systems are incredibly sensitive to geosmin and can detect concentrations as low as five parts per trillion in the air. Further, other organisms are also quite sensitive to geosmin. Earthworms and many other soil dwelling invertebrates are attracted to loci of concentrated geosmin. This attraction may act to recruit soil biota into areas of intense biological decomposition, and, thus, further accelerate and facilitate this vital biogeochemical process. The recruited soil invertebrates may also act to transport and disperse the relatively immobile Streptomyces throughout the soil volume thus increasing the presence and impact of these important bacteria.
Talking about scent of the air after a rain with Javier (Dr. Gomez) and Maria (Sra. Franco de Gomez) led to an unexpected connection. The area around the city of Guadalajara, Mexico (Javier’s hometown) is known for its “after a rain” scent. This observation is even noted in a popular song about Guadalajara. A possible explanation for this may involve the distinct, seasonal rainfall patterns of this area of Mexico and the abundance of clay minerals in the soils of this area. Almost ninety percent of the yearly rain falls during four months of the year (June through September). In the other eight months of the year the rain comes in “odd, quick drizzles” (to quote from a local blog posting). It is possible that these very occasional drizzles would aerosolize the accumulated petrichor and geosmin from the area’s clay-rich soils and rocks and set up a persisting aroma of rain on dry earth. This lingering scent sounds to me like a great reason to visit Guadalajara (along with all of the museums, the great food, the mariachi music, and the futbol!).