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New research from an international team of scientists suggests that the instantly recognizable earthy odor after rain is released by bacteria trying to attract a particular arthropod as a way to spread its spores.
Smell is an example of a 500-million-year-old chemical communication that evolved to help a particular type of bacteria spread.
Scientists have long been fascinated by the unique smell that appears when it rains.
The smell is particularly evident when the first rains of a season hit dry ground. Two Australian researchers named the scent petrichor, after an influential study in 1960 suggested that certain plants produce a particular oil during dry spells, and that it is then released into the air when it rains.
One of the main components of petrichor is an organic compound called geosmin. Scientists have long known that a common genus of bacteria, known as Streptomyces, produces geosmin. Virtually all Streptomyces species release geosmin when they die, but exactly why the bacteria generate this distinctive scent has not been clear so far.
The fact that they all produce geosmin suggests that it confers a selective advantage on the bacteria, otherwise they would not. So we suspect they were pointing at something and the most obvious would be some animal or insect that could help distribute the Streptomyces spores.
Mark Buttner, author of the new research.
Through a series of laboratory and field experiments, the researchers discovered that geosmin specifically attracts a type of tiny arthropod called springtail.
By studying the antennae of springtails, the researchers found that organisms can directly sense geosmin. The researchers suggest that the two organisms evolved together, Streptomyces serving as food for springtails, while springtails later shed bacterial spores helping to seed new colonies of Streptomyces.
There is a mutual benefit. Collembola eat Streptomyces, so geosmin attracts them to a valuable food source. And, springtails distribute spores, both stuck on their bodies and in their feces, which are full of viable spores, so the Streptomyces disperse. This is analogous to birds that eat the fruits of plants. They get food but they also distribute the seeds, which benefits the plants.
This symbiotic relationship is key to the survival of Streptomyces, as the bacterium is known to produce certain antibiotic compounds that make it toxic to other organisms such as the fruit fly or nematodes. Collembola, on the other hand, generate a series of novel enzymes that can detoxify the antibiotics produced by Streptomyces.
This new discovery suggests that one of the main elements of that iconic moist earth smell is supported by a nearly 500 million year relationship between bacteria and arthropods, mediated by an extraordinarily specific mode of chemical communication.
More information: www.nature.com