TFew herald the arrival of spring as surely as Petrichor. The earthy smell that wafts in after a rain comes primarily from compounds called terpenes. For example, geosmin is the most commonly associated terpene. Streptomyces Other bacteria and fungi also produce it, and it is found in soils and freshwater bodies around the world. Its ubiquity has long fascinated scientists, not least because it was unclear why different producers make it.
Liana Zarroubi, a doctoral candidate at Simon Fraser University in British Columbia, Canada, first encountered this mystery in 2015 when she was an undergraduate at Concordia University in Montreal. She remembers taking Brandon Findlay’s class on chemical ecology and being intrigued by the extra questions. In his final exam, he asked, “What do you think geosmin does and how do you test it?” She decided to join his lab as a master’s student to investigate that question. “I thought it was really interesting,” she says.
What is geosmin and what does it do?
After months of reviewing the literature and testing hypothesis after hypothesis, she and Findlay discovered that 2-methylisoborneol (2-MIB), another terpene responsible for geosmin and petrichol, indirectly We looked at whether we could deter predators. Although these chemicals themselves are not toxic to animals, other compounds produced by bacteria are, so geosmin and 2-MIB can induce hungry birds to eat elsewhere. Like the colors of many poisonous insects, it can be an apocalyptic signal.
Zarroubi said initial experiments with bacteria-eating amoebas were unsuccessful. She explains that this creature is a very slow predator and that geosmin is very unstable. Unlike the amoeba, which takes weeks to reach the bacteria in a researcher’s lab equipment, geosmin breaks down within days or even hours. “So we thought of faster predators, like nematodes.”
First, the researchers tested: C.Elegance It responds to the presence of geosmin. The researchers found that while the chemical did not appear to affect the worms’ health, it had a dramatic effect on the worms’ movement, causing them to move faster and more frequently. It turned out that it started to change direction. Mutant worms defective in detecting soluble and volatile odorants showed no such behavioral changes, suggesting that wild-type animals were sniffing and tasting the compounds. There is.
Next, the researchers said: C.Elegance and Streptomyces coelicolorinject bacteria that produce both geosmin and 2-MIB into a Petri dish. Overall, the researchers found that the worms avoided the bacteria. But when researchers engineer bacteria so that they don’t produce the chemicals, or the worms can’t detect those chemicals, the worms consume the bacteria more frequently, and the microbes similarly The toxic metabolites produced can make you sick. “Geosmin thus acts as an apocalypse, honestly and reliably advertising its producer’s unpalatability, with mutual benefits for predator and prey,” the authors write in the paper.
Although the chemical did not seem to affect the worms’ health, it had a dramatic effect on their movement.
Microbial aposematic signaling
Findlay said this is the first time aposematic signaling has been documented in bacteria. He added that it’s not surprising that geosmin and 2-MIB give good apose signals. These compounds are composed of hydrocarbons arranged in rings or chains and are highly compatible with cellular receptors. However, they degrade so quickly that they cannot accumulate in the environment or travel far. That means we can now reveal exactly the organisms that produce them. “As a chemical messenger, it [them] It’s very, very valuable,” Findlay says.
This study is just one of a few recent papers identifying possible functions for geosmin and 2-MIB. For example, a study by scientists at the Swedish Agricultural University demonstrated that the two chemicals actually attract springtails, which feed on the bacteria that produce them, and that the toxins have no negative effects. The Collembola then spreads bacterial spores in its feces and carries them onto its body, helping the microorganisms move to the new environment.
Some fly species have discovered ways to interpret petrichor’s odor signals. While working at the Max Planck Institute for Chemical Ecology in Germany in 2012, sensory neuroecologist Marcus Stensmill published the following paper: paper This shows that Drosophila melanogaster is repelled by geosmin even when geosmin is added to vinegar. “Flies love vinegar,” says Stensmill, now at Lund University in Sweden. “Anything that can make you less attractive has to be important.” This aversion is specifically tuned to geosmin and can detect concentrations of the chemical as low as 1 in 1 billion. The research team showed that it is controlled by a single receptor.
It is not clear why flies do not prefer geosmin. Fly larvae may be susceptible to toxins produced by various geosmin manufacturers, Stensmill suggests. Competition for food may also be involved. Some molds etc. Penicilliumwhich produce geosmin by feeding on yeast that grows on rotting fruit. Drosophila larvae also feed on yeast, so the presence of mold, signaled by geosmin, means that larvae laid on certain fruits may starve to death.
follow up the study Found a female in Stens Mill Aedes aegypti Mosquitoes with very similar geosmin-specific receptors respond quite differently. “They loved it,” he says. This makes sense given that mosquitoes are insensitive to toxins produced by bacteria and, in fact, mosquito larvae feed on the bacteria that produce geosmin. Stensmar points out that in his study, female mosquitoes preferred spawning sites where geosmin was present. “If you just look at mosquitoes and flies, which are not so distantly related, this compound seems very important,” he says. “But they have different meanings.”
Stensmill says it’s likely geosmin can be detected in a huge number of animals. Even humans are highly sensitive to geosmin and can smell it even at the lowest concentrations. 1/400 trillionth. “We have examples of nematodes, we have infections from insects, we have infections from humans. In between, we have every animal phylum that can react to this chemical, and in some way, we have it.” In fact, some animals react to compounds in ways that seem completely unrelated to bacteria. the study Research in the 1990s suggested that geosmin may help European glass eels find fresh water, and Stensmill speculates that this function may have been used by our ancestors as well. I am doing it. Concentrations of chemicals are low, but not high appear Suppresses the stinging behavior of bees.
Geosmin may have more secrets. For example, Professor Zarroubi points out that the fungal strains that produce this chemical do not seem to use the same genetic pathways as bacteria to produce it, and that geosmin production has probably evolved multiple times independently. It is pointed out that it means Findlay added that this study will help scientists look at apothematism in a new way, from the perspective of the predator, not just the prey that sends out a “don’t eat” signal. Apothematic signals depend on “both the sender and the receiver of the signal,” he says. “In our case, we have pretty complete control over the genetics of these worms. So we can investigate evolution from both sides and from multiple angles. We’re very excited about that.” Masu.”
FAQ
What is the smell of rain called?
- Petrichor is the smell of earth after the rain. It is primarily derived from compounds called terpenes.
What is the difference between Geosmin and Petrichor?
- Geosmin is a terpene produced by bacteria (mainly bacteria). Streptomyces) Fungi found in soil and freshwater bodies. Other terpenes, such as geosmin and 2-methylisoborneol (2-MIB), are soluble and volatile odorants responsible for the smell of rain, petrichor.
Why do microorganisms produce geosmin?
- Although geosmin is not toxic, researchers have observed that geosmin may deter predators such as nematodes and flies from ingesting geosmin-producing microorganisms that make the toxin. This is called implicit signaling, similar to how the bright colors of poisonous insects and frogs act as warning signals to predators.
This article was originally published on August 1, 2022. Updated on January 18, 2025. Deanna McNeillPh.D.
