timeThere are few more reliable harbingers of spring than Petriccio. This is the earthy aroma that lingers after a heavy rain, and it comes primarily from compounds called terpenes. For example, geosmin is a terpene most commonly associated with: Streptomyces Bacteria, although other bacteria and fungi also produce it, and it is found in soil and freshwater bodies around the world. Its ubiquity has long fascinated scientists — not least because it’s unclear why its various manufacturers make it.
Liana Zaroubi, a doctoral student at Simon Fraser University in British Columbia, Canada, first learned about the mystery in 2015 while an undergraduate at Concordia University in Montreal. Take the course and recall your interest in additional questions. She eventually joined his lab as a master’s student so she could study the problem. “I think it’s very interesting,” she said.
What is geosmin and its effects?
After months of reviewing the literature and testing hypothesis after hypothesis, she and Findlay considered geosmin and another terpene that contributes to lithosmin, 2-methylisoborneol (2-MIB). , whether it can indirectly deter predators. Although the chemicals themselves are not toxic to animals, other compounds produced by the bacteria are, so geosmin and 2-MIB may be a warning signal, much like the coloration of many poisonous insects tells hungry birds to feed elsewhere.
The first round of experiments with bacteria-eating amoeba went poorly, Zalubi said. She explains that these organisms are very slow predators, and geosmin is very unstable. In the researchers’ experimental setup, it took weeks for the amoeba to come into contact with the bacteria, but geosmin degraded within days or even hours. “So we thought about faster predators, such as nematodes.”
First, the researchers tested whether Nematodes Will react to the presence of geosmin. They found that while the chemical did not appear to affect the nematodes’ health, it greatly affected their movement, causing them to move faster and change direction more frequently. Mutant worms defective in detecting soluble and volatile odorants did not show this behavioral change, suggesting that wild-type animals were smelling or tasting the compound.
Next, the researchers plopped Nematodes and Streptomyces coelicolora bacterium that produces geosmin and 2-MIB, was placed in a petri dish. The team found that, in general, the worms avoided the bacteria. But if the researchers engineered the bacteria so that they didn’t produce the chemicals, or so that the worms couldn’t detect the chemicals, the nematodes consumed the bacteria more frequently and became sick from the toxic metabolites produced by the microbes. “Geosmin thus functions as a warning signal, honestly and reliably advertising the unpalatable nature of its producer and providing mutual benefits to predators and prey,” the authors write in their paper.
While the chemical didn’t appear to affect the nematodes’ health, it greatly affected their movement.
Microbial warning signs
Findlay said this is the first time a control signal has been recorded in bacteria. He adds that it’s not surprising that geosmin and 2-MIB produce good alert signals: These compounds are composed of hydrocarbons arranged in rings or chains, making them ideal for cell receptors. But because they degrade so quickly, they cannot accumulate or travel very far in the environment, meaning they can reveal exactly what organisms produced them here and now. “As a chemical messenger, this allows [them] Very, very valuable,” Findlay said.
This study is just one of a few recent papers identifying the possible functions of geosmin and 2-MIB. For example, research by scientists at the Swedish University of Agricultural Sciences shows that these two chemicals actually attract springtails, which feed on the bacteria that produce them without suffering the ill effects of the toxins. In turn, the springtails disperse bacterial spores in their fecal pellets and carry them on their bodies, helping the microorganisms migrate to new environments.
Some flies have also found ways to interpret petrichor scent signals. In 2012, while working at the Max Planck Institute for Chemical Ecology in Germany, sensory neuroecologist Marcus Stensmyr published a paper showing that fruit flies were repelled by geosmin even when it was added to vinegar. “Flies absolutely love vinegar,” says Stensmire, now at Lund University in Sweden. “Anything that can reduce attractiveness must be important.” He said the team showed that this disgust is controlled by a single receptor that modulates specifically for geosmin and is able to detect concentrations as low as One part per billion of chemical substance.
It’s unclear why flies don’t like geosmin. Stensmire said fly larvae may be sensitive to toxins produced by various geosmin producers. This may also be related to competition for food. Some molds, e.g. PenicilliumPlants that produce geosmin eat the yeast that grows on rotting fruit. Since fruit fly larvae also eat yeast, the presence of mold indicated by geosmin means that larvae that land on a particular fruit may starve.
Stensmyr’s follow-up study found that women Aedes aegypti Mosquitoes have very similar geosmin-specific receptors, but their responses are completely different. “They love it,” he said. This makes sense because mosquitoes are not susceptible to toxins produced by bacteria, and in fact, mosquito larvae eat the bacteria that produce geosmin. Stensmire noted that in his study, female mosquitoes prioritized egg-laying sites where geosmin was present. “If you just look at mosquitoes and flies, which are not too distantly related, this compound seems to be very important,” he said. “But it has a different meaning.”
It’s likely that a large number of animals can detect geosmin, Stensmeier said. Even humans are highly sensitive to it and can smell geosmin in concentrations as low as 400 parts per trillion. “We have the nematode example, we have the insect example, we have the human example; we have a whole phyla of animals in between that may also respond to this chemical or may respond in this way or that way. In fact, some animals’ responses to the compound appear to have nothing to do with bacteria, and research in the 1990s suggested that geosmin might help European glass eels find freshwater, and Stensmire speculated that human ancestors might have used it as well. Low but not high concentrations of the chemical appear to inhibit the stinging behavior of bees.
Geosmin may hold more secrets. For example, Zarrubi noted that the fungal strains that produce the chemical do not appear to use the same genetic pathways as bacteria to make the chemical, meaning geosmin production may have evolved independently multiple times. Findlay added that the study could help scientists look at vigilance behavior in a new way: from the perspective of a predator, not just the prey signaling “don’t eat me.” Warning signals depend on “the sender and receiver of the signal,” he said. “In our case, we have complete control over the genetics of these nematodes. So we can look at this evolution from both sides, from multiple angles. I’m very excited about that.
FAQ
What is the smell of rain called?
- Petrichor is the earthy scent after the rain. It comes primarily from compounds called terpenes.
What is the difference between geosmin and petrichor?
- Geosmin is a terpene produced by bacteria (mainly Streptomyces) and fungi found in soil and freshwater bodies. Geosmin and other terpenes, such as 2-methylisoborneol (2-MIB), are soluble volatile odorants that produce the smell of rain.
Why do microorganisms produce geosmin?
- Although geosmin is not toxic, researchers have observed that it prevents predators such as nematodes or flies from eating geosmin-producing microorganisms and producing toxins. This is called a warning signal, similar to the bright colors of poisonous insects and frogs that signal to predators.
This article was originally published on August 1, 2022. Deanna McNeil,PhD.
