teaThere are few surer harbingers of spring here than Petrico. The earthy scent you smell after a heavy rain comes mainly from chemical compounds called terpenes. For example, Geosmin is a terpene most commonly associated with: strep throat Bacteria are found in soil and fresh water around the world, although they also give rise to other bacteria and fungi. Its ubiquity has long fascinated scientists. Especially since it wasn’t clear why the various producers were making them.
Liana Zaroubi, a doctoral student at Simon Fraser University in British Columbia, Canada, first approached this mystery in 2015 when she was an undergraduate at Concordia University in Montreal. She recalls taking a chemical ecology class with Brandon Findlay and being intrigued by the bonus questions. In his final exam he asked, “What do you think geosmin does, and how would you test it?” She joined 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 hypotheses, she and Findlay determined whether 2-methyl-isoborneol (2-MIB), another terpene that contributes to geosmin and petrichor, could indirectly deter predators. I have considered whether or not. These chemicals themselves are not toxic to animals, but other compounds the bacteria make are, so geosmin and 2-MIB may be fatty signals, like the coloration of many toxic insects, that tell hungry birds to eat elsewhere.
Early experiments with bacteria-eating amoebas did not go well, Zaroubi says. The organism is a very slow predator and geosmin is very unstable, she explains. While the amoeba takes weeks to reach the bacteria in the researchers’ experimental setup, geosmin breaks down within days or even hours. “So we thought about faster predators, like nematodes.”
First, the researchers tested whether: C. elegans Responds to the presence of geosmin. They found that although the chemicals did not seem to affect the health of the nematodes, they had a drastic effect on their movements, causing them to move much faster and change direction more often. Mutant worms defective in detecting soluble and volatile odorants showed no such behavioral changes, suggesting that wild-type animals were smelling or tasting the compounds.
Next, the researchers C. elegans and Streptomyces coelicolorAdd bacteria that produce both geosmin and 2-MIB to a Petri dish. Overall, the bugs avoid bacteria, the team found. But when researchers manipulate the bacteria so they don’t produce the chemicals, or the worms so they can’t sense these chemicals, the nematodes consume the bacteria more often and become sick from the toxic metabolites the microbes produce. “Geosmin serves as an aposematic signal that honestly and reliably advertises the unpalatability of producers and provides mutual benefit to predators and prey,” the authors wrote in the paper.
Although the chemicals did not appear to affect the health of the nematodes, they had a significant impact on their movement.
Microbial lipolysis signal
This is the first time fatty signaling has been documented in bacteria, Findlay said. He added that it is not surprising that geosmin and 2-MIB should produce good fat signals. These compounds, which are made up of hydrocarbons arranged in rings or chains, fit very well into cell receptors. But they also break down very quickly, so they cannot accumulate in the environment or travel far. That means revealing precisely the organisms that create them, right here and now. “As a chemical messenger it [them] It’s very, very worthwhile,” says Findlay.
This study is just one of a handful of recent papers that have identified possible functions for geosmin and 2-MIB. For example, a study by scientists at the Swedish University of Agricultural Sciences found that two chemicals actually attract springtails to eat the bacteria that produce them, without the side effects of the toxins. Springtails then spread bacterial spores in their feces, carrying them on their bodies and helping the microorganisms move to new environments.
Some fly species have also found ways to interpret the Petricot’s odorous signals. In 2012, while working at the Max Planck Institute for Chemical Ecology in Germany, sensory neurophysiologist Marcus Stensmyr published the following paper. paper Even adding geosmin to vinegar has been shown to repel fruit flies. “Flies really love vinegar,” says Stensmyr, who is now at Lund University in Sweden. “Anything that can make it less attractive is going to be important.” The team showed that this aversion is governed by a single receptor that is specifically tuned to geosmin and can detect the chemical at concentrations as low as 1 part per billion.
It’s unclear why flies don’t like geosmin. Fly larvae are likely sensitive to the toxins produced by various geosmin manufacturers, Stensmyr suggests. It may also have to do with competition for food. Some molds as follows penicilliumThose that produce geosmin feed on yeast growing on rotting fruit. Because fruit fly larvae also feed on yeast, the presence of fungi indicated by geosmin means that larvae laid on certain pieces of fruit may starve to death.
follow up research I found the woman in Stensmyr Aedes aegypti mosquito Mosquitoes, which have very similar geosmin-specific receptors, respond completely differently. “They loved it,” he says. This makes sense, considering that mosquitoes are insensitive to the toxins produced by bacteria, and that mosquito larvae actually eat the bacteria that produce geosmin. Stensmyr notes that in his study, female mosquitoes prioritized oviposition sites where geosmin was present. “If we look only at mosquitoes and flies, which are not so distantly related, this compound seems to be very important,” he says. “But it has a different meaning.”
Stensmyr says it’s likely that a huge number of animals can detect geosmin. Even humans are very sensitive to geosmin. 400 parts per set. “We have examples in nematodes, we have examples in insects, we have examples in humans. In between, there are many different animal phyla that can react to these chemicals or use them in some way.” In fact, some animals react to compounds in ways that seem completely unrelated to bacteria. research Since the 1990s, Geosmin has speculated that a feature that Stensmyr suspects may have helped European glass eels find fresh water may also have been used by human ancestors. and low, but not high, concentrations of chemicals. appear It inhibits the stinging behavior of bees.
Geosmin may have more secrets yet. For example, Zaroubi points out that fungal strains that produce the chemical do not appear to use the same genetic pathways as bacteria, possibly meaning that geosmin production evolved independently multiple times. Findlay added that this research could help scientists look at aposematism in a new way, from the perspective of a predator rather than a prey, which generates a don’t-eat-me signal. Aposematic signals depend on “both the sender and receiver of the signal.” “In our case, we have complete control over the genetics of these nematodes. We can therefore examine the evolution of both sides from more than one angle. I’m very excited about it.”
FAQ
What do you call the smell of rain?
- Petrichor is the earthy scent that comes after rain. It mainly comes from chemical compounds called terpenes.
What is the difference between geosmin and petrichor?
- Geosmin is a terpene produced (primarily) by bacteria. strep throat) and fungi found in soil and fresh water. Geosmin and other terpenes, such as 2-methyl-isoborneol (2-MIB), are water-soluble and volatile odorants responsible for the rain odor, petrichor.
Why do microorganisms produce geosmin?
- Although geosmin is not toxic, researchers have observed that geosmin can deter predators such as nematodes and flies from consuming geosmin-producing microorganisms, which produce the toxin. This is called lipotropic 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 by . Deanna McNeildoctor.
