The typical smell of geosmin rises when rain falls on dry earth or when we dig up moist soil. But a research team has only now clarified which of our more than 400 different olfactory receptors we use to perceive this typical earthy smell. They discovered that a receptor called OR11A1 is responsible for geosmin. This can detect the earthy smell of this substance and a second scent, also produced by microbes in the soil. This means that we humans are also among the mammals that have this “earth sensor”. This underlines the importance that this smell has and has had – both for better and for worse.
Geosmin is a volatile compound with a distinct “earthy” to “musty” smell. It is created primarily when soil becomes moist and the microbes it contains become active. But some plants, such as beetroot, also produce the odorant geosmin. “While the smell of geosmin suits beetroot, its occurrence in foods such as fish, beans, cocoa, water, wine or grape juice is problematic. In these foods, it severely impairs the sensory quality and acceptance,” explains co-author Stephanie Frank from the Leibniz Institute for Food Systems Biology at the Technical University of Munich. The odorant geosmin, a terpenoid compound, responsible for these musty smells and tastes was first isolated and chemically identified in 1965. Today we know that just a tiny amount of four to ten nanograms of geosmin per liter is enough for us to perceive the typical musty smell. This corresponds to about one teaspoon of geosmin in the water of 200 Olympic-sized swimming pools.
One of 616 receptors
But which odor receptor we humans use to perceive the scent was previously unknown. No wonder: humans have a total of around 400 different genes for odor receptors, which in turn are responsible for around 600 different receptor variants in the nasal mucosa. Of this wealth of odor sensors, however, only around 20 percent of the chemical compounds to which they react are known. The team led by Frank and lead author Lena Ball from the Leibniz Institute has now specifically searched for the receptor for geosmin. To do this, they created 616 different variants of human odor receptors in cell cultures and tested each one to see whether it responded to geosmin. They genetically modified the test cells so that they acted like small biosensors for the odor and lit up when bound using a special fluorescent marker. The analyses showed that only a single receptor in the human nasal mucosa, called OR11A1, responds to physiologically relevant concentrations of geosmin.
In a counter test, the team then investigated whether OR11A1 reacts to other odorants that are chemically related to geosmin. Of the 177 substances tested, only 2-ethylfenchol, which also smells earthy, was able to significantly activate the receptor. “2-ethylfenchol and geosmin are both produced by soil-dwelling microorganisms and can contaminate food,” explain Ball and her colleagues. Many animals are known to react to the earthy scent of geosmin and co. Camels and elephants use this scent, for example, to find watering holes, and the mosquito species Aedes aegypti uses it to find suitable wet breeding grounds for laying its eggs. Conversely, geosmin can also serve as a warning signal, for example to detect spoiled food. Fruit flies, for example, avoid fruit with this smell. “This shows that geosmin functions as a chemical signaling substance in the animal kingdom and certainly also in humans,” says Ball.
From kangaroo rat to polar bear
To find out more about the history of the human geosmin receptor, Ball and her colleagues also took a closer look at similar receptors in other mammals. “Since geosmin is an important signaling substance in the animal kingdom, we also examined how the olfactory receptors of kangaroo rats, mice, rhesus monkeys, Sumatran orangutans, polar bears and camels that are genetically most closely related to the human receptor react to geosmin,” explains Ball. “We wanted to find out whether the highly selective recognition of geosmin by the same receptor has been preserved over 100 million years of mammalian evolution.” The kangaroo rats (Dipodomys), which are found in many desert regions, have a particularly keen nose for the earthy smell of geosmin. Their receptor reacts around a hundred times more sensitively to the scent than ours. The researchers attribute this to the fact that these small desert dwellers use the smell as an indicator for both water and their food, which includes geosmin-containing cactus fruits.
What is astonishing, however, is the fine sense of geosmin in polar bears. “Surprisingly, the polar bear’s OR11A1 receptor showed a similarly high sensitivity to geosmin as that of camels and mice,” report Ball and her colleagues. At first glance, this seems strange, since the bears living on the coasts of the Arctic have enough water around them. However, the team suspects that the polar bears use the earthy smell of sediment washed into the sea as a guide: the geosmin gradient in the water could serve as a signal for them to find their way back to the coast. “The new findings about the highly sensitive odor receptors of some animals once again emphasize the biological relevance of geosmin as a signaling substance,” says senior author Dietmar Krautwurst from the Leibniz Institute. Knowledge about the geosmin receptors could also help to develop new detection systems that can be used to monitor the quality of food or drinking water better than before.
Source: Lena Ball (Leibniz Institute for Food Systems Biology, Munich) et al., Journal of Agricultural and Food Chemistry, doi: 10.1021/acs.jafc.4c01515