Calgary is generally not a bad place to live. It was recently ranked #5 on a list of the world’s best cities to live in (a fact which bemuses most Calgarians – the general consensus is that the judges must have only visited in the summer). One of the climatological quirks that make Calgary appealing is its sunshine. It’s the sunniest city in Canada, and one of the sunniest in North America. Sure, some of these sunny days happen to coincide with −40 temperatures, but where else in the world is it so easy to get sunburn and frostbite at the same time?
Winter in Calgary is a welcoming time of year. Credit: Glenn Little
The downside of Calgary’s weather, for some people, is the absence of rain. Our rain tends to come in dramatic, mid-summer thunderstorms that move between rain and hail for short periods of time, and then disappear (often having destroyed a few cars, knocked over a few deck umbrellas, and shredded a few gardens). There is no relaxing pitter-patter of a gentle late-night drizzle, no afternoon showers to ease the heat of the day. It’s Biblical inundation or nothing. Which is too bad, because it means we don’t get to experience petrichor – the scent after rain.
I bet you didn’t know that earthy, fresh smell had a name, but it does. Petrichor is the name given to the characteristic scent produced by arid soil after a rainstorm. It means “essence of the soil”, but let’s be poetic, and call it the essence of the Earth. Two geologists named the scent in the 1960s, in a paper published in Nature, but it has many other names too. In Lucknow, India the scent is collected from clay-disks inundated with rain during the monsoon season. It is mixed with sandalwood oil and sold in vials as matti ka attar – earth perfume. In the parched red clay deserts of Western Australia, the Anangu people call it panti wiru, simply meaning ‘good smell’. It is the harbinger for a wave of greenness and fresh vegetative growth to overtake the desert – signalling a time of plenty. Panti wiru is associated with youth, health, fecundity, and new beginnings.
Most desert plants wait in eager anticipation for any rain. The smallest rainfall can trigger an explosion in growth almost over night. This picture is from Death Valley, California.
The odour comes from actinomycetes (a group of bacterium that we met earlier). Actinomycetes are members of the actinobacteria – a tremendously large phylum of bacteria, which are among the most common organisms on Earth. Actinobacteria are found everywhere: in the ocean, in lakes and streams, and in the soil. They are responsible for breaking down dead and decaying plant and animal life, recycling nutrients into an ecosystem so they can be used to fund new growth. Actinobacteria are also the source of the antibiotic actinomycin – which sort-of morally balances out the fact that they are also responsible for tuberculosis, leprosy, and diphtheria.
When wet, actinomycetes exude a number of chemical compounds, including one responsible for the smell of rain – geosmin. Geosmin was identified in 1965 (clearly the 1960s were the best time for “scent of rain research”), and has since been isolated from many different species of bacteria – but researchers have yet to determine its purpose (if it even has one). But that’s not to say other species haven’t learned to take advantage of its distinctive smell. The glass eel (Anguilla anguilla) spawns in the ocean, but must navigate to fresh-water estuaries to grow and develop (as the ocean is a frightening and unfriendly place for a little fish, but given the frequency with which glass eels are eaten, freshwater may not be that much safer of an option). To do this, they use their nose (technically they’re called nares, but the principal is similar to smelling).
A glass eel. When it reachers freshwater, the food it ingests contain pigments which give it colour – but as a juvenile, it is translucent.
Geosmin is produced by actinomycetes on the banks of rivers and flushed downstream. Eventually, dilute amounts of the scent makes its way into estuaries and deltas, before being pushed out to sea. Glass eels detect the trace amounts of geosmin, and follow the scent gradient until they find the river, then migrate upstream to grow safely, until they are big enough to move back to the ocean and have babies of their own. The amount of geosmin in freshwater is a concern for fishermen too. Too much geosmin in the water creates fish that have a musty, off-flavour that many people find unappealing. But too little geosmin in the water, and fish can be flavourless. It’s less of a problem in wild-caught fish, but aquaculture systems that use recycled water can have a build-up of geosmin that cause fish to taste like a mudpie. Basically, bacterial sweat is determining the flavour of your salmon. Something to keep in mind next time you go fishing.
Neil Griffin
Literature Cited
Bear IJ and RG Thomas. 1964. Nature of argillaceous odour. Nature 201: 933-935.
Farmer LJ, JM McConnell and DJ Kilpatrick. 2000. Sensory characteristics of farmed and wild Atlantic salmon. Aquaculture 187: 105-125.
Gerber NN and HA Lechevalier. 1965. Geosmin, an earthy-smelling substance isolated from actinomycetes. Appl. Microbiol. 13: 935-938.
Tosi L and C Sola. 2010. Role of geosmin, a typical inland water odour, in guiding glass eel, Anguilla anguilla (L.) migration. Ethology 95: 177-185.
Young D. 2005. The smell of greeness: cultural synaesthesia in the Western Desert. Etnofoor 18: 61-77.
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