Recently I rediscovered some notebooks and a flash drive from early elementary school, in which I found many pages filled by a kid who had just discovered the fun in writing. These were generally a mix of equal parts cute and embarrassing. On the latter-leaning side of things, there was a lot of writing strewn with exclamation points about first-time experiences and associated smells. "We finally saw snow! It smelled like white" (?) or "My brother and I went swimming in the sea for the first time. He is mad right now because I told him he smells like the fish aisle of Vons" or "The Twolumne [sic] flowers smelled so good but then I think the bee came inside our car because we were smelling them without his permission." And so on.
At the core of all these records of trying things and going places for the first time was a child's wide-eyed wonderment with just about anything and everything. Such nostalgia for a state of mind! With the singular link that exists between scent and memory, I wished I could smell exactly what I had on those childhood days so that I could re-embody that utter fascination.
But as many will say, this is not unlike being in science, where discovering or learning things for the first time happens all the time (and here is the two-fold joy of being a newbie at something within science – the privilege of getting to experience amazement that much more frequently). So, instead of asking for some kind of futuristic scratch-n-sniff sticker, in the name of nostalgia I went searching for connections between those pleasurable scents and the small things that we consider on this blog.
To start with, the classic example is geosmin, the molecule responsible for the earthy smell of soil that is most famously synthesized by actinomycetes like Streptomyces. The wonderful heady smell that follows rainfall, petrichor, is the result of rain droplets hitting the earth and casting geosmin up into the air as they aerosolize. Geosmin's connection to lifeforms spans all sizes: it attracts springtails upon whose backs the microbes' spores can hitch a ride for dissemination, and as Roberto wrote here a couple of years ago, there is a remarkable evolutionary side story as to why the human nose might have such a keen receptivity for geosmin (even at picomolar concentrations, which blows a shark's sensitivity to blood out of the water).
It is terpenes like geosmin and many other molecules of microbial make that imbue so much of the world with scents. Commercial perfumes, too, are principally scented by terpenes and their derivatives and other molecules that are aromatic (aromatic both in the sense of smell and in the sense of the molecules' cyclic, planar, pi-conjugated nature, the latter historically named after the former). Terpenes run the gamut of adjectives: citrusy (limonene), floral (linalool, geraniol), earthy (myrcene), minty (menthol), woodsy (cedrol, santalol, borneol, pinene…).
Meanwhile, fruity smells are generally thanks to various fatty aldehydes and to ketones like melonal and raspberry ketone. (This is why a warning sign of diabetes, ketoacidosis, is accompanied by fruity-smelling breath.) Such sweet-smelling compounds are also responsible for that distinctly welcoming smell of libraries – or rather, the smell of old books that have been exposed to light, heat, or microbes. Constituents of paper like lignin and rosin decompose into volatile organic compounds including 2-ethylhexanol, benzaldehyde, and vanillin, which contribute a sweet, almondy, vanilla-like aroma. (The rosin contains acids whose hydrophobicity makes paper more amenable to writing, at the expense of lifespan – hence, "acid-free archival paper.")
The smell of the oceanside, on the other hand, is thanks to a molecule with no particular reputation for pleasing the nose. Dimethyl sulfide (DMS), which is also behind the smell of overcooked cabbage in its less dilute form (good things really do come in small packages), plays an important role in ocean ecology. Phytoplankton like dinoflagellates and coccolithophores generate dimethylsulfonioproprionate (DMSP) for osmoregulation and predator deterrence. Seafaring bacteria like Roseobacter, whose cells stick together to form little rosettes, then take this microalgal DMSP and convert it into DMS. Higher up above the waves, DMS signals to seabirds that they are in the right place for plentiful prey, and there it also seeds cloud formation – cloud-maker is another role that falls within the repertoire of ocean bacteria. It's a sweet image, little rosette flowers making scents and making clouds.
The smell of flowers, too, is within the purview of microbial influence. Flowers of plants grown in sterile conditions emit a different collection of aromatic volatiles than do flowers of plants grown with epiphytic microbes. Some molecules are missing, some molecules have altered concentrations, some new molecules appear. The mechanistic detail behind these changes are not well known, but it's likely that the downstream effect is altered flora-fauna partnerships for those as delicately specific as that of Ficus figs and wasps. What seems to be an ellipsis between such flora and fauna is (unsurprisingly) filled with bacterial language.
So, back to terpenes. Terpenes have been called the "world's most spoken language," and for good reason. Geosmin and other terpenes act as conductors for the symphonic ebb and flow of life rooted in soil. Within the earth, Serratia plymuthica rewires its metabolic profile in response to fungal terpenes, and the protist predators of various terpene-producing bacteria snoop on those terpene messages to home in on the whereabouts of their prey. Nematodes are dissuaded from snacking on bacteria when they detect geosmin, and have been observed to form memories associating smells with unpleasant experiences like pathogen exposure and hunger. The lives of plants and insects are intertwined through terpene signaling, silently hollering across fields and forests through molecules that signal distress and defense. From worms to humans, it is fascinating how memories can be so inextricably interwoven with specific scents – whole novels are written within a single scented molecule. Volatile organic compounds seem to be the Esperanto that never was, something that transcends biological boundaries to link microbe to plant to human, link tangible to intangible.
And to circle back to that elementary school notebook entry about snow smelling "white": the more apt description would have been that the snow smelled black. It turns out that the distinct "smell" of impending snow is in part the absence of usual smells, analogous to the physicist's black being the absence of color. When the temperature falls, molecules in the air slow down, and so aromatic compounds don't travel as far as in warmer weather and will make less frequent contact with olfactory receptors in the nose. This drop in receptor-ligand binding is registered as an altered scent.
Bottling up scents (or lack of scents) might be the closest thing to bottling up memories. That something as incorporeal as scent and memory can be so inextricably tied to life at all scales is fascinating; it seems there's nothing like awareness of such grand-scale biology to simultaneously make one feel important but also unimportant existing within the network of things humming about in, on, and above the earth. What a happy, comforting feeling.