Last week an uninvited guest showed up for breakfast. As I poured maple syrup over my son’s waffle, Plop! A perfect dime-sized fungus colony spilled out to crown that waffle like a malevolent pat of butter. The maple syrup had been sitting out, unrefrigerated for, um, quite a while.
A 10 cm custard cup of maple syrup, with guest (click for a closer look)
What would you do? I asked my 4-year-old whether he could touch his nose with his tongue, and used mommy stealth to swiftly replace the contaminated waffle with a new one (with honey on it). Crisis averted. Finally, and I’m sure you would’ve done the same, I packed up the offending maple syrup for a rendezvous with my microscope. Sometimes uninvited guests turn out to be pretty darned interesting, if you take the trouble to get to know them.
A fungus that can grow in maple syrup is not your average everyday mold, it’s a xerophile. Xerophiles grow in places that are too dry and hostile for your average fungus. OK, maple syrup is wet, but it’s also extremely high in sugar. All that sugar has the effect of pulling water out of cells, and the vast majority of fungi can’t grow in maple syrup at all. No matter how much they might like it in smaller quantities–the water in maple syrup isn’t “available” to them.
Water activity (aw) is a measure of “available water.” Water itself has an aw of 1.0 (all the water in water is available!); for dried milk powder aw=0.2 (hardly any water is available). To protect food from spoilage by molds and bacteria without refrigeration, you want to reduce water activity below 0.8. You can do that by drying out the food, or brining it with either a high salt or a high sugar treatment. According to this site, maple syrup typically has a water activity of about 0.87 to 0.88, pretty hostile to most molds. Only a few xerophiles can live in it,1 including our surprise guest, Wallemia sebi.
What a handsome fungus! Wallemia sebi is a xerophilic mold that specializes in growing on things of low water activity, like dried fruits and jams, and salted meats and nuts. It grows in salterns (the evaporating beds in which sea salt is produced), and its bumpy little spores are found fairly often in indoor air.
A nice study by Zalar and colleagues2 reveals that Wallemia is a distinctly weird mold. It’s so weird, and so distantly related to most other molds, the authors erected a whole new class of fungi just for it, class Wallemiomycetes. Within this entire class there are only three species, Wallemia sebi, W. muriae, and W. ichthyophaga. Just for reference, you and I and my dog and almost every furry creature in the world are Mammals–that’s a class too.
The Wallemiomycetes are distantly related to the usual xerophilic suspects, Aspergillus and Penicillium. In fact, they’re distantly related to just about every other fungus we know. Zalar et al. found that they’re out there by themselves on a very long evolutionary branch. They emerged very early, just as the major basidiomycete groups were evolving. It’s hard to imagine Wallemia as a basidiomycete, because it has never been seen make a sexual fruiting body, and that’s how most basidiomycetes are classified. Mysterious and beautiful, that’s Wallemia.
A lot of people think that a mold is a mold is a mold, but that’s just not so. The mold that’s rotting your lemon is not the same one that’s growing in your maple syrup, or eating your strawberries. In fact, your lemon, maple, and strawberry molds each belong to a different phylum of fungi. Proust said it:
The real voyage of discovery consists not in seeking new landscapes but in having new eyes.
- To avoid moldy maple syrup, producers heat-sterilize it before selling it to you. Once you open the bottle, the fridge is your friend. The fridge doesn’t change the aw, but the low temperatures in there slow or stop the growth of most fungi. By the way, fake maple syrup resists molds through the miracle of chemical preservatives (usually sodium benzoate and sorbic acid). You don’t use fake maple syrup, do you?
- Zalar, P., G.S. de Hoog, H.-J. Shroers, J.M. Frank, N. Gunde-Cimerman. 2005. Taxonomy and phylogeny of the xerophilic genus Wallemia (Wallemiomycetes and Wallemiales, cl. et ord. nov.) Antonie van Leeuwenhoek 87: 311-328.
- ERRATUM: When I first wrote the article, I overlooked an important new publication on Wallemia (thanks, Else). It sheds a little more light on Wallemia’s relationships, and includes a thorough discussion of Wallemia’s strange characteristics. Here’s the citation: PB Matheny, JA Gossmann, P Zalar, TKA Kumar, and DS Hibbett. 2006. Resolving the phylogenetic position of the Wallemiomycetes: an enigmatic major lineage of Basidiomycota. Can. J. Bot. 84: 1794-1805.
Thanks to Kent Loeffler, who took the photos of Wallemia growing in little 10 cm custard cups full of maple syrup.