A stowaway strain of yeast, crossing the Atlantic centuries ago, may be responsible for a cool quarter-trillion-dollar beverage industry today.

Lager beer, which currently accounts for more than half the global beer market, relies on the yeast Saccharomyces pastorianus. Lager brewers ferment their product at relatively cool temperatures, then condition it under refrigeration, and S. pastorianus can tolerate lower temperatures than can ale-producing yeasts. The so-called cryo-tolerant lager yeast is a hybrid of the ale yeast S. cerevisiae and another yeast species that had long eluded conclusive identification.

Now a group of researchers based in Argentina, the U.S. and Portugal says they have identified the wild yeast that combined with S. cerevisiae to form the cryo-tolerant lager yeast S. pastorianus. The team reported its finding in a study published online August 22 in Proceedings of the National Academy of Sciences.

The wild yeast, which has been named Saccharomyces eubayanus, was discovered in the beech forests of South America's Patagonia region, thousands of kilometers from the Bavarian cellars where lager brewing took hold in the 15th century. Patagonia's beeches and the yeast species they host endure relatively low average temperatures, which might explain why the newly described S. eubayanus yeast would lend cryo-tolerance to its hybrid yeast offspring.

The Patagonian yeast could have hitched a ride on food, on pieces of wood or in beverages once sailing vessels began crossing the Atlantic, says study co-author Chris Todd Hittinger, an evolutionary geneticist at the University of Wisconsin–Madison. The yeast may have even stowed away in the digestive tracts of fruit flies. "Several generations of fruit flies hanging out on a sailing ship would get you pretty far," he says.

Of course, there may be an ecological niche for S. eubayanus closer to southern Germany that simply has not been located yet. "What we can say is that Europe is probably the most exhaustively sampled place in the world for wild and industrial yeasts," Hittinger says. "Our collaboration has searched on five separate continents, and we haven't turned it up—nor have others."

The process of brewing lager seems to predate a plausible transatlantic crossing of S. eubayanus, however, so it may be that early lagers were produced with standard ale yeast working below its optimal fermentation temperature. "The moment S. eubayanus comes on the scene and forms a hybrid, that would have immediately outcompeted S. cerevisiae in those environments," Hittinger says.

The researchers sequenced the S. eubayanus genome and found that it was distinct from any previously described species. The genetic analysis showed the newfound strain to be a 99.5 percent match with the missing piece of the hybrid lager yeast S. pastorianus—the part of the hybrid not accounted for by well-studied, warm-fermentation ale yeast.

That makes S. eubayanus a good candidate for the missing hybrid progenitor, says Ursula Bond, a microbiologist at Trinity College Dublin. "With such a recent hybridization event, one might expect to find 100 percent identity between the genomes," she says. "However, with the discovery of the dynamic nature of these genomes, which can undergo chromosome rearrangements and gene loss in response to environmental stresses, recent genetic changes may account for the differences."

Hittinger and his colleagues found the beech forest yeast as part of a larger effort to trace the biodiversity of Saccharomyces yeasts and wild yeasts in general. "We were certainly aware of the problem" of the missing half of the lager-yeast hybrid, he says. "You're always aware of the half dozen or dozen problems that your particular research campaign might impinge upon."

But now that the wild yeast has been located and identified, it may soon have some practical import, thanks to study co-author Diego Libkind of the National University of Comahue in Bariloche, Argentina. "Diego tells me that they're actually in talks with an Argentinean brewing company to make some beers using S. eubayanus and hybridizing it with S. cerevisiae to reenact the process that happened hundreds of years ago," Hittinger says. "Whether those beers taste any good or not, I think that's an open question. And we're all awaiting the results on that."