Does a wine taste of its soil or of its microbes?

Both, probably, but the microbes' share is a live argument rather than a settled fact. Terroir is the French idea that a wine carries the character of the place it came from: the environment, the farming and the growing site. The AOC system that rules French wine presumes that the land gives the grapes a unique quality, yet how much terroir matters is debated in the wine industry. Microbes make the debate harder.

Here is the twist. A typical berry carries anywhere from 160 to 100,000 colony-forming units of wild yeast, but the yeast that most people think of as wine yeast, Saccharomyces cerevisiae, is quite rarely found in the vineyard. The wild passengers are mostly other genera, Kloeckera, Candida and Pichia, and they have very low tolerance to alcohol. They can start a fermentation in the harvest bin, but they die as the alcohol builds, often at 3 to 5% by volume. A Saccharomyces strain then finishes the job, and very often it is a strain that has taken up residence in the winery itself over the years.

So, is the yeast a piece of the vineyard's terroir, or of the cellar's? Some winemakers feel that resident yeast contributes to a wine's terroir, and classified Bordeaux estates tout their resident "chateau" strains. Wineries often even return leftover pomace and lees to the vineyard as compost to encourage favorable strains. Many others prefer to control the fermentation with predictable cultured yeast.

There is a cost to letting nature drive. Compared with cultured yeast, ambient yeasts risk off-flavors, higher volatile acidity or a stuck fermentation. Wild yeasts can produce high-quality, unique-flavored wines, but they can also contribute to spoilage.