**1. Introduction**

Wine is a complex beverage, produced through the interplay of grape and microbial metabolomes during the process of fermentation. While the majority of modern commercial wine fermentation is performed using inoculated commercial strains of the major wine yeas<sup>t</sup> *Saccharomyces cerevisiae*, a significant proportion of commercial wine fermentations are now being performed using uninoculated grape must. In these situations, the fermentation is conducted by a consortium of wine yeas<sup>t</sup> and bacteria that establish themselves either from the grape surface or from the winery via shared equipment or other vectors such as insects [1].

In the very early stages of fermentation, apiculate yeasts, and yeast-like fungi which reside on the surface of intact grape berries or winery equipment and include the genera *Aureobasidium*, *Rhodotorula*,

*Pichia*, *Candida*, *Hanseniaspora* and *Metschnikowia*, represent the majority of the microbiota [2]. However, the majority of these yeasts succumb very early after fermentation commences. Mildly fermentative yeasts, such as *Hanseniaspora uvarum*, *Candida stellata*, *Metschnikowia pulcherrima, Torulaspora delbrueckii* and *Lachancea thermotolerans* have been shown to proliferate and survive well into the fermentation, but reduce in numbers as ethanol levels increase above 6% [3–6].

Vineyard geography, environment and managemen<sup>t</sup> practices, and harvest, juice/must processing and fermentation conditions can all a ffect yeas<sup>t</sup> population dynamics during wine fermentation [7–13]. Of those fermentation conditions that are readily modulated by winemakers, the addition of the antimicrobial sulphur dioxide (SO2) represents the most broadly available intervention practice. Previous microbiological studies have shown that species (and strains) of the major wine yeasts can respond di fferently to the application of SO2. Typically, commercial strains of *S. cerevisiae* display high tolerance to SO2, while "wild" yeasts display lower tolerances and are therefore thought to be broadly selected against through the application of moderate amounts of SO2 prior to the start of fermentation [14–16].

In order to explore the e ffect of SO2 addition on the yeas<sup>t</sup> microbiota during uninoculated Chardonnay wine, meta-barcoding (phylotyping) analysis was used to assess the population dynamics of wine produced across two successive vintages using a range of pre-ferment SO2 levels.
