# Yeast Diversity during Spontaneous Fermentations and
## 3 Results and Discussion
### Yeast Population Dynamics during Spontaneous Fermentation  
The influence of grape variety and fermentation process on yeast population dynamics were analysed. Indigenous yeast exhibited distinct patterns of distribution across grape varieties in this particular vineyard (Figure 2A). While _S. cerevisiae_ and _H. uvarum_ were found from all fermentations, their abundance varied greatly between grape varieties. _P. kluyveri_ dominated the yeast population in Pinot Noir fermentation whilst its presence was in low abundance with other fermentations (Figure 2A). _S. pustorianus_ also possessed a wide distribution, with exception only in Cinsault, Merlot and Cabernet Sauvignon fermentations. Notably, albeit of low abundance, _M. pulcherrina_ and _M. aff. fructicola_ were only discovered in grape mash from Semillon and Riesling, respectively (Figure 2A), indicating their potential role in expressing distinct characteristics of wines made from the corresponding grape cultivar. In agreement with our study, several recent studies have also shown the role of grape varieties in differentiation of yeast populations during spontaneous fermentation [8; 21]. Notedly, these studies revealed a picture of distinctive yeast populations with influence from grape varieties at regional (more than 100 Km; [PERSON] et al. [8]) or sub-regional scales (8-12 km; [PERSON] et al. [21]), whilst our study estimated the distribution of yeast patterns within an individual vineyard. Given the small scale of the studied area in our study, local conditions, including weather and topography may not be involved in modulating yeast communities between grape varieties. Factors like soil properties, harvest date, and animal vectors that can transfer yeast across the vineyard, (reviewed by [PERSON] et al. [6]) might explain the intra-vineyard variation of yeast populations, but these analyses were beyond the scope of the current study. Evolution of the yeast community was carefully evaluated throughout the fermentation (Figure 2B). At the start of fermentation, the grape must possessed higher species diversity, with _H. uvarum_ dominating the yeast population (Figure 2B). Several previous studies also proved the high abundance of _H. uvarum_ harboured in the grape juice/must [16; 21], and discussed its role in improving wine aroma formation [41]. As fermentation proceeded, species diversity declined coinciding with increased population of _S. cerevisiae_, whose abundance finally reached 93.97% by the end of fermentation (Figure 2B). Decreased species diversity during fermentation can be due to the relatively poorer tolerance of many non-_Saccharomyces_ to oenological associated stressors, in particular ethanol, compared to that of _S. cerevisiae_[47]. Nonetheless, non-_Saccharomyces H. uvarum_ and _P. kluyveri_ that have been reported as having average resistance to typical winemaking stressors [48] were seen throughout some fermentations (Figure 2B). The effect of SO2 addition on the yeast community in spontaneous fermentation was also investigated in this study. Due to its antimicrobial function, SO2 is commonly used as an inexpensive and readily-implementable strategy to prevent the growth of undesirable microorganisms during winemaking. In this study, supplementation of SO2 before the onset of fermentation reduced yeast diversity during spontaneous fermentation, particularly at the beginning of fermentation (Figure 2B), which was in line with the results of [PERSON] et al. [49] and [PERSON] et al. [26]. Compared to the SO2-free group, SO2 addition resulted in the loss of some non-_Saccharomyces_ yeasts when fermentation commenced, for example, _M. pulcherrina_, _M. aff. fructicola_ in Semillon juice (Figure 2C,D) and _C. zemplinia_ in Cinsault must (Figure 2E,F). These yeasts have been reported to be able to ferment at extremely low SO\({}_{2}\) concentrations, offering paradigms for improving wine aroma complexity by releasing desirable secondary metabolites [50].