S. cerevisiae serves as keystone species for spoilage resistance in experimental synthetic wine yeast communities

Species diversity is a commonly stated contributor to the fate of an invader, and thus community resistance, in both microbial and non-microbial communities. Termed the "diversity-invasion hypothesis", a positive relationship between diversity and resistance to invasion is observed when an introduced species exhibits lower levels of survival in resident communities with higher species richness. The diversity-invasion hypothesis is an attractive perspective with convincing theory and examples, yet an "invasion paradox" of contrasting results means that a positive role of diversity against invasion is still not a certainty and under debate. In this study we investigated the relationship between resistance to invasion and resident community species richness versus species identity (i.e., keystone species). Using synthetic communities comprised of combinations of four wine yeasts (Saccharomyces cerevisiae, Lachancea thermotolerans, Torulaspora delbrueckii, Starmerella bacillaris), we tracked over 21 days the presence of introduced Brettanomyces bruxellensis spoilage yeast and Lactiplantibacillus plantarum lactic acid bacteria to ask the following: 1. Does yeast community species richness impact the establishment of B. bruxellensis yeast and L. plantarum bacteria during wine fermentation? 2. How does yeast species identity influence such establishment? We found that species identity rather than richness drove the prevention of establishment of B. bruxellensis and L. plantarum, with S. cerevisiae playing a critical keystone species role. Aside from spoilage prevention by S. cerevisiae, the four resident yeast species demonstrated a strict dominance ranking of competitive exclusion regardless of background community composition. Our research lends evidence against the commonly predicted positive relationship between species richness and resistance to invasion. Furthermore, as spontaneously fermented natural wines and diverse starter cultures gain popularity, our findings support a remaining importance of S. cerevisiae in preventing B. bruxellensis spoilage..