Global distribution of IRC7 alleles in Saccharomyces cerevisiae populations: a genomic and phenotypic survey within the wine clade.

The adaptation to the different biotic and abiotic factors of wine fermentation has led to the accumulation of numerous genomic hallmarks in Saccharomyces cerevisiae wine strains. IRC7, a gene encoding a cysteine-S-ß-lyase enzyme related volatile thiols production in wines, has two alleles: a full-length allele (IRC7F ) and a mutated one (IRC7S ), harbouring a 38 bp-deletion. Interestingly, IRC7S -encoding a less active enzyme - appears widespread amongst wine populations. Studying the global distribution of the IRC7S allele in different yeast lineages, we confirmed its high prevalence in the Wine clade and demonstrated a minority presence in other domesticated clades (Wine-PDM, Beer and Bread) while it is completely missing in wild clades. Here, we show that IRC7S -homozygous (HS) strains exhibited both fitness and competitive advantages compared with IRC7F -homozygous (HF) strains. There are some pieces of evidence of the direct contribution of the IRC7S allele to the outstanding behaviour of HS strains (i.e., improved response to oxidative stress conditions and higher tolerance to high copper levels); however, we also identified a set of sequence variants with significant co-occurrence patterns with the IRC7S allele, which can be co-contributing to the fitness and competitive advantages of HS strains in wine fermentations.

Occurrence and enological properties of two new non-conventional yeasts (Nakazawaea ishiwadae and Lodderomyces elongisporus) in wine fermentations.

The microbial diversity of wine alcoholic fermentation is not restricted to the presence and activity of Saccharomyces yeast strains. Some non-Saccharomyces species have been described as part of the fermentative microbiota, specially found in the initial steps of wine fermentations. These species may play roles from wine spoilage to flavor quality enhancement. From a large number of wine fermentations (429 wine samples), analyzed by ITS-amplicon sequencing to define their mycobiome, 2 non-conventional yeast species (Nakazawaea ishiwadae and Lodderomyces elongisporus) were detected, in a very limited number of samples but in significant levels of relative abundance. One strain of each species was isolated and their technological and enological potential have been characterized in this work. Compared with the Saccharomyces cerevisiae Viniferm Revelacion wine strain, the studied N. ishiwadae BMK17.1 and L. elongisporus BMK12.5 strains showed, as expected, a lower tolerance and growth fitness in high ethanol concentrations. However, N. ishiwadae BMK17.1 was able to grow also at 15% ethanol and L. elongisporus BMK12.5 at 10% reaching, in the latter case, slightly higher efficiency rates than S. cerevisiae at this level. Contrary to most non-Saccharomyces yeasts, these species were able to growth in presence of high doses of potassium-metabisulfite, reaching in both cases higher efficiency rates than S. cerevisiae. A notable affinity of L. elongisporus BMK12.5 for high pH values was clearly observed. Their fermentation kinetics and the final chemical-analytical characterization were studied in micro-fermentation assays, using synthetic grape must. L. elongisporus BMK12.5 was able to complete, in single inoculation, the sugar fermentation after 19 days, but, N. ishiwadae BMK17.1 left about 80 g/L sugars at this time. Co-inoculation assays (in a 1:100 proportion of S. cerevisiae:non-Saccharomyces strains) finished sugar consumption with similar kinetics than the S. cerevisiae single inoculation, in the case of L. elongisporus BMK12.5 co-inoculation, and with lower kinetics when using N. ishiwadae BMK17.1. A remarkable malic acid consumption and a low acetic acid production was associated with L. elongisporus BMK12.5 fermentations, together with a high production of 3-methyl-1-butanol and 2-phenylethanol, and the release of high amounts of proteins into the wines. N. ishiwadae BMK17.1, although unable to finish the fermentation itself, showed a high production of oligosaccharides and volatile compounds such as isobutanol or isobutyric acid. This work reports, for the first time, the occurrence and enological potential of two strains pertaining to the non-conventional yeast genera Lodderomyces and Nakazawaea.