Résumé
Fermentative aromas play an important role in the organoleptic profile of wines. These compounds derive for a significant part from the break-down of nitrogen sources naturally found in grape juice, but their production also depends on the yeast strains driving the fermentation. The sequential inoculation of non-Saccharomyces yeasts with Saccharomyces cerevisiae is becoming more widespread in order to diversify wine styles. Multiple interactions between yeasts may occur, including competition for nutrients that directly impacts fermentation performance and aroma production. In a previous study, three unconventional non-Saccharomyces yeasts (Pichia burtonii, Kluyveromyces marxianus, Zygoascus meyerae) were selected in synthetic medium for displaying a behaviour different from that of S. cerevisiae in terms of nitrogen consumption and fermentative aroma production. These 3 strains and S. cerevisiae were inoculated individually in a synthetic grape juice medium. The uptake of nitrogen sources was monitored together with fermentation and population kinetics. In parallel, total RNA was extracted at specific time points corresponding to mid-lag phase and time where half of the maximal population was reached. At these time points, the uptake of certain nitrogen sources either ceased or started. RNA sequencing was used to investigate these behaviours at a molecular level. Data showed that S. cerevisiae and K. marxianus exhibited a similar behaviour with regard to nitrogen uptake and this was reflected in the expression of genes encoding nitrogen transporters and metabolism. On the other hand, Z. meyerae and P. burtonii assimilated very little nitrogen overall and their molecular response was different from that of the other two species. Unlike K. marxianus, Z. meyerae and P. burtonii are weak competitors with S. cerevisiae as they displayed different preferences for nitrogen sources. All three non-Saccharomyces yeasts however produced positive aroma compounds showing their oenological potential. The study demonstrated that evaluating possible nutrient competition is essential in the context of multi species inoculations.