Gombert, A. | bioRxiv Science

bioRxiv2021

Blocking mitophagy does not improve fuel ethanol production in Saccharomyces cerevisiae

Ethanol fermentation is frequently performed under conditions of low nitrogen. In Saccharomyces cerevisiae, nitrogen limitation induces macroautophagy, including the selective removal of mitochondria, also called mitophagy. Shiroma and co-workers (2014) showed that blocking mitophagy by deletion of the mitophagy specific gene ATG32 increased the fermentation performance during the brewing of Ginjo sake. In this study, we tested if a similar strategy could enhance alcoholic fermentation in the context of fuel ethanol production from sugarcane in Brazilian biorefineries. Conditions that mimic the industrial fermentation process indeed induce Atg32-dependent mitophagy in cells of S. cerevisiae PE-2, a strain frequently used in the industry. However, after blocking mitophagy, no differences in CO2 production, final ethanol titres or cell viability were observed after five rounds of ethanol fermentation, cell recycling and acid treatment, as commonly performed in sugarcane biorefineries. To test if S. cerevisiaes strain background influences this outcome, cultivations were carried out in a synthetic medium with strains PE-2, Ethanol Red (industrial) and BY (laboratory), with and without a functional ATG32 gene, under oxic and oxygen restricted conditions. Despite the clear differences in sugar consumption, cell viability and ethanol titres, among the three strains, we could not observe any improvement in fermentation performance related to the blocking of mitophagy. We conclude with caution that results obtained with Ginjo sake yeast is an exception and cannot be extrapolated to other yeast strains and that more research is needed to ascertain the role of autophagic processes during fermentation. ImportanceBioethanol is the largest (per volume) ever biobased bulk chemical produced globally. The fermentation process is very well established, and industries regularly attain nearly 85% of maximum theoretical yields. However, because of the volume of fuel produced, even a small improvement will have huge economic benefits. To this end, besides already implemented process improvements, various free energy conservation strategies have been successfully exploited at least in laboratory strains to increase ethanol yields and decrease by-product formation. Cellular housekeeping processes have been an almost unexplored territory in strain improvement. Shiroma and co-workers previously reported that blocking mitophagy by deletion of the mitophagy receptor gene ATG32 in Saccharomyces cerevisiae led to a 2.12% increase in final ethanol titres during Japanese sake fermentation. We found in two commercially used bioethanol strains (PE-2 and Ethanol Red) that ATG32 deficiency does not lead to an improvement in cell viability or ethanol levels during fermentation with molasses or in a synthetic complete medium. More research is required to ascertain the role of autophagic processes during fermentation conditions.

microbiology↗

bioRxiv2020

A biorefinery concept for the production of fuel ethanol, probiotic yeast and whey protein from a by-product of the cheese industry

1.Agroindustrial by-products and residues can be transformed into valuable compounds in biorefineries. Here we present a new concept: production of fuel ethanol, whey protein and probiotic yeast from cheese whey. An initial screening under industrially relevant conditions, involving thirty Kluyveromyces marxianus strains, was carried out using spot assays to evaluate their capacity to grow on cheese whey or on whey permeate (100 g lactose/L), under aerobic or anaerobic conditions, in the absence or presence of 5% ethanol, at pH 5.8 or pH 2.5. The four best growing K. marxianus strains were selected and further evaluated in a miniaturized industrial fermentation process using reconstituted whey permeate (100 g lactose/L) with cell recycling (involving sulfuric acid treatment). After five consecutive fermentation cycles, the ethanol yield on sugar reached 90% of the theoretical maximum in the best cases, with 90% cell viability. Cells harvested at this point displayed probiotic properties such as capacity to survive the passage through the gastrointestinal tract and capacity to modulate innate immune response of intestinal epithelium, both in vitro. Furthermore, the CIDCA 9121 strain was able to protect against histopathological damage in an animal model of acute colitis. Our findings demonstrate that K. marxianus CIDCA 9121 is capable of efficiently fermenting the lactose present in whey permeate to ethanol and that the remaining yeast biomass has probiotic properties, enabling an integrated process for the obtainment of whey protein, fuel ethanol and probiotics from cheese whey. 2. ImportanceCheese whey is the liquid remaining following the precipitation and removal of milk casein during cheese-making. This by-product represents about 85-95% of the milk volume and retains 55% of milk nutrients so it can be exploited as a source of valuable end products. However, at a global level around 50% of cheese whey is wasted, representing an important environmental impact and indicating the need to develop alternative processes to recover value. Kluyveromyces marxianus is capable of fermenting lactose, generally regarded as safe, and has been explored separately as an ethanol producer and as a viable bioactive microorganism. The significance of our research is to establish the proof of concept that a biorefinery for fuel ethanol production using whey and K. marxianus can also be exploited to obtain viable probiotic biomass, conferring an added value to the process and providing an alternative to reduce environmental impact.

microbiology↗