Bioethanol is a safe and renewable source of energy that continues to be a research focus, since fossil fuels have been linked to global warming and nuclear energy sources are affected by the increased safety concerns following the 2011 nuclear power plant accident in Japan. In general, bioethanol is converted from a biomass by yeast fermentation. The production efficiency of this bioethanol is not sufficiently high, and its practical use as a substitute for fossil fuels and nuclear energy is thus limited. For the industrial production of bioethanol, the yeast fermentation of biomass cultures containing high concentration sugar, NaCl, and ethanol is necessary, but this might induce phenomena in which the stresses arising in the yeasts weaken their cells during fermentation. As described herein, we isolated 1028 strains of yeasts from natural aquatic environments: Japan's Tama River and Lake Kasumigaura. Among them, 412 strains were fermentative yeasts and 31 strains showed high fermentation ability under a 30% sorbitol + 10% ethanol condition. These strains were identified as Torulaspola delbrueckii, Wickerhamomyces anomalus, Candida glabrata, Pichia kudriavzevii, Saccharomyces cf. cerevisiae/paradoxus, and Lachancea kluyveri. The strains T. delbrueckii, W. anomalus, and C. glabrata also showed tolerance against 15% NaCl. Most importantly, S. cf. cerevisiae/paradoxus H28 and L. kluyveri F2-67 produced 57.4 g/L and 53.9 g/L ethanol from molasses (sucrose 104.0 g/L, fructose 33.4 g/L, and glucose 24.8 g/L) within 48 hrs at 25˚C, respectively.

In this study, yeasts with broad range of pH tolerance were isolated and characterized from natural aquatic environments in Japan. Only a few basic and application studies of alkali-tolerant yeasts have been reported, despite the unmet industrial needs. First, we surveyed alkali-tolerant yeasts from natural aquatic environments at pH 7.6-9.4. We isolated 35 yeast strains that grew in pH 9.0 medium, from seven genera and nine species: 25 strains (N1, N2, through N6, N9, K1, K3 through K19) were Rhodotorula mucilaginosa; one (N7) was Rhodosporidium fluvial; one (N8) was Scheffersomyces spartinae; two (N10 and N13) were Wicherhamonyces anomalus; one (N11) was Cyberlindnera saturnus; one (S1) was Candida sp.; two (S2 and S4) were Candida intermedia; one (S3) was Candida quercuum; and one (K2) was Cryptococcus liquefacience. We examined the effects of pH on the growth of representative yeast strains. Strains K12 and S4 showed high growth at pH 3-10. Strains N7, N8, N10, N11, and S3 showed high growth at pH 3-9. Strains K2 and S1 showed high growth at pH 4-8. All nine of these strains had neutralizing activities from acidic media at pH 3-5 to pH 6-8. We previously isolated acid-tolerant yeasts (Cryptococcus sp. T1 [1] and Candida intermedia CeA16 [2]) from extremely acidified environments; they showed high growth at pH 3-9 and neutralizing activities of acidic media by releasing ammonium ions. Thus, alkali-tolerant yeasts and acid-tolerant yeasts were found to be similar species and have both high growth at a broad range of pH and neutralizing activities of acid media. Previously, we also isolated acid-tolerant, acid-neutralizing yeasts How to cite this paper:

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