Schwanniomyces etchellsii, acid-thermotolerant yeasts from urban city soil

Acid-tolerant yeasts are one of the important keys to producing ethanol from acidic substrates, especially from molasses and agricultural waste. In this study, selected cultivars of yeasts isolated from a variety of locations such as botanical gardens in Thailand urban areas, which are often found highly polluted in the air such as carbon dioxide which is a cause of acid rain. There is limited information about how tolerant yeasts, are or their functional properties related to the environment. Yeast species were determined by using the 18S rDNA sequence guide. The level of acid tolerance was evaluated by adding to the culture medium lactic acid (300-900 mM), acetic acid (100-400 mM), and propionic acid (25-100 mM). 18S rDNA analysis has shown a %similarity of the nucleotide sequence higher than 98.65% compared to the database. Schwanniomyces etchellsii strains found in urban city soil were notable for their tolerance of lactic acid up to 100 mM. There are two main types of yeasts in overall acid tolerance: S. etchellsii, which is recognized as an osmotic pressure-resistant species that is highly resistant to fermentation inhibitors and produces ethanol; and Schizosaccharomyces pombe, which cell wall has been reported to be characterized by accumulation of α-(1,3)-glucan and malic acid can be used in metabolic pathways. The results show that S. pombe, isolated from rice paddy fields, can grow efficiently in acetic and propionic acid up to 400 mM and 100 mM, respectively. This species could be cultured in ethanol at a concentration of 12.5% (v/v). Moreover, it presented high ethanol and acetic acid production of 14.5-15.9 g/L and 7-10 g/L, respectively, with or without acidic conditions. In comparison, S. etchellsii, isolated from the botanical garden soil, which is grown in acetic, propionic, and lactic acid, was also indicated to be an organic acid-tolerant species.