Prioritizing Wild Yeast Strains for Macroalgal Bioethanol Production

Abstract

Macroalgal biomass for bioethanol production has proved to be a viable alternative to feedstocks of first-generation (food crops rich in starch) and second-generation biofuel (agricultural residues and woody biomasses rich in lignocellulosic components). Production of bioethanol from biomass involves fermentation of mixed monosugars such as glucose, xylose, galactose, rhamnose, mannose, and fucose, and abundant monomer is found in algal biomass as well as lignocellulosic biomass. The inability of commonly used Saccharomyces cerevisiae to ferment xylose (pentose) sugar has led to the exploration of robust yeast strains that can utilize mixed sugars to produce ethanol. This study focuses on the isolation of yeast strains from various fruits and fermented products to determine efficacy in ethanol production using synthetic and macroalgal sugar. Two strains prioritized based on ethanol yield are Meyerozyma caribbica (isolated from cashew-fermented juice) and Pichia kudriavzevii (isolated from toddy). Strain P. kudriavzevii is thermotolerant (at 45 °C), whereas M. caribbica is tolerant to high salinity and produced ethanol of 2.6 g/L from 5.95 g/L of sugar, achieving 88.8% fermentation efficiency. P. kudriavzevii strain exhibits ethanol tolerance up to 4%. Fermentation of synthetic glucose produced 1.35 g/L and 1.44 g/L ethanol by M. caribbica and P. kudriavzevii strains with fermentation efficiencies of 83.6% and 94.8% respectively. M. caribbica strain fermented xylose and produced 1.4 g/L of ethanol achieving 14.9% fermentation efficiency, while simultaneous saccharification and fermentation process using P. kudriavzevii strain exhibited efficiency of 65.1% and 80.9% for Enteromorpha intestinalis and Ulva lactuca respectively. Cellulolytic activity of the prioritized strains was determined to carry out consolidated bioprocess.