Direct ethanol fermentation of the algal storage polysaccharide laminarin with an optimized combination of engineered yeasts

•A novel laminarinase, Gly5M, was isolated from a macroalgae-assimilating marine bacterium Saccharophagus degradans.•Gentiobiose was the major degradation product from laminarin by Gly5M-displaying yeast.•Gentiobiose was converted into glucose and assimilated by β-glucosidase (BG)-displaying yeast.•Coculture of Gly5M- and BG-displaying yeasts improved ethanol production rather than individual culture of each strain.

Laminarin is the algal storage glucan and represents up to 35% of the dry weight of brown macroalgae. In this study, a novel laminarinase, Gly5M, was first found using focused proteome analysis of a laminarin-assimilating marine bacterium, Saccharophagus degradans, and the encoding gene was isolated. A Gly5M-displaying yeast strain was prepared with the cell surface display system using Saccharomyces cerevisiae. It showed a laminarin-degrading activity on the cell surface and caused the dominant accumulation of gentiobiose. The obtained gentiobiose was converted into glucose and could be assimilated by an Aspergillus aculeatus β-glucosidase (BG)-displaying yeast strain. When Gly5M- and BG-displaying yeasts were anaerobically cultivated together in fermentation medium containing 20 g/L laminarin as a sole carbon source, the coculture system with the combination of optimized ratios of the 2 yeast strains directly produced 5.2 g/L ethanol. This coculture system of the 2 engineered yeast strains would be a platform for the use of laminarin and contribute to the complete utilization of brown macroalgae.