Assessment of saccharification and fermentation of brown seaweeds to identify the seasonal effect on bioethanol production

Abstract

Brown seaweeds such as the kelps are attractive sources of biomass for bioethanol production, but fully optimised saccharification and fermentation conditions have yet to be established. To address this, various saccharification methods using dilute and concentrated acid and enzymes were trialled on three kelp species, Laminaria digitata, Laminaria hyperborea and Saccharina latissima, collected through a full seasonal cycle. Enzymatic hydrolysates were then fermented using Saccharomyces cerevisiae and Pichia angophorae to identify seasonal variations in ethanol yields. Highest glucose yields were achieved using concentrated acid, followed by enzymatic and dilute acid saccharification, respectively. The effect of seasonality showed that the highest glucose and ethanol yields were from kelps harvested during the autumn months and lowest during winter and spring months. However, the season at which biomass was collected did not have any measurable impact on the method of saccharification. Differences in ethanol yields between seaweed species were found with P. angophorae producing more ethanol from L. digitata and L. hyperborea hydrolysates, whilst S. cerevisiae was better for fermentation of S. latissima hydrolysates. It was observed that ethanol conversion yields with S. cerevisiae were higher than the theoretical maximum based on the yield of glucose identified, suggesting that other sugars in addition to glucose were co-fermented. For glucose liberation from seaweeds, terrestrial-derived cellulose and hemicellulose enzyme blends were suitable, but for liberation of all sugar monomers from seaweed polymers, other hydrolytic enzymes need to be investigated. In addition, fermentative microorganisms that are more tolerant of salinity and polyphenols are still required and ideally, be strains that can be engineered to ferment all carbohydrate sources present in kelps.