Discovery of the novel metabolic pathway of the non-fermentable 3,6-anhydro-L-galactose from red macroalgae

Algae are emerging as third generation biomass feedstocks for producing fuels and chemicals mainly due to their advantages over lignocellulose. Especially, marine macroalgae have been receiving much attention. It is because they possess large amounts of carbohydrates which can be used to produce fermentable sugars for producing fuels and chemicals. With regard to Gellidium amansii that is one species of red macroalgae (Rhodophyta), approximately 80% of its biomass is composed of carbohydrates, and approximately 60% of the carbohydrates is agarose. Agarose is a linear polysaccharide, in which D-galactose and 3,6-anhydro-L-galactose (AHG) are alternately linked. Although agarose is considered a valuable source of fermentable sugar, its major sugar, AHG, is known to be not fermentable by terrestrial microorganisms, such as Escherichia coli, yeasts, and others. Due to the high abundance of AHG in red macroalgal biomass, the inability of fermenting AHG impedes the overall utilization red macroalgal biomass as the biomass feedstock. The issue of AHG is as critical as that of xylose from lignocellulose, since xylose is not inherently fermentable by Saccharomyces cerevisiae. We have recently discovered the novel metabolic pahtway of AHG using a marine miacroorganism by the intergrated analyses of metabolome and transcriptome. Our results paved the road to the bioconversion and ferementation of AHG into common metabolic intermediates. This is the first report on the metabolic pathway discovery of AHG and one of the fewest examples of novel metabolic pathway discoveries by metabolome and transcriptome analyses.