Evaluation of Ulva lactuca hydrolysates as a feedstock for clostridial fermentations to produce 1,2-propanediol

Macroalgae are an abundant and underutilized renewable feedstock that can be exploited for the production of various low- and high-value biomolecules. The study herein describes a mild acid and base process to hydrolyze the green macroalgae Ulva lactuca into simpler, fermentable carbohydrates, with an emphasis on rhamnose. Hydrolysis experiments involving up to 5% v/v sulfuric acid and 5% w/v sodium hydroxide at temperatures between 25 and 100 °C demonstrated that the highest recovery of fermentable carbohydrates was generally obtained using 2.5% v/v sulfuric acid at 75 °C. Two Clostridium species ( Clostridium strain AK1 isolated from SW Iceland, and Clostridium beijerinckii strain DSM 791) were used to ferment L-rhamnose to 1,2-propanediol, both as a single substrate and as part of macroalgal hydrolysates. Additionally, the impact of culture conditions (pH and initial substrate concentration) on rhamnose fermentation was investigated in batch culture for both strains. Generally, pH did not influence the production of 1,2-propanediol and both strains partially degraded rhamnose at very low (10 mM) initial substrate concentrations. A kinetic investigation of rhamnose utilization using strain AK1 showed that the pentose is degraded much slower as compared with glucose with 1,2-propanediol production lagging and reaching a maximum concentration of 7.7 mM. When Clostridium strain AK1 was cultivated on U. lactuca hydrolysates and non-pretreated U. lactuca , the maximum yields were 7.9 mM 1,2-propanediol. This is the first report of the production of 1,2-propanediol from macroalgal biomass using a moderately thermophilic Clostridia.