L-Rhamnose metabolism in Clostridium beijerinckii DSM 6423.

Macroalgae (or seaweeds) are considered potential biomass feedstocks for the production of renewable fuels and chemicals. Their sugar composition is different from that of lignocellulosic biomasses, and in green species, including Ulva lactuca , the major sugars are L-rhamnose and D-glucose. C. beijerinckii DSM 6423 utilized these sugars in an U. lactuca hydrolysate to produce acetic acid, butyric acid, isopropanol, butanol and ethanol (IBE) and 1,2-propanediol. D-glucose was almost completely consumed in diluted hydrolysates, while L-rhamnose or D-xylose were only partially utilized. In this study, the metabolism of L-rhamnose by C. beijerinckii DSM 6423 was investigated to improve its utilization from natural resources. Fermentations on D-glucose, L-rhamnose and a mixture of D-glucose and L-rhamnose were performed. On L-rhamnose, the cultures showed low growth and sugar consumption, and produced 1,2-propanediol, propionic acid and n-propanol in addition to acetic and butyric acids, whereas on D-glucose, IBE was the major product. On a D-glucose/L-rhamnose mixture, both sugars were converted simultaneously and L-rhamnose consumption was higher, leading to high levels of 1,2-propanediol (78.4 mM), in addition to 59.4 mM of butanol and 31.9 mM isopropanol. Genome and transcriptomics analysis of D-glucose- and L-rhamnose-grown cells revealed the presence and transcription of genes involved in L-rhamnose utilization, but also in bacterial microcompartment (BMC) formation. These data provide useful insights into the metabolic pathways involved in L-rhamnose utilization and the effects on the general metabolism (glycolysis, early sporulation, stress-response) induced by growth on L-rhamnose. IMPORTANCE A pre-requisite for a successful biobased economy is the efficient conversion of biomass resources into useful products, such as biofuels, bulk- and specialty chemicals. In contrast to other industrial microorganisms, natural solvent-producing Clostridia utilize a wide range of sugars, including C5, C6 and deoxy-sugars, for production of long-chain alcohols (butanol, 2,3-butanediol), isopropanol, acetone, n-propanol and organic acids. Butanol production by Clostridia from first generation sugars is already a commercial process, but for the expansion and diversification of the A/IBE process to other substrates, more knowledge is needed on the regulation and physiology of fermentation of sugar mixtures. Green macroalgae, produced in aquaculture systems, harvested from the sea or from tides, can be processed into hydrolysates containing mixtures of D-glucose and L-rhamnose, which can be fermented. The knowledge generated in this study will contribute to the development of more efficient processes for macroalgae fermentation, and of mixed sugar fermentation in general.