KEY FOR EFFICIENT BIOETHANOL PRODUCTION PROCESS: BETTER UTILIZATION OF MICROORGANISMS

Second-generation bioethanol, produced from polysaccharides (mainly cellulose) found in lignocellulosic biomasses, is a promising alternative energy source. Its production process involves enzymatic hydrolysis of the polysaccharides into fermentable sugars, which are then used for alcoholic fermentation. In addition, a pretreatment step is usually required for improving the enzymatic hydrolysis (i.e., removing lignin from plant cell walls). Despite the enormous effort conducted during the last decades, there are two technical/economical bottlenecks that render bioethanol production economically unfavorable: (i) the relatively high cost of biomass pretreatment and (ii) enzyme production. In the current work, with the aim of overcoming these hurdles, the application of two microorganisms were studied: Pleurotus ostreatus and Clostridium thermocellum as follows:

(A) Biological pretreatment of lignocellulosic biomass using Pleurotus ostreatus mutant strains. Biological pretreatment of lignocellulosic biomass involves mineralized of lignin using an appropriate microorganism. Unfortunately, the microorganism consumes a portion of the cellulose. Thus, a selective strain (that can mineralized lignin with only minimal cellulose consumption) would be required. Here, two mutant strains of the white rot fungus P. ostreatus PC9 were evaluated for selective lignin mineralization and pretreatment efficiency and were found to differ in their selectivity of lignin mineralization.

(B) Assembly of an optimized cellulolytic enzyme cocktail, based on the cellulosome of Clostridium thermocellum. A diverse set of enzymes is required for efficient hydrolysis of lignocellulosic biomass. In this context, the cellulolytic machinery of C. thermocellum, one of the best-known and efficient cellulose-degrading bacteria, was studied. The bacteria was grown on varied carbon sources, producing diverse populations of cellulosomes. The activity of those cellulosomes, as well as their subunits composition, was analyzed. The results obtained should be implemented in the future for fabrication of efficient designer cellulosomes, for formulation of recombinant cellulolytic cocktails or for engineering C. thermocellum strains with improved lignocellulosic biomass-converting abilities.