Abstract
Anaerobes harbor some of the most efficient biological machinery for cellulose degradation, especially thermophilic bacteria, such as Acetivibrio thermocellus and Thermoclostridium stercorarium, which play a fundamental role in transferring lignocellulose into ethanol through consolidated bioprocessing (CBP). In this study, we compared activities of two cellulase systems under varying kinds of hemicellulose and cellulose. A. thermocellus was identified to contribute specifically to cellulose hydrolysis, whereas T. stercorarium contributes to hemicellulose hydrolysis. The two systems were assayed in various combinations to assess their synergistic effects using cellulose and corn stover as the substrates. Their maximum synergy degrees on cellulose and corn stover were, respectively, 1.26 and 1.87 at the ratio of 3:2. Furthermore, co-culture of these anaerobes on the mixture of cellulose and xylan increased ethanol concentration from 21.0 to 40.4 mM with a high cellulose/xylan-to-ethanol conversion rate of up to 20.7%, while the conversion rates of T. stercorarium and A. thermocellus monocultures were 19.3% and 15.2%. The reason is that A. thermocellus had the ability to rapidly degrade cellulose while T. stercorarium co-utilized both pentose and hexose, the metabolites of cellulose degradation, to produce ethanol. The synergistic effect of cellulase systems and metabolic pathways in A. thermocellus and T. stercorarium provides a novel strategy for the design, selection, and optimization of ethanol production from cellulosic biomass through CBP.
Highlights
IntroductionLignocellulosic biomass is the most abundant renewable resource on earth, yet its structural complexity has hampered its exploitation for cellulosic ethanol and biochemicals
Lignocellulosic biomass is the most abundant renewable resource on earth, yet its structural complexity has hampered its exploitation for cellulosic ethanol and biochemicals.Efficient conversion of lignocellulosic biomass to liquid transport fuels such as ethanol is one of the most promising and important methods among alternatives to fossil fuels because of its potential sustainability, security, and rural economic benefits [1]
The results indicated that Acetivibrio thermocellus (At) (72 CAZymes in 29 glycoside hydrolase (GH) families) and Thermoclostridium stercorarium (Ts) (68 CAZymes in 37 GH families) possess almost equal numbers of genes encoding GH family enzymes, but they are significantly different in terms of enzyme type and quantity in each GH family (Table 1)
Summary
Lignocellulosic biomass is the most abundant renewable resource on earth, yet its structural complexity has hampered its exploitation for cellulosic ethanol and biochemicals. Efficient conversion of lignocellulosic biomass to liquid transport fuels such as ethanol is one of the most promising and important methods among alternatives to fossil fuels because of its potential sustainability, security, and rural economic benefits [1]. The economic production of cellulosic ethanol at large scale is still a challenge, such as generation of inhibitors by pre-treatment, expensive enzyme cost and co-utilization of xylose [4–6]. CBP, in a single vessel or reactor with low process complexity, which simultaneously combines lignocellulosic biomass hydrolysis and fermentation of full-spectrum of the liberated sugars, is a promising strategy in energy conversion by reducing biological processing costs and an economical approach to the production of cellulosic ethanol [7,8].
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