Abstract
BackgroundHigh cost of traditional substrates and formation of by-products (such as acetone and ethanol) in acetone–butanol–ethanol (ABE) fermentation hindered the large-scale production of biobutanol. Here, we comprehensively characterized a newly isolated solventogenic and xylanolytic Clostridium species, which could produce butanol at a high ratio with elimination of ethanol and conversion of acetone to more value-added product, isopropanol. Ultimately, direct butanol production from hemicellulose was achieved with efficient expression of indigenous xylanase by the novel strain via consolidated bioprocessing.ResultsA novel wild-type Clostridium sp. strain NJP7 was isolated and characterized in this study, which was capable of fermenting monosaccharides, e.g., glucose into butanol via a fermentative acetone–isopropanol–butanol pathway. With enhancement of buffering capacity and alcohol dehydrogenase activities, butanol and isopropanol titer by Clostridium sp. strain NJP7 was improved to 12.21 and 1.92 g/L, respectively, and solvent productivity could be enhanced to 0.44 g/L/h. Furthermore, with in situ extraction with biodiesel, the amount of butanol and isopropanol was finally improved to 25.58 and 5.25 g/L in the fed-batch mode. Meanwhile, Clostridium sp. strain NJP7 shows capability of direct isopropanol–butanol production from hemicelluloses with expression of indigenous xylanase. 2.06 g/L of butanol and 0.54 g/L of isopropanol were finally achieved through the temperature-shift simultaneous saccharification and fermentation, representing the highest butanol production directly from hemicellulose.ConclusionThe co-production of isopropanol with butanol by the newly isolated Clostridium sp. strain NJP7 would add on the economical values for butanol fermentation. Furthermore, the high isopropanol-butanol production with in situ extraction would also greatly enhance the economic feasibility for fermentative production of butanol–isopropanol in large scale. Meanwhile, its direct production of butanol–isopropanol from polysaccharides, hemicellulose through secretion of indigenous thermostable xylanase, shows great potential using lignocellulosic wastes for biofuel production.
Highlights
High cost of traditional substrates and formation of by-products in acetone–butanol–ethanol (ABE) fermentation hindered the large-scale production of biobutanol
Since xylanase activities could be determined by the size of orange digestion halos formed on xylan plates using Congo red staining, violet colonies were picked and inoculated individually to reduced mineral medium spiked with 30 g/L of glucose for butanol production
By detecting with GC-flame ionization detector (FID) and HPLC, the main metabolic products were identified as acetone, isopropanol, butanol, acetic acid, and butyric acid, indicating that strain NJP7 synthesizes biobutanol via a fermentative acetone–isopropanol–butanol (AIB) pathway
Summary
High cost of traditional substrates and formation of by-products (such as acetone and ethanol) in acetone–butanol–ethanol (ABE) fermentation hindered the large-scale production of biobutanol. We comprehensively characterized a newly isolated solventogenic and xylanolytic Clostridium species, which could produce butanol at a high ratio with elimination of ethanol and conversion of acetone to more value-added product, isopropanol. Butanol can be produced anaerobically by several solventogenic bacteria belonging to the genus of Clostridium. The most extensively studied species are C. acetobutylicum and C. beijerinckii, which produce butanol, acetone, and ethanol with a typical ratio of 6:3:1 in a so-called acetone–butanol–ethanol (ABE) fermentation process [1, 2]. Solventogenic Clostridium species have been used for large-scale ABE fermentation, butanol production is still considered less economical than ethanol fermentation using yeasts. Disruption of the genes responsible for acetone formation led to less acetone production, and decreased butanol production with the accumulation of volatile acids (e.g., acetate and butyrate) [2,3,4]
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