Simultaneous fermentation of glucose and xylose to butanol by Clostridium sp. strain BOH3.

Abstract

Cellulose and hemicellulose constitute the major components in sustainable feedstocks which could be used as substrates for biofuel generation. However, following hydrolysis to monomer sugars, the solventogenic Clostridium will preferentially consume glucose due to transcriptional repression of xylose utilization genes. This is one of the major barriers in optimizing lignocellulosic hydrolysates that produce butanol. Unlike studies on existing bacteria, this study demonstrates that newly reported Clostridium sp. strain BOH3 is capable of fermenting 60 g/liter of xylose to 14.9 g/liter butanol, which is similar to the 14.5 g/liter butanol produced from 60 g/liter of glucose. More importantly, strain BOH3 consumes glucose and xylose simultaneously, which is shown by its capability for generating 11.7 g/liter butanol from a horticultural waste cellulosic hydrolysate containing 39.8 g/liter glucose and 20.5 g/liter xylose, as well as producing 11.9 g/liter butanol from another horticultural waste hemicellulosic hydrolysate containing 58.3 g/liter xylose and 5.9 g/liter glucose. The high-xylose-utilization capability of strain BOH3 is attributed to its high xylose-isomerase (0.97 U/mg protein) and xylulokinase (1.16 U/mg protein) activities compared to the low-xylose-utilizing solventogenic strains, such as Clostridium sp. strain G117. Interestingly, strain BOH3 was also found to produce riboflavin at 110.5 mg/liter from xylose and 76.8 mg/liter from glucose during the fermentation process. In summary, Clostridium sp. strain BOH3 is an attractive candidate for application in efficiently converting lignocellulosic hydrolysates to biofuels and other value-added products, such as riboflavin.