(485bn) Characterization of biofuel-producing microbes isolated from the Homestake Gold mine

Authors: 
Dixon, D. J., SD School of Mines and Technology
Hughes, S. R., USDA-ARS, NCAUR
Bischoff, K., USDA-ARS, NCAUR
Sani, R. K., South Dakota School of Mines & Technology


Lignocellulosic biomass, an abundant and renewable carbon source, has the potential to replace starch as a feedstock for the fermentative production of fuels including ethanol, organic acids. Extensive use of this feedstock is problematic, because hydrolysis of cellulose and hemicellulose yields a mixer of hexoses and pentoses, and commercial fermentation biotechnology is based on a relatively small number of microorganisms that are generally limited in their ability to utilize the pentose sugars. In this study we, therefore, collected soil samples from 1.5 km deep subsurface of Homestake Gold mine (known as Deep Underground Science and Engineering Laboratory - DUSEL). Soil samples were enriched specifically for microorganisms capable of fermenting xylose and glucose. After enrichment, xylose- and glucose-fermenting microbes were isolated and characterized using culture-independent molecular phylogenetic analysis (16S rRNA gene). Single-substrate kinetics of selected isolates indicated that the specific growth rates of all the bacteria using xylose as a substrate were greater than on glucose as a substrate. Ethanol was the major fermentation product of these novel isolates, with a yield of 0.61 g/g consumed xylose-1, which is close to the theoretical yield of 0.72 g/g consumed xylose-1. Despite the formation of the by-products (acetic acid, succinic acid and lactic acid), one of xylose fermenting strain produced twice as much ethanol (2.86 g/l from 5 g/l initial concentration of substrate) within 68 hours as a result of the higher xylose consumption rate. Due to increased demand of biodiesel, microbial conversion of glycerol to 1-3 Propanediol (PDO, a monomer to produce polypropylene terephthalate) has recently attracted more attention. Therefore, in addition to xylose and glucose fermenting bacteria, soil samples were also enriched for glycerol fermenting bacteria. Three isolates were characterized using 16S rRNA gene analyses, and found distantly related to Clostridium genus. Batch experiments showed the PDO was major fermented product with yields between 0.63 and 0.65 mole PDO per mole glycerol. Our data suggest that these newly isolated bacterial strains have the potential to be developed as biocatalysts for the conversion of agricultural residues into fuels and other value-added fermentation products.