Abstract
The fiber in corn kernels, currently unutilized in the corn to ethanol process, represents an opportunity for introduction of cellulose conversion technology. We report here that Clostridium thermocellum can solubilize over 90% of the carbohydrate in autoclaved corn fiber, including its hemicellulose component glucuronoarabinoxylan (GAX). However, Thermoanaerobacterium thermosaccharolyticum or several other described hemicellulose-fermenting thermophilic bacteria can only partially utilize this GAX. We describe the isolation of a previously undescribed organism, Herbinix spp. strain LL1355, from a thermophilic microbiome that can consume 85% of the recalcitrant GAX. We sequence its genome, and based on structural analysis of the GAX, identify six enzymes that hydrolyze GAX linkages. Combinations of up to four enzymes are successfully expressed in T. thermosaccharolyticum. Supplementation with these enzymes allows T. thermosaccharolyticum to consume 78% of the GAX compared to 53% by the parent strain and increases ethanol yield from corn fiber by 24%.
Highlights
The fiber in corn kernels, currently unutilized in the corn to ethanol process, represents an opportunity for introduction of cellulose conversion technology
C. thermocellum is unable to ferment most of the carbohydrate present in hemicellulose, which represents a large portion of lignocellulosic biomass
We evaluate the performance of C. thermocellum with and without coculture partners in fermenting corn fiber
Summary
The fiber in corn kernels, currently unutilized in the corn to ethanol process, represents an opportunity for introduction of cellulose conversion technology. To identify the corn fiber carbohydrates solubilized but not utilized by C. thermocellum in monoculture and in coculture with T. saccharolyticum or T. thermosaccharolyticum, the fermentation broths were analyzed by NMR spectroscopy and MALDI-TOF mass spectrometry. The side chain 1 was identified as the disaccharide β-D-Xylp-(1,2)-α-LAraf-(1,3) and represented 5% of the total carbohydrate in the TS-Coculture Broth (Fig. 1, Supplementary Table 1).
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