The three major components of plant biomass, cellulose, hemicellulose and lignin, are highly recalcitrant and deconstruction involves thermal and chemical pretreatment. Microbial conversion is a possible solution, but few anaerobic microbes utilize both cellulose and hemicellulose and none are known to solubilize lignin.Herein, we show that the majority (85%) of insoluble switchgrass biomass that had not been previously chemically treated was degraded at 78 C by the anaerobic bacterium Caldicellulosiruptor bescii.Remarkably, the glucose/xylose/lignin ratio and physical and spectroscopic properties of the remaining insoluble switchgrass were not significantly different than those of the untreated plant material. C. bescii is therefore able to solubilize lignin as well as the carbohydrates and, accordingly, lignin-derived aromatics were detected in the culture supernatants. From mass balance analyses, the carbohydrate in the solubilized switchgrass quantitatively accounted for the growth of C. bescii and its fermentation products, indicating that the lignin was not assimilated by the microorganism. Immunoanalyses of biomass and transcriptional analyses of C. bescii showed that the microorganism when grown on switchgrass produces enzymes directed at key plant cell wall moieties such as pectin, xyloglucans and rhamnogalacturonans, and that these and as yet uncharacterized enzymes enable the degradation of cellulose, hemicellulose and lignin at comparable rates. This unexpected finding of simultaneous lignin and carbohydrate solubilization bodes well for industrial conversion by extremely thermophilic microbes of biomass that requires limited or, more importantly, no chemical pretreatment. Broader contextThe three major components of plant biomass are cellulose (a glucose polymer), hemicellulose (a polymer of xylose and a variety of other sugars) and lignin (a complex polymer of aromatic units). The sugar polymers are potential feedstocks for the production of biofuels by anaerobic microorganisms. However, plant biomass is highly recalcitrant and harsh and inefficient chemical treatments are required to solubilize the biomass and release the sugars. Moreover, no anaerobic microorganism is known that can degrade the highly recalcitrant lignin. Herein it is shown that switchgrass, a model plant for bioenergy production, can be degraded at moderate temperatures (78 C) by an anaerobic bacterium that solubilizes lignin as well as cellulose and hemicellulose. The microorganism produces a range of both known and as yet uncharacterized enzymes that degrade at comparable rates all of the major components of the plant cell wall. Such thermophilic microbes could potentially be developed to enable the direct conversion of plant biomass to biofuels without the need for any chemical pretreatment.

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bCaldicellulosiruptor bescii grows optimally at 78°C and is able to decompose high concentrations of lignocellulosic plant biomass without the need for thermochemical pretreatment. C. bescii ferments both C 5 and C 6 sugars primarily to hydrogen gas, lactate, acetate, and CO 2 and is of particular interest for metabolic engineering applications given the recent availability of a genetic system. Developing optimal strains for technological use requires a detailed understanding of primary metabolism, particularly when the goal is to divert all available reductant (electrons) toward highly reduced products such as biofuels. During an analysis of the C. bescii genome sequence for oxidoreductase-type enzymes, evidence was uncovered to suggest that the primary redox metabolism of C. bescii has a completely uncharacterized aspect involving tungsten, a rarely used element in biology. An active tungsten utilization pathway in C. bescii was demonstrated by the heterologous production of a tungsten-requiring, aldehyde-oxidizing enzyme (AOR) from the hyperthermophilic archaeon Pyrococcus furiosus. Furthermore, C. bescii also contains a tungsten-based AOR-type enzyme, here termed XOR, which is phylogenetically unique, representing a completely new member of the AOR tungstoenzyme family. Moreover, in C. bescii, XOR represents ca. 2% of the cytoplasmic protein. XOR is proposed to play a key, but as yet undetermined, role in the primary redox metabolism of this cellulolytic microorganism.T hermophilic bacteria of the genus Caldicellulosiruptor are currently under intense investigation due to their ability to decompose lignocellulosic plant biomass anaerobically at high temperature, thereby potentially mitigating costly thermochemical pretreatment steps (1, 2). One of these species, Caldicellulosiruptor bescii, has an optimal growth temperature of 78°C and is the most thermophilic cellulose degrader known to date. It is able to ferment high concentrations of cellulosic feedstock primarily to hydrogen gas, lactate, acetate, and CO 2 (3, 4). Species from this genus can degrade cellulose (and also xylan), using novel multidomain glycosyl hydrolases, representing a new paradigm in cellulose conversion by anaerobic thermophiles (2). Moreover, the recent development of a genetic system for C. bescii creates potential for using this and related species for consolidated biomass processing in the production of liquid fuels (5).Developing metabolic engineering strategies for any microorganism obviously requires an in-depth understanding of their primary metabolism. Evidence that C. bescii may have a completely uncharacterized aspect to its primary redox metabolism came from an analysis of its genome sequence for molybdoenzymes (6). These are present in virtually all forms of life, serving diverse roles in primary metabolism of carbon, nitrogen and sulfur (7). As expected, we found that the C. bescii genome contains genes necessary for the synthesis of the pyranopterin cofactor that coordinates molybdenum (Mo) in such enzymes (7). Accordin...

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