Efficient Degradation of Lignocellulosic Plant Biomass, without Pretreatment, by the Thermophilic Anaerobe “ Anaerocellum thermophilum ” DSM 6725

Abstract

Very few cultivated microorganisms can degrade lignocellulosic biomass without chemical pretreatment. We show here that "Anaerocellum thermophilum" DSM 6725, an anaerobic bacterium that grows optimally at 75°C, efficiently utilizes various types of untreated plant biomass, as well as crystalline cellulose and xylan. These include hardwoods such as poplar, low-lignin grasses such as napier and Bermuda grasses, and high-lignin grasses such as switchgrass. The organism did not utilize only the soluble fraction of the untreated biomass, since insoluble plant biomass (as well as cellulose and xylan) obtained after washing at 75°C for 18 h also served as a growth substrate. The predominant end products from all growth substrates were hydrogen, acetate, and lactate. Glucose and cellobiose (on crystalline cellulose) and xylose and xylobiose (on xylan) also accumulated in the growth media during growth on the defined substrates but not during growth on the plant biomass. A. thermophilum DSM 6725 grew well on first-and second-spent biomass derived from poplar and switchgrass, where spent biomass is defined as the insoluble growth substrate recovered after the organism has reached late stationary phase. No evidence was found for the direct attachment of A. thermophilum DSM 6725 to the plant biomass. This organism differs from the closely related strain A. thermophilum Z-1320 in its ability to grow on xylose and pectin. Caldicellulosiruptor saccharolyticus DSM 8903 (optimum growth temperature, 70°C), a close relative of A. thermophilum DSM 6725, grew well on switchgrass but not on poplar, indicating a significant difference in the biomass-degrading abilities of these two otherwise very similar organisms.

Direct conversion of plant biomass to ethanol by engineered Caldicellulosiruptor bescii

“…2), and indeed no ethanol production is detected from this strain (18). The phylogenetically related, thermophilic Firmicute C. thermocellum, however, encodes an NADH-dependent AdhE (Cthe0423), which is one of the most highly expressed genes (24) and abundant proteins (25) in C. thermocellum and is the key enzyme in ethanol production in this organism.…”

“…C. thermocellum is one promising candidate for consolidated bioprocessing because it is naturally cellulolytic, able to hydrolyze cellulose at 2.5 g·L −1 ·h −1 , and produces ethanol as one fermentation product, but it has not yet been engineered to produce ethanol at high yield and lacks the ability to ferment hemicellulosic sugars (13,17). Caldicellulosiruptor bescii, on the other hand, is the most thermophilic cellulolytic bacterium so far described, growing optimally at ∼80°C with the ability to use a wide range of substrates, such as cellulose, hemicellulose, and lignocellulosic plant biomass without harsh and expensive chemical pretreatment (17,18), efficiently fermenting both C 5 and C 6 sugars derived from plant biomass (17,18). C. bescii uses the EmbdenMeyerhof-Parnas pathway for conversion of glucose to pyruvate, and the predominant end-products are acetate, lactate, and hydrogen ( Fig.…”

“…C. bescii uses the EmbdenMeyerhof-Parnas pathway for conversion of glucose to pyruvate, and the predominant end-products are acetate, lactate, and hydrogen ( Fig. 2) (18). A mutant strain of C. bescii (JWCB018) was recently isolated in which the lactate dehydrogenase gene (ldh) was disrupted spontaneously via insertion of a native transposon (19,20).…”

Metaproteomics of cellulose methanisation under thermophilic conditions reveals a surprisingly high proteolytic activity

“…This taxon contains cellulolytic members, such as C. obsidiansis or C. bescii (formerly Anaerocellum thermophilum). These do not possess cellulosomes, their cellulolytic system being based on secreted multifunctional Metaproteomics of cellulose methanisation F Lü et al Metaproteomics of cellulose methanisationenzymes; they usually utilise a broad range of plant materials, including crystalline cellulose, cellulose, hemicelllose, starch and pectin, with a very high hydrogen yield (van de Werken et al, 2008;VanFossen et al, 2009;Yang et al, 2009;Hamilton-Brehm et al, 2010;Lochner et al, 2011). Among the non-redundant protein groups identified for Caldicellulosiruptor, four were probably related to hemicellulose degradation (two beta-mannanases UniRef50_Q9KWY5, one acetyl xylan esterase UniRef50_F8F4V8 and one endoxylanase UniRef50_ E4Q5G9).…”

Single gene insertion drives bioalcohol production by a thermophilic archaeon

“…Thermoanaerobacter strain X514 was cultivated at 65°C on complex medium used for cultivation of thermophilic heterotrophic anaerobes (modified DSMZ 516 medium) with 5 g·L −1 cellobiose as electron donor (34). P. furiosus (DSM 3638) was routinely grown at the indicated temperatures with 5 g·L −1 maltose and 2 g·L −1 yeast extract as described previously (13).…”