Heat-Tolerant Bacteria Efficiently Degrade Non-Pretreated Biomass

The Science

Presenting the possibility of eliminating the pretreatment step from cellulosic biofuel production, a hot springs bacterium known as Caldicellulosiruptor bescii has shown that it can efficiently degrade crystalline cellulose, xylan (a hemicellulose), and various types of non-pretreated biomass including hardwoods such as poplar, high-lignin grasses such as switchgrass, and low-lignin grasses such as Bermuda grass. With an optimal growth temperature of 75°C, C. bescii was able to break down 65% of switchgrass biomass without pretreatment. This bacterium is the most heat-tolerant biomass degrader known (withstanding temperatures up to 90°C), and it primarily produces hydrogen as an end product when grown on plant biomass. BESC researchers have discovered another hot springs bacterium (Caldicellulosiruptor obsidiansis), isolated from Yellowstone National Park, that thrives at 78°C and can ferment all the simple sugars in cell-wall polysaccharides into diverse products including ethanol. Combining the functional capabilities of C. bescii and C. obsidiansis theoretically could yield organisms that both deconstruct and ferment plant biomass at temperatures above the boiling point of ethanol (78.4°C). Producing ethanol in the vapor phase could greatly reduce the inhibitory effects of ethanol on cell growth.

The Impact

Very few cultivated microorganisms can degrade lignocellulosic biomass without chemical pretreatment. Plant feedstocks vary considerably in their compositions. The main components of plant biomass and the sources of the fermentable sugars, cellulose and hemicellulose, are combined with lignin, which can occupy 20% (wt/wt) or more of the plant cell wall. The development of technologies to efficiently degrade plant biomass therefore faces considerable obstacles. The discovery or engineering of new microorganisms with the ability to convert the components of lignocellulosic biomass into sugars is therefore of high priority.

Summary

A. thermophilum DSM 6725 has the ability to grow on plant biomass with a high lignin content and high crystallinity of cellulose; it is insensitive to inhibitors present in poplar biomass; its cells remain vital and produce hydrogen, which is an alternative biofuel to ethanol, for prolonged periods (20 days); it is able to hydrolyze highly crystalline cellulose almost completely with glucose and cellobiose as major products; and it grows on spent biomass efficiently. These unique properties might be of utility in any applied biomass conversion process.

Principal Investigator

James G. Elkins
Oak Ridge National Laboratory

Co-Principal Investigator

Michael W. W. Adams
Oak Ridge National Laboratory

References

Yang, S. J., I. Kataeva, S. D. Hamilton-Brehm, N. L. Engle, T. J. Tschaplinski, C. Doeppke, M. Davis, J. Westpheling, and M. W. W. Adams. 2009. “Efficient Degradation of Lignocellulosic Plant Biomass, Without Pretreatment, by the Thermophilic Anaerobe ‘Anaerocellum thermophilum’ DSM 6725,” Applied and Environmental Microbiology 75(14), 4762–69. DOI:10.1128/AEM.00236-09.

Hamilton-Brehm, S. D., J. J. Mosher, T. Vishnivetskaya, M Podar, S. Carroll, S. Allman, T. J. Phelps, M. Keller, and J. G. Elkins. 2010. “Caldicellulosiruptor obsidiansis sp. nov., an Anaerobic, Extremely Thermophilic, Cellulolytic Bacterium Isolated from Obsidian Pool, Yellowstone National Park,” Applied and Environmental Microbiology 76(4), 1014–20. DOI:10.1128/AEM.01903-09.