Heat-Tolerant Bacteria Efficiently Degrade Non-Pretreated Biomass

Summary

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.

References

C. bescii (formerly called Anaerocellum thermophilum DSM 6725) findings are from Yang, S. J., et al. 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.

The discovery of C. obsidiansis was reported in Hamilton-Brehm, S. D., et al. 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.