Thermophilic bacteria can degrade woody plant biomass from poplar and switchgrass, without any high-temperature chemical treatment-a key step toward developing renewable petroleum substitutes, according to Michael Adams of the University of Georgia, Athens and the Oak Ridge National Laboratory in Oak Ridge, Tenn., and his collaborators there and at the National Renewable Energy Laboratory (NREL) in Golden, Colo. Details are published in the July Applied and Environmental Microbiology (75:4762-4769).

The challenge of deconstructing cellulose and lignocellulose lies in their physical and chemical properties, which make these materials "recalcitrant" to the degradative efforts of microorganisms and insects such as termites. For instance, cellulose's crystalline structure renders it insoluble, a deterrent to the degradative effects of enzymes that work in aqueous solutions. Further, even chemically treated lignocellulose requires 100-fold more enzyme to be degraded into component sugars than does starch, according to experts at NREL.

Nonetheless, Anaerocellum thermophilum DSM 6725, which is active at 75°C, degrades switchgrass, metabolizing both crystalline cellulose and xylan. Although other microorganisms metabolize cellulose and xylan, the authors describe their report as the first to document amicroorganism degrading switchgrass. "The organism grew just as well on the insoluble biomass after we had removed any soluble material," Adams says. It "also grew on what is termed ‘spent' biomass, which is the insoluble biomass left over after the organism has already grown to a high cell density."

To more fully degrade these recalcitrant materials, the researchers subject them to successive inoculation and culturing, an approach that helps to overcome a stalling out that occurs with diminishing returns in terms of completing the degradative process, according to Adams. This plateau effect might be due to quorum sensing, he says. In any case, after the third round of culture, A. thermophilum had converted more than 60% of the original switchgrass into soluble material.

Adams and his collaborators are identifying the enzymes and other proteins that partake in this degradative process. "The organism appears to use free enzymes, and does not produce a so-called cellulosome, the large extracellular multiprotein complex that degrades cellulose in some other anaerobic bacteria," he says. Meanwhile, a team at the Department of Energy Joint Genome Institute (JGI) in Walnut Creek, Calif., determined the genomic sequence of A. thermophilum.

Those enzyme-driven degradative steps are part of the early phase of an anticipated process for making petroleum substitutes. The breakdown products from biomass, including hydrogen, acetate, lactate, glucose, and cellobiose need to be converted to something else before they could serve as a substitute for gasoline. Toward that end, the research is "positive . . . although more incremental than breakthrough," says James D. McMillan of NREL, who was not involved in the research. For example, the genes or genetic pathways embodied in A. thermophilum might be used to develop strains that could not only degrade biomass but also produce a substitute fuel such as ethanol or some other alcohol, he says.

Another shortcoming with A. thermophilum at this point is that it is not effective unless the biomass it digests is first milled into particles of less than 1 mm in diameter, which is "somewhat expensive," McMillan notes. Moreover, these experiments were "done on extremely dilute suspensions of particles," containing 0.5% weight per volume, whereas a commercial process could be expected to involve suspensions that are at least 20 times more concentrated.

This research remains in the "discovery" phase, and has not reached a "proof of concept of practical applicability," McMillan says. Nonetheless, it could point the way to producing commercially viable fuels from biomass crops, including poplar trees and switchgrass that could be grown on marginal lands that are unsuited for food crops.

David Holzman
David Holzman is the Microbe Journal Highlights Editor.

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