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 degrees 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 degrees 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 degrees 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.

FIG. 1.

Growth of A. thermophilum DSM 6725 on crystalline cellulose and xylan. Cell growth on unprocessed crystalline cellulose (solid symbols) and xylan (open symbols) was monitored by measuring cell density (circles) and pH (triangles) (A) and hydrogen (squares), lactate (triangles), and acetate (circles) (B).

FIG. 2.

Growth of A. thermophilum DSM 6725 on unprocessed switchgrass and poplar. Cell growth on unprocessed switchgrass (solid symbols) and poplar (open symbols) was monitored by measuring cell density (circles) and pH (triangles) (A) and hydrogen (squares), lactate (triangles), and acetate (circles) (B).

FIG. 3.

Utilization of the insoluble forms of poplar, switchgrass, xylan, and crystalline cellulose by A. thermophilum DSM 6725. The amounts of substrate remaining after cell growth on the insoluble forms of poplar (P; diamonds), switchgrass (S; squares), xylan (X; circles), and crystalline cellulose (C; triangles) were determined by dry weight.

FIG. 4.

End product analyses after prolonged growth of A. thermophilum DSM 6725 on the insoluble forms of poplar, switchgrass, and crystalline cellulose. Hydrogen (solid symbols) and lactate (open symbols) in cultures grown on the insoluble forms of poplar (squares), switchgrass (triangles), and crystalline cellulose (circles) were measured. H, hydrogen; L, lactate; P, poplar; S, switchgrass; C, crystalline cellulose.

FIG. 5.

Growth of A. thermophilum DSM 6725 on unspent, first-spent, and second-spent insoluble switchgrass (A) and insoluble poplar (B). Cells were grown on unspent (0; squares), first-spent (1; triangles), and second-spent (2; circles) switchgrass or poplar.

FIG. 6.

Growth of A. thermophilum DSM 6725 and C. saccharolyticus DSM 8903 on insoluble and soluble fractions of switchgrass (A) and poplar (B). A. thermophilum (solid symbols) and C. saccharolyticus (open symbols) were grown on the insoluble (triangles) and soluble (squares) fractions of switchgrass or poplar.