Biological lignocellulose solubilization: comparative evaluation of biocatalysts and enhancement via cotreatment

We examined the ability of microorganisms to solubilize cellulosic biomass with no pretreatment save autoclaving under controlled but not industrial conditions.  Glucan solubilization of mid-season harvested switchgrass after 5 days ranged from 24% for Caldicellulosiruptor bescii  to 65% for Clostridium thermocellum, with intermediate values for Clostridium clariflavum, Clostridium cellulolyticum, a thermophilic horse manure enrichment, and simultaneous saccharification and fermentation (SSF) featuring b-glucosidase-supplemented fungal cellulase and yeast.  Solubilization yields were about twice as high for C. thermocellum as compared to fungal cellulase regardless of enzyme loading, the presence or absence of microbes, incubation temperature (for fungal cellulase), substrate particle size, and initial substrate loading; solubilization yields were also about twice as high for mid-season harvested switchgrass than for senescent switchgrass.  The following trends were observed from data drawn from six conversion systems and three substrates: essentially equal fractional solubilization of glucan and xylan, an inert non-carbohydrate fraction, and higher solubilization for three of the four thermophilic cultures tested as compared to the two mesophilic systems tested.  Brief (5 minute) ball milling of solids remaining after fermentation of senescent switchgrass by C. thermocellum roughly doubled carbohydrate solubilization upon reinnoculation as compared to a control without milling.  Greater particle size reduction and solubilization were observed for milling of partially-fermented solids than for unfermented solids.  Our results provide mechanistic insights into biomass deconstruction and also suggest that processing with little or no pretreatment might be possible by combining nature’s best cellulolytic microbes with physical disruption after initiation of fermentation, much as occurs in ruminants.