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Abstract
Background Feedstock recalcitrance is the most important barrier impeding cost-effective production of cellulosic biofuels. Pioneer commercial cellulosic ethanol facilities employ thermochemical pretreatment and addition of fungal cellulase, reflecting the main research emphasis in the field. However, it has been suggested that it may be possible to process cellulosic biomass without thermochemical pretreatment using thermophilic, cellulolytic bacteria. To further explore this idea, we examine the ability of various biocatalysts to solubilize autoclaved but otherwise unpretreated cellulosic biomass under controlled but not industrial conditions.ResultsCarbohydrate solubilization of mid-season harvested switchgrass after 5 days ranged from 24 % for Caldicellulosiruptor bescii to 65 % for Clostridium thermocellum, with intermediate values for a thermophilic horse manure enrichment, Clostridium clariflavum, Clostridium cellulolyticum, and simultaneous saccharification and fermentation (SSF) featuring a fungal cellulase cocktail and yeast. Under a variety of conditions, solubilization yields were about twice as high for C. thermocellum compared to fungal cellulase. Solubilization of mid-season harvested switchgrass was about twice that of senescent switchgrass. Lower yields and greater dependence on particle size were observed for Populus as compared to switchgrass. Trends observed from data drawn from six conversion systems and three substrates, including both time course and end-point data, were (1) equal fractional solubilization of glucan and xylan, (2) no biological solubilization of the non-carbohydrate fraction of biomass, and (3) higher solubilization for three of the four bacterial cultures tested as compared to the fungal cellulase system. Brief (5 min) ball milling of solids remaining after fermentation of senescent switchgrass by C. thermocellum nearly 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. Physical disruption of cellulosic feedstocks after initiation of fermentation, termed cotreatment, warrants further study.ConclusionsWhile the ability to achieve significant solubilization of minimally pretreated switchgrass is widespread, a fivefold difference between the most and least effective biocatalyst—feedstock combinations was observed. Starting with nature’s best biomass-solubilizing systems may enable a reduction in the amount of non-biological processing required, and in particular substitution of cotreatment for pretreatment.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0412-y) contains supplementary material, which is available to authorized users.
Highlights
Feedstock recalcitrance is the most important barrier impeding cost-effective production of cellulosic biofuels
Lynd and Grethlein showed that hydrolysis of dilute acid pretreated hardwood with cell-free broth from the thermophilic anaerobic bacterium Clostridium thermocellum was similar to previous reports with T. reesei cellulase [5]
Solubilization of mid-season switchgrass at 100 % elongation was assessed in batch serum vials for Caldicellulosiruptor bescii cultivated at 75 °C; Clostridium thermocellum, Clostridium clariflavum, and an enrichment from horse manure cultivated at 60 °C; Clostridium cellulolyticum cultivated at 35 °C; and simultaneous saccharification and fermentation (SSF) featuring a fungal cellulase cocktail (Novozyme Ctec2 supplemented with Htec2) and Saccharomyces cerevisiae incubated at 37 °C
Summary
Feedstock recalcitrance is the most important barrier impeding cost-effective production of cellulosic biofuels. Mediated processing of cellulosic biomass is a promising route to sustainable production of fuels and chemicals, but requires improved approaches to producing soluble intermediates from this recalcitrant feedstock. By the late 80s, hundreds of papers on hydrolysis of wood using fungal cellulase and various thermochemical pretreatments had appeared in the literature. At this time, Lynd and Grethlein showed that hydrolysis of dilute acid pretreated hardwood with cell-free broth from the thermophilic anaerobic bacterium Clostridium thermocellum was similar to previous reports with T. reesei cellulase [5]. Solubilization is thought to be enhanced by the alternating microbial solubilization and mechanical disruption that occurs during rumination [11, 12]
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