Lignocellulosic biomass, such as switchgrass, is an abundant renewable source of energy which can be used to make biofuels. Conversion of lignocellulose to biofuel requires an initial pretreatment step which degrades/depolymerizes the lignin and exposes the plant polysaccharides to hydrolytic enzymes. The released sugars are then fermented to biofuel. In nature, lignin degradation is the result of combined action of fungi and bacteria. However, bacteria and fungi cocultures have not been much explored for the pretreatment of plant biomass. The goal of present study was to evaluate the feasibility of using cocultures of bacteria and fungi for the pretreatment of switchgrass. Three previously isolated lignin-degrading bacterial strains; Pseudomonas sp. YS-1p, Arthrobacter sp. RT-1, and Alcaligenes sp. 3K, and two fungal strains Phanerochaete chrysosporium RP78 and Myceliophthora thermophila M77 were selected for this study. The pretreatment of switchgrass was performed in 250 ml flasks containing 6.7% switchgrass inoculated with bacteria alone, fungi alone, and a co-culture of fungi and bacteria and the flasks were incubated at 37 0C for 42 days. Population dynamics monitored using qPCR of the culture samples collected periodically during incubation exhibited both synergistic and antagonistic interactions among microorganisms. Proteome analysis of culture supernatants at the end of experiment (42 days) revealed the presence of large number of carbohydrates degrading enzymes in the cultures inoculated with only fungi and bacteria+fungi. In addition, laccase and few accessory enzymes related to lignin degradation, and some carbohydrate esterases involved in breaking linkages between lignin and carbohydrates were also found. On the other hand, proteome of culture supernatants inoculated with bacterial strains had lignin degradation and aromatic ring degradation related enzymes, and relatively fewer carbohydrate degrading enzymes. To determine the saccharification ability of the microbe-pretreated switchgrass, we hydrolyzed switchgrass with commercially available cellulases. Interestingly, our results showed that switchgrass treated with bacterial strains resulted in better saccharification compared to fungi treated switchgrass and the switchgrass treated with co-culture of bacteria and fungi. We believe this could have been due to the inhibition of cellulase enzymes in later two pretreatment systems. This work shows the ability of bacterial cocultures to be used for the pretreatment of lignocellulosic biomass. Future research may unveil the bacterial communities with stronger lignin degradation ability for better pretreatment of biomass.
Hadia Mukhtar (Wed,) studied this question.