Microbial pretreatment makes use of microorganisms and their enzyme systems to breakdown lignin and/or hemicellulose present in lignocellulosic biomass. So far, the isolated and identified lignocellulolytic microor­ganisms mainly include fungi and a few bacterial strains. Fungi including brown-, white-, and soft-rot fungi are the predominant organisms responsible for lignocellulose degradation, and among the fungi, the Basidiomycetes that cause both white and brown rots are the most rapid degraders (Bennet et al., 2002; Loguercio-Leite et al., 2008; Rabinovich et al., 2004; Sanchez, 2009; ten Have and Teunissen, 2001). Several Basidiomycetes such as P. chrysosporium, C. subvermispora, Phlebia subserialis, Pleurotus ostreatus, and Irpex lacteus have been shown to efficiently degrade lignin in different lignocellulosic materials (Hatakka and Usi-Rauva, 1983; Keller et al., 2003; Sawada et al., 1995; Taniguchi et al., 2005; Zeng et al., 2011).

Natural Microorganisms and Practical Applications in Bioconversion

Application of White-Rot Fungus in Treatment of Different Biomasses

CORN STOVER

When corn stover is pretreated with C. subvermispora for downstream bioethanol production, lignin is selec­tively degraded up to 31.59% with a limited cellulose loss of less than 6% during an 18-day pretreatment. Longer pretreatment time was found to increase lignin removal, resulting in correspondingly higher glucose yields from enzymatic hydrolysis. The highest overall ethanol yield of 57.80% was obtained with 35-day — pretreated corn stover (Wan and Li, 2010).

In a later study, the effectiveness of C. subvermispora pretreatment on different types of feedstocks, including corn stover, wheat straw, soybean straw, switchgrass, and hardwood was tested. After an 18-day pretreat­ment, corn stover, switchgrass, and hardwood were effectively delignified, leading to a two — to threefold in­crease in glucose yield over those of the untreated raw materials. In contrast, wheat straw and soybean straw did not show glucose yield increase after undergoing the same pretreatment, suggesting the importance of us­ing a specific strain for pretreatment of specific biomass (Wan and Li, 2011).

Pretreatments of corn stover with the white-rot fungus

I. lacteus CD2 also resulted in significant lignin degrada­tion with limited cellulose loss (Zeng et al., 2011). Pre­treatment of corn stover with Cyathus stercoreus led to a three — to fivefold improvement in enzymatic cellulose digestibility (Keller et al., 2003). Pretreatment of corn sto­ver with a newly isolated white-rot fungus, Trametes hirsuta yj9, led to selective lignin degradation up to 71.49% and a significant increase in enzymatic digestibil­ity of 73.99% after a 42-day pretreatment (Sun et al., 2011). Pretreatment of corn stover fractions (leaves, cobs, and stalks) with the white-rot fungus C. subvermispora showed that the leaves were the least recalcitrant to fungal pretreatment with a 45% lignin degradation as well as higher carbohydrate degradation after 30 days of pretreatment. However, corn cobs produced the high­est sugar yield after fungal pretreatment (Cui et al., 2012).

SOFTWOOD

The effect of pretreatment on the softwood Pinus den- siflora by three white-rot fungi, Ceriporia lacerata, Stereum hirsutum, and Polyporus brumalis, has been investigated. Among the three white-rot fungi tested, S. hirsutum selectively degraded the lignin rather than the holocellu — lose component. Consistently, extracellular enzymes from S. hirsutum showed higher activity of ligninase and lower activity of cellulase than those from the other white-rot fungi. In addition, the available pore size and surface area in the pretreated wood were increased, possibly due to degradation of lignin and a small portion of hemicellulose by the secreted enzymes. Sugar yield of the S. hirsutum pretreated wood also greatly increased compared to a nonpretreated sample, indi­cating S. hirsutum might be a potentially effective fungus for use in biological pretreatment of woody biomass (Lee et al., 2007).