A Combined Biological Pretreatment Of Wheat Straw Using Native Wood-Rotting Fungi For Improving Its Biodegradability

Abstract

Lignocellulosic biomass represents a highly abundant feedstock for the production of chemical compounds (organic acids, xylitol, furfural, cresols, catechol, among others) and biofuels (bioethanol, biomethane, biohydrogen, among others). Lignocellulosic biomass is basically composed of cellulose, hemicellulose, and lignin. Cellulose is a major fraction (35-50% dry mass) of lignocellulosic biomass, which can be converted into glucose through enzymatic hydrolysis, and can subsequently be fermented into ethanol. However, lignocellulosic biomass is highly recalcitrant to enzymatic hydrolysis, because cellulose is sheathed by lignin and hemicellulose; this physical barrier hinders enzyme access to cellulose during hydrolysis. In addition, cellulose fibers show highly resistant crystalline regions. Therefore, a feasible process for obtaining cellulose-derived products from lignocellulosic biomass requires a pretreatment step to increase cellulose accessibility and biodegradability. In biological pretreatment, white-rot fungi have been used for their ability to selectively degrade lignin, whereas brown-rot fungi have been used for their ability to cause extensive degradation and depolymerization of cellulose and hemicellulose via Fenton reactions, with minimal assimilation of degradation products in early decay stages. For that reason, the main objective of this thesis was to evaluate a wheat straw pretreatment, comprising the combined action of white-rot fungus, Ganoderma lobatum, and brown-rot fungus, Gloeophyllum trabeum, for improving its biodegradability. These fungi were selected in a preliminary screening of 14 Chilean strains of wood-rotting fungi. In a first step, the effects of metal ions, Fe2+ and Mn2+, as enzyme inducers and Fenton reactants, and NO3- as additional nitrogen source, were evaluated in order to find optimal culture conditions to promote an adequate wheat straw degradation by the application of a fungal pretreatment. In general, Mn2+ had the strongest positive effect on lignin degradation by G. lobatum, whereas Fe2+ had the strongest positive effect on decreasing total crystallinity index of wheat straw by G. trabeum. NO3- decreased the weight loss caused by both tested fungi during wheat straw degradation. Afterwards, the effect of pretreatment by single-fungus-culture under optimal culture conditions was evaluated on enzymatic hydrolysis of wheat straw. The highest glucose yields from enzymatic hydrolysis were detected in pretreated wheat straw, by G. lobatum and G. trabeum single-cultures for 20 and 10 days of incubation, which increased the yields by 43.6% and 26.1% compared to untreated wheat straw, respectively. Based on these results, the effect of fungal co-culture was evaluated using different inoculation strategies, including co-inoculation and sequential inoculation. Co-culture using co-inoculation, both fungi were simultaneously inoculated in wheat straw and incubated for 20 days, resulted in higher glucose yield (70% higher than untreated wheat straw) when compared with single cultures, but no synergistic effect between fungi was observed. In contrast, the sequential inoculation of G. lobatum, incubated for 10 days, followed by G. trabeum, incubated for 10 more days, showed a synergic effect on enzymatic hydrolysis. The synergistic effect between G. lobatum and G. trabeum observed in co-culture using sequential inoculation was mainly related to their abilities to synergistically degrade lignin and modify the crystalline regions in the wheat straw lignocellulosic matrix, respectively. This co-culture showed the highest glucose yield (191.5 mg glucose g-1 wheat straw), which was 2.8-fold higher than untreated wheat straw. Although the glucose yield was higher than other biological pretreatments previously reported, it is still not comparable with the currently physicochemical methods. Nevertheless, new studies have been focusing on the use of biological pretreatments as a complementary step to physicochemical pretreatments in order to increase its efficiency. In this sense, a biological pretreatment with high glucose yield is a good alternative as a complementary method.