Structural change in wood by brown rot fungi and effect on enzymatic hydrolysis

Abstract

The effects of biological pretreatment on Pinus radiata and Eucalyptus globulus, were evaluated after exposure to two brown rot fungi Gloephylum trabeum and Laetoporeus sulphureus. Changes in chemical composition, structural modification, and susceptibility to enzymatic hydrolysis in the degraded wood were analyzed. After eight weeks of biodegradation, the greatest loss of weight and hemicellulose were 13% and 31%, respectively, for P. radiata with G. trabeum. The content of glucan decreased slightly, being the highest loss of 20% for E. globulus with G. trabeum. Consistent with degradation mechanism of these fungi, lignin was essentially undegraded by both brown rot fungi. Both brown rot fungi cause a sharp reduction in the cellulose degree of polymerization (DP) in the range between 58% and 79%. G. trabeum depolymerized cellulose in both wood faster than L. sulphureus. Also, structural characteristic of crystalline cellulose were measured by using two different techniques – X-ray diffraction (XRD) and infrared spectroscopy (FT-IR). The biological pretreatments showed an effect on cellulose crystallinity structure, a decrease between 6% and 21% was obtained in the crystallinity index (CrI) calculated by IR, no changes were observed in the XRD. Material digestibility was evaluated by enzymatic hydrolysis, the conversion of cellulose to glucose increased with the biotreatment time. The highest enzymatic hydrolysis yields were obtained when saccharification was performed on wood biopretreated with G. trabeum (14% P. radiata and 13% E. globulus). Decreasing in DP and CrI, and hemicellulose removal result in an increase of enzymatic hydrolysis performance. Digestibility was better related to DP than with other properties. G. trabeum can be considered as a potential fungus for biological pretreatment, since it provides an effective process in breaking the wood structure, making it potentially useful in the development of combined pretreatments (biological–chemical). A viable alternative to pretreatment process that can be used is a bio-mimetic system, similar to low-molecular complexes generated by fungi such as G. trabeum combined pretreatments (biological–chemical).

Introduction

Lignocellulosic biomass is particularly abundant in nature and has a high potential for bioconversion. Lignocellulosic biomass offers a sustainable alternative to fossil fuels as source of carbon. Every year, large accumulation of this biomass results not only in deterioration of the environment but, also in loss of potentially valuable materials that can be processed for the production of biofuels, energy, and value-added chemicals [1], [2].

Cellulose, lignin and hemicellulose, the key components of lignocellulosic biomass, are closely associated with each other at the plant cell level. This close association and the partly cellulose crystalline nature, reduces cellulose reactivity towards enzymatic hydrolysis in biomass [3]. Thus, lignocellulose digestibility is affected by physical and chemical structural features [4], [5]. Making pretreatments, in order to break biomass structural features, necessary to enhance biomass digestibility. The effects of the pretreatment have been described as a disruption of the cell-wall matrix including the connection between carbohydrates and lignin, depolymerizing and solubilizing hemicellulose polymers, as well as changing the degree of cellulose crystallinity. These effects make cellulose more accessible to the enzymes that convert carbohydrate polymers into fermentable sugars [6], [7], [8].

In biological pretreatments, wood-destroying microorganisms that attack naturally wood, degrading lignin and holocellulose, are allowed to grow on the biomass, producing lignin–holocellulose complex breaking. The most serious kind of microbiological decay of wood is caused by fungi as they can cause rapid structural failure. Brown rot fungi are the most destructive type of wood decay [9]. These fungi can be naturally breaking down the highly ordered cellulose crystalline structure [9], [10]. Brown rot fungi preferentially degrade wood polysaccharides, and partially oxidize lignin [11]. They degrade holocellulose causing a rapid decrease in degree of polymerization with a low mass loss, and degradation products are produced faster than they are utilized [9], [12]. In the initial step of wood decay, enzymes are known not to be involved due their size that is larger than pores of the cell wall. Brown rot fungi employ a low-molecular-weight biochemical agent, which is capable of penetrating cell-wall pores, depolymerizing cellulose, thus making it accessible to further degradation [13]. Thus, the initial depolymerization is caused by producing small, diffusible, extracellular oxidants (free radicals), operating at a distance from the hyphae [11], [14]. This initial stage of decay involves the action of extracellular Fenton system (Fe²⁺ + H₂O₂) to generate hydroxyl radical (OH) [11].

Wood strength decays rapidly due to brown rot fungi initial attack. The purpose in this present study is evaluating the effect of the structural changes in wood by Gloephylum trabeum and Laetoporeus sulphureus on enzymatic hydrolysis of Pinus radiata and Eucalyptus globulus.