Screening of fungi capable of highly selective degradation of lignin in rice straw

Abstract

In order to screen for novel fungus with powerful delignification capability on rice straw, fungal samples found on woody surfaces were collected from several regions in China and were isolated and screened for their lignin-degrading properties on rice straw. After the individual isolates were cultivated on rice straw as the sole carbon substrate for 10 days, gravimetric fiber extraction analysis was used to determine the change in holocellulose and lignin content. Isolate 812 exhibited a high degree of lignin degradation at 34.7% while maintaining low holocellulose degradation at 2.1%, which performed better than the traditional white-rot fungus Phanerochaete chrysosporium with 28.3% lignin degradation and 28.4% holocellulose degradation. Isolate 812 was identified as Fusarium moniliforme based on its 18S rDNA gene sequence and its morphological characteristics. Ligninolytic enzyme assay indicated that the fungus produced lignin peroxidase and manganese peroxidase during solid-state and liquid fermentation. Above results indicated that we had screened a F. moniliforme isolate with highly efficient delignification capability and very slight damage to holocellulose. The newly isolated fungus could be used in biodegradation of lignocellulosic feedstock.

Introduction

Energy plays an essential role in a modern economy and protecting a sustainable energy security has become a major policy of China. According to the 2009 Annual Bioindustry Report in China, 92% of China's energy came from fossil fuel in 2008 (Zhang, 2010). The call for ‘green energy’ comes amidst an impending depletion of traditional fossil fuels and rising oil prices. With measures aimed at reducing the fossil fuel dependency (by increasing non-fossil fuel consumption to 15% by 2020 in China), the research spotlight centers around renewable resources and its energy derivatives. For the purpose of this research, China's abundant stock of rice straw (IRRI, 2011) will be the renewable biomass of interest.

To utilize the energy existed in rice straw, a process called delignification is required in order to break down the lignocellulose structure and provide access for enzymatic saccharification of the cellulosic component. Lignocellulose refers to the lignin–cellulose complex found in plants and is the reason for plants' rigid characteristics. Technologies for the conversion of cellulose to ethanol are already plentiful (Tengerdy and Szakacs, 2003; Shrestha et al., 2008; Rasmussen et al., 2010; Wan and Li, 2010). Other products such as methane and butanol are also possible (Fernando et al., 2006; Zheng et al., 2010). However, it is the lignin component of the lignin–cellulose complex that makes enzymatic saccharization of the cellulose difficult. Chemical and physicochemical pretreatment methods such as strong acid and steam explosion, respectively, break the lignin structure for enzymatic contact with the cellulose. However, such methods also bring about higher operational costs and hazardous waste (Sun and Cheng, 2002). Hence biological pretreatment, or biodegradation, serves as an attractive option that is both energy-saving and environmentally friendly (Scott et al., 1998). Therefore, this research will be focusing on the use of biological degradation through the use of lignin-degrading species of fungi.

The use of fungi for biodegradation has already been investigated for several decades, with the major player being of the white-rot species (e.g. Phanerochaete chrysosporium) (Eriksson and Kirk, 1985; Arora et al., 2002; Taniguchi et al., 2005; Zhang et al., 2007; Shi et al., 2008). The problem that persists today is the slow cultivation of the fungal species on wood and the sensitivity to growth conditions. These wood-rotting fungi employ a ligninolytic system that differs between different species in that the enzymes involved may either be one or the combination of the following: lignin peroxidases (LiP), manganese peroxidases (MnP), and laccase (Leonowicz et al., 1999; ten Have and Teunissen, 2001). The effectiveness of such a system of extracellular enzymes may also be dependent on the type of used substrate, such as wood or agricultural straws, owing to the differences in chemical compositions and structure of wood and straw (Kirk and Cullen, 1998; Li et al., 2008). Therefore, more research can be made to screen for highly selective isolates of fungi capable of degrading rice straw.

In this paper, 57 isolates obtained from several regions in China were screened for effective degradation of lignin in rice straw. Degradation was studied under solid-state fermentation (hereafter referred to as SSF). Enzyme activity under SSF and liquid fermentation were also studied.