Microbial enzyme systems for biomass conversion: emerging paradigms

323 ISSN 1759-7269 10.4155/BFS.09.25 © 2010 Future Science Ltd †Author for correspondence 1Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA 2Bioenergy Science Center (BESC), Oak Ridge National Laboratory, TN 37830, USA 3Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Ramat Aviv 69978, Israel 4Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel Tel.: +972 8 934 2373; Fax: +972 8 946 8256; E-mail: [email protected] Biofuels derived from lignocellulose, the most abundant source of organic material on our planet, are an attractive alternative to current petroleum-based fuels, due to their potential for sustainability as well as reduction of greenhouse gas emissions. Plants have evolved mechanisms over millennia to protect the structural forms of polysaccharides from which their cell walls are comprised. Therefore, only a small fraction of microorganisms possess the ability to degrade cellulose efficiently. Fungi and bacteria are the dominant micro organisms responsible for lignocellulose degradation in the biosphere. These microbes show significant diversity in their surviving environments and can be found in mesophilic as well as thermophilic ecosystems where plant matter is abundant, such as forest and pasture soils, hot spring pools and decaying plant debris (Table 1) [1]. In these decay communities, degradation of the plant cell wall is accomplished by complex suites of hydrolytic enzymes that all cellulolytic microbes secrete outside of their cell wall. In this article, we will review the major paradigms of microbial enzyme systems for biomass conversion. Lignocellulosic biomass: an abundant resource Lignocellulosic biomass can be defined as crop residues, short rotation transgenic trees (e.g., poplars), woody grasses (e.g., switchgrass), forestry waste, construction waste, waste from pulp and paper production, agricultural residues and municipal solid waste. A recent study by Oak Ridge National Laboratory (ORNL) determined that 1.3 billion tons of lignocellulosic biomass was theoretically available in the USA alone [2]. Although some challenges remain for biomass-based biofuels processes, the sheer availability of biomass does not appear to be a limiting factor. Rather, the high cost of conversion is recognized as the major deterrent for commercialization. Crop residues consist primarily of plant tissues that are composed of various types of cell walls and these plant tissues pose significant resistance to chemical, enzymatic and microbial deconstruction. Plant cell wall biomass represents the structural forms of monosaccharides, as opposed to starch and amylopectin, which represent the storage forms [3]. Biofuels (2010) 1(2), 323–341 Microbial enzyme systems for biomass conversion: emerging paradigms