Environmental applications of fungal and plant systems: Decolourisation of textile wastewater and related dyestuffs

Abstract

Only a few groups of microorganisms are capable of bringing about the biodegradation of recalcitrant organic polluting matter, lignin and other aromatic compounds being a case in point. Due to its intrinsic properties, such as insolubility and chemical complexity, lignin protects structural plant cell wall carbohydrates (cellulose and hemicellulose) from microbial attack and enzymatic hydrolysis (Reid 1995; Prez et al. 2002). From the chemical point of view, lignin is an amorphous heteropolymer derived from three phenylpropanoid monomers: p-coumaryl alcohol and its two methoxysubstituted derivatives, p-coniferyl alcohol and p-sinapyl alcohol. These basic units are randomly joined together by different types of carbon-carbon and ether linkages (Adler 1977; Douglas 1996). Despite natural high structural variability, lignin is essentially an aromatic nonphenolic substrate, since a typical lignin only contains 10-20% of phenolic hydroxyl radicals (Tuor et al. 1995; Youn et al. 1995). Fungi, in general, but more specifically the wood-decaying basidiomycetes of white-rot type, are the most efficient microorganisms that perform the depolymerization and even complete lignin mineralization to carbon dioxide and water (Reid 1995; Leonowicz et al. 1999; ten Have and Teunissen 2001). The high performance exhibited by white-rot fungi (WRF) could be mainly ascribed to the production of a powerful extracellular ligninolytic system. Three main types of different oxidoreductase activities can be found in this enzymatic system: polyphenoloxidases, peroxidases and auxiliary H2O2- generating oxidases (Breen and Singleton 1999; Leonowicz et al. 1999). More specifically, the key ligninolytic enzymes synthesised by WRF are laccase (polyphenoloxidase; EC 1.10.3.2), manganese-dependent peroxidase (MnP; EC 1.11.1.13) and lignin peroxidase (LiP; EC 1.11.1.14). However, several ligninolytic fungal species only produce two of them, with the most usual combination of activities being laccase and MnP (Tuor et al. 1995; Tekere et al. 2001). Since the lignin-degrading system of basidiomycetous WRF has typically a broad substrate specificity, both whole cultures and their ligninolytic enzymes were found to be useful for the bioremediation of a wide number of environmental pollutants, ranging from natural compounds to (perhaps a bit surprisingly) xenobiotic ones, including textile dyes (Field et al. 1993; Reddy 1995; Fu and Viraraghavan 2001; Jarosz-Wilkolazka et al. 2002; Wesenberg et al. 2003). © 2007 Springer-Verlag Berlin Heidelberg.