ABSTRACT

Soil is a complex ecosystem with very diverse habitats. It harbors all major groups of fungi, including the motile fungi of the classes Zygomycetes and Basidiomy-cetes. Fungal populations have been estimated by determining the number of colony-forming units, which range from 104 to 106/g of soil. Hyphal lengths range from 100 to 1000 meters/g, and biomass ranges from 37 to 184 g of dry weight/m2 [1]. A total of 144 samples of sandy soil from Ipanema Beach, Rio de Janeiro, Brazil produced 4285 colonies of yeast and almost 7000 colonies of fungi. Filamentous fungi were identified in 1334 colonies, and represented 34 genera and 170 species. The most common genera were Aspergillus (30.4%), Penicillium (16.2%), Fusarium (12.6%), Trichoderma (6.4%), Paecilomyces (3.7%), Cladosporium (3.1%), and Acremonium (1 %) [2]. These impressive num­bers point to the dominance of fungi in many soil habitats. Mycologists estimate that there are over 1 million species of filamentous fungi populating virtually every ecosystem on earth [3]. Soil contains not only fungal cells, but also their secretions. Oxidative enzymes of fungal origin have been detected in grassland and forest soil samples [4].Saprophytic microorganisms have a fundamental role in the earth’s carbon

Reprinted from Publication No. 98501 of the Idaho Agricultural Experiment Station. 379

cycle. They release C 02 bound in plant material back to the atmosphere, pre­venting the accumulation of carbon in organic matter of biological origin. The most studied and best understood saprophytic fungi are those that cause wood decay and deterioration. Ultrastructural microscopic studies of decaying wood reveal several major types of decay: white-rot, brown-rot, soft-rot, and bacterial erosion. The molecular mechanism of white-rot decay has been examined exten­sively and is generally well understood. Brown-rot decay is not as well under­stood, and from microscopic and morphological investigations looks quite differ­ent from white-rot decay. Residues of brown-rotted lignin make up a significant fraction of most forest soils. The soft-rot and bacterial mechanisms of lignocellu-lose degradation are the least understood [5,6]. Lignocellulose, thought to be the most abundant form of biomass on earth, evolved in land-inhabiting plants as one of the main components of plant material. This composite polymer constitutes a major organic reservoir within the biospheric carbon cycle [7]. The chem istry of lignocellulose and its structural role in woody plant tissue have been reviewed in numerous publications [8-10]. Biosynthetic and biodegra-dative processes o f lignocellulosic and hem icellulosic polym ers have also been

the subject o f frequent reviews [5,11-13]. The hemicelluloses, polysaccharides that can be extracted from plants by aqueous alkali, are composed o f both linear and branched heteropolymers o f D-xylose, L-arabinose, D-mannose, D-glucose, D-galactose, and D-glucoronic acid. W hite-rot fungi appear to attack angiosperm (hardwood) trees preferentially, whereas brown-rot fungi mainly decay conifer­ous (softwood) trees [14]. Hardwood and softwood are distinguished by the dif­ferent structures o f their lignin elements. Gymnosperm lignin is built predom i­ nantly from coniferyl alcohol monomers (guaiacyl lignin). Angiosperm lignin is assembled mostly from a com bination o f coniferyl and sinapyl alcohol subunits (guaiacyl-syringyl lignin). Grass lignin consists primarily o f copolym erized mo­ nomers of sinapyl alcohol, coniferyl alcohol, and /?-coumaryl alcohol (syringylguaiacyl-/?-hydroxyphenyl lignin). Because o f these different m onomeric struc­ tures and because the proportions o f the three alcohols differ within the lignins o f higher plant species and within their organs and even across a single wood­cell wall [15], lignin, among naturally occurring random polymers, is one o f the most recalcitrant to microbial degradation.To polymerize lignin monomers, plants use peroxidases, ubiquitous en­zymes found in virtually all green plants, in many fungi, and in aerobic bacteria. Plant peroxidases provide an efficient mechanism for removing toxic peroxides and for immobilizing toxic phenolic metabolites in plant cell walls. Lignocellu­lose can be viewed not only as a mechanical necessity developed by land plants,but also as the response to the oxidative stress faced by plants after they left the aquatic environment in their evolutionary journey [16]. Saprophytic fungi, the only group of microorganisms known to completely mineralize lignin to C 02 and H20 , also use highly specialized peroxidases to depolymerize and mineralize lignin.