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American Society for Testing and Materials

Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation

Cellulose is the main polymeric component of the plant cell wall, the most abundant polysaccharide on Earth, and an important renewable resource. Basidiomycetous fungi belong to its most potent degraders because many species grow on dead wood or litter, in environment rich in cellulose. Fungal cellulolytic systems differ from the complex cellulolytic systems of bacteria. For the degradation of cellulose, basidiomycetes utilize a set of hydrolytic enzymes typically composed of endoglucanase, cellobiohydrolase and β-glucosidase. In some species, the absence of cellobiohydrolase is substituted by the production of processive endoglucanases combining the properties of both of these enzymes. In addition, systems producing hydroxyl radicals based on cellobiose dehydrogenase, quinone redox cycling or glycopeptide-based Fenton reaction are involved in the degradation of several plant cell wall components, including cellulose. The complete cellulolytic complex used by a single fungal species is typically composed of more than one of the above mechanisms that contribute to the utilization of cellulose as a source of carbon or energy or degrade it to ensure fast substrate colonization. The efficiency and regulation of cellulose degradation differs among wood-rotting, litter-decomposing, mycorrhizal or plant pathogenic fungi and yeasts due to the different roles of cellulose degradation in the physiology and ecology of the individual groups.

Degradation of cellulose by basidiomycetous fungi.

The production of organic acids by fungi has profound implications for metal speciation, physiology and biogeochemical cycles. Biosynthesis of oxalic acid from glucose occurs by hydrolysis of oxaloacetate to oxalate and acetate catalysed by cytosolic oxaloacetase, whereas on citric acid, oxalate production occurs by means of glyoxylate oxidation. Citric acid is an intermediate in the tricarboxylic acid cycle, with metals greatly influencing biosynthesis: growth limiting concentrations of Mn, Fe and Zn are important for high yields. The metal-complexing properties of these organic acids assist both essential metal and anionic (e.g. phosphate) nutrition of fungi, other microbes and plants, and determine metal speciation and mobility in the environment, including transfer between terrestrial and aquatic habitats, biocorrosion and weathering. Metal solubilization processes are also of potential for metal recovery and reclamation from contaminated solid wastes, soils and low-grade ores. Such ‘heterotrophic leaching’ can occur by several mechanisms but organic acids occupy a central position in the overall process, supplying both protons and a metal-complexing organic acid anion. Most simple metal oxalates [except those of alkali metals, Fe(III) and Al] are sparingly soluble and precipitate as crystalline or amorphous solids. Calcium oxalate is the most important manifestation of this in the environment and, in a variety of crystalline structures, is ubiquitously associated with free-living, plant symbiotic and pathogenic fungi. The main forms are the monohydrate (whewellite) and the dihydrate (weddelite) and their formation is of significance in biomineralization, since they affect nutritional heterogeneity in soil, especially Ca, P, K and Al cycling. The formation of insoluble toxic metal oxalates, e.g. of Cu, may confer tolerance and ensure survival in contaminated environments. In semiarid environments, calcium oxalate formation is important in the formation and alteration of terrestrial subsurface limestones. Oxalate also plays an important role in lignocellulose degradation and plant pathogenesis, affecting activities of key enzymes and metal oxidoreduction reactions, therefore underpinning one of the most fundamental roles of fungi in carbon cycling in the natural environment. This review discusses the physiology and chemistry of citric and oxalic acid production in fungi, the intimate association of these acids and processes with metal speciation, physiology and mobility, and their importance and involvement in key fungal-mediated processes, including lignocellulose degradation, plant pathogenesis and metal biogeochemistry.

Fungal production of citric and oxalic acid: importance in metal speciation, physiology and biogeochemical processes.

Interactions of heavy metals with white-rot fungi

Incipient decay of wood by brown-rot fungi causes measurable strength losses in wood before measurable weight loss occurs. Previous studies have shown that the high levels of strength loss that occur during incipient brown-rot decay may be related to loss in hemicellulose. This study investigates the effect of decay on hemicellulose composition and the relationship of decay to the mechanical properties of the wood. An in vitro test method was used to allow progressive sampling of southern pine exposed to monocultures of brown-rot fungi. The wood was subsequently analyzed by mechanical testing and chemical analysis. The results demonstrated a ratio of strength to weight loss of approximately 4:1. The chemical data indicated that early strength loss (up to 40%) was associated with loss of arabinan and galactan components. Subsequent strength loss (greater than 40%) was associated with the loss of the mannan and xylan components. Significant loss of glucan (representing cellulose) was only detected at greater than 75 percent modulus of rupture loss.

Relationships between mechanical properties, weight loss and chemical composition of wood during incipient brown rot decay.

https://www.fpl.fs.fed.us/documnts/pdf2003/green03b.pdf

Copper tolerance of brown-rot fungi: time course of oxalic acid production

https://www.fpl.fs.fed.us/documnts/pdf2013/fpl_2013_kirker001.pdf

The role of extractives in naturally durable wood species

Nanotechnology has the potential to affect the field of wood preservation through the creation of new and unique metal biocides with improved properties. This study evaluated leachability and efficacy of southern yellow pine wood treated with copper, zinc, or boron nanoparticles against mould fungi, decay fungi, and Eastern subterranean termites. Results showed that nanocopper with and without surfactant, nanozinc, and nanozinc plus silver with surfactant resisted leaching compared with metal oxide controls. Nearly all nanoboron and boric acid was released from the treated wood specimens during leaching. Mould fungi were moderately inhibited by nanozinc oxide with surfactant, but the other nanometal preparations did not significantly inhibit mould fungi. Mass loss from Gloeophyllum trabeum was significantly inhibited by all copper preparations, while Antrodia sp. was not inhibited by nanometal treatments. Nanometals imparted high resistance in southern yellow pine to the white-rot fungus, Trametes versicolor. Unleached specimens treated with nanoboron or nanozinc plus surfactant caused 100% and 31% mortality, respectively. All specimens treated with nanozinc or nanozinc plus silver inhibited termite feeding, but the copper treatments were less effective against termites. Nanozinc possessed the most favorable properties: leach resistance, termite mortality, and inhibition of termite feeding and decay by the white-rot fungus.

https://www.fpl.fs.fed.us/documnts/pdf2009/fpl_2009_kartal001.pdf

Do the unique properties of nanometals affect leachability or efficacy against fungi and termites? International biodeterioration & biodegradation

https://www.fpl.fs.fed.us/documnts/pdf2007/fpl_2007_yang001.pdf

Carol A. Clausen

Papers

Relationships between mechanical properties, weight loss and chemical composition of wood during incipient brown rot decay.

Copper tolerance of brown-rot fungi: time course of oxalic acid production

The role of extractives in naturally durable wood species

Do the unique properties of nanometals affect leachability or efficacy against fungi and termites? International biodeterioration & biodegradation

Antifungal effect of essential oils on southern yellow pine