Production and Degradation of Oxalic Acid by Brown Rot Fungi

Upcycling of black currant pomace for the production of a fermented beverage with Wolfiporia cocos

Pomace as a side stream from black currant juice production is mostly discarded, even though it is rich in nutrients like protein, fiber, sugars, anthocyanins, polyphenols, and other secondary metabolites. Fungi from the division of Basidiomycota have a great enzymatic toolbox to recycle these complex mixtures of nutrients. In particular, the edible medicinal fungus Wolfiporia cocos has been described as a suitable biocatalyst to form pleasant aroma compounds in fermentation processes. Therefore, medium optimization, upscaling, and filtration were performed to produce a beverage based on black currant pomace fermented with W. cocos. A trained panel described the beverage as highly pleasant, reminiscent of honey, flowers and berries with a well-balanced sour and sweet taste. The flavor compounds linalool (citrus), geraniol (flowery), phenylacetic acid (honey), methyl phenylacetate (honey), eugenol (clove), and 2-phenylethanol (rose) were produced during fermentation and the concentrations exceeded their respective odor thresholds. The produced beverage was evaluated with 8.0 ± 1.4 from 10 for the question of whether panelists would buy the product. Fungal fermentation with the edible fungus W. cocos enabled the production of a highly pleasant beverage and additionally may reduce waste by using pomace and table sugar as sole ingredients.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05677-4.

Which Is the Best Substrate to Regenerate? A Comparative Pot Experiment for Tree Seedling Growth on Decayed Wood and in Soil

Dead wood is an important microsite for seedling regeneration in forest ecosystems. Although recent studies have found important associations between fungal wood decay type (white rot and brown rot) and both density and species composition of regenerating seedlings, its abiotic and biotic mechanisms are unknown. In the present study, pot experiments were conducted with the seedlings of two ectomycorrhizal tree species (Abies veitchii and Betula ermanii) and two arbuscular mycorrhizal tree species (Chamaecyparis obtusa and Cryptomeria japonica) to evaluate their growth using three substrates: brown rot wood, white rot wood, and soil. Results showed that the shoot growth of B. ermanii grown in white rot wood was greater than in other substrates, but this effect disappeared in sterilized substrates, suggesting some biotic positive effects occur in white rot wood. The seedling weights of Cr. japonica and Ch. obtusa were found to be greater in soil than in wood, and this may be partly attributable to the high mycorrhizal rate of their roots in soil. Colonization of arbuscular and ectomycorrhizal fungi was restricted to the seedlings in unsterilized soil. These results demonstrate the importance of the biological mechanisms affecting seedlings’ preferences for a variety of regeneration microsites and illustrate the need for future experiments to include larger sets of seedling species.

Wild Strawberry-like Flavor Produced by the Fungus Wolfiporia cocos─Identification of Character Impact Compounds by Aroma Dilution Analysis after Dynamic Headspace Extraction

Brown-rot fungi are particularly suitable for the sustainable and cost-efficient biotechnological production of natural flavors. In this study, Wolfiporia cocos was employed for the fermentation of European black currant pomace supplemented with aspartate in surface cultures to produce a flavor reminiscent of wild strawberries. Aroma dilution analysis (ADA) by means of dynamic headspace extraction was developed as a suitable technique for solid samples. The character impact compounds were quantified by stable isotope dilution analysis and standard addition and validated by recombination experiments. (R)-Linalool (1879 μg kg^-1, ADA 2¹¹), methyl anthranilate (2206 μg kg^-1, 2¹⁰), 2-aminobenzaldehyde (771 μg kg^-1, 2⁵), and geraniol (138 μg kg^-1, 2⁵) were identified as key aroma compounds. Recombination experiments demonstrated that the combination of the four analyzed compounds was responsible for the odor impression reminiscent of wild strawberries.

Extraction of Vanillin Following Bioconversion of Rice Straw and Its Optimization by Response Surface Methodology

Value-added chemicals, including phenolic compounds, can be generated through lignocellulosic biomass conversion via either biological or chemical pretreatment. Currently vanillin is one of the most valuable of these products that has been shown to be extractable on an industrial scale. This study demonstrates the potential of using rice straw inoculated with Serpula lacrymans, which produced a mixture of high value bio-based compounds including vanillin. Key extraction conditions were identified to be the volume of solvent used and extraction time, which were optimized using response surface methodology (RSM). The vanillin compounds extracted from rice straw solid state fermentation (SSF) was confirmed through LC-ESI MS/MS in selective ion mode. The optimum concentration and yield differed depending on the solvent, which was predicted using 60 mL ethyl acetate for 160 min were 0.408% and 3.957 μg g⁻¹ respectively. In comparison, when ethanol was used, the highest concentration and yields of vanillin were 0.165% and 2.596 μg g⁻¹. These were achieved using 40 mL of solvent, and extraction time increased to 248 min. The results confirm that fungal conversion of rice straw to vanillin could consequently offer a cost-effect alternative to other modes of production.

Recent Advances in Applications of Acidophilic Fungi to Produce Chemicals

Processing of fossil fuels is the major environmental issue today. Biomass utilization for the production of chemicals presents an alternative to simple energy generation by burning. Lignocellulosic biomass (cellulose, hemicellulose and lignin) is abundant and has been used for variety of purposes. Among them, lignin polymer having phenyl-propanoid subunits linked together either through C-C bonds or ether linkages can produce chemicals. It can be depolymerized by fungi using their enzyme machinery (laccases and peroxidases). Both acetic acid and formic acid production by certain fungi contribute significantly to lignin depolymerization. Fungal natural organic acids production is thought to have many key roles in nature depending upon the type of fungi producing them. Biological conversion of lignocellulosic biomass is beneficial over physiochemical processes. Laccases, copper containing proteins oxidize a broad spectrum of inorganic as well as organic compounds but most specifically phenolic compounds by radical catalyzed mechanism. Similarly, lignin peroxidases (LiP), heme containing proteins perform a vital part in oxidizing a wide variety of aromatic compounds with H₂O₂. Lignin depolymerization yields value-added compounds, the important ones are aromatics and phenols as well as certain polymers like polyurethane and carbon fibers. Thus, this review will provide a concept that biological modifications of lignin using acidophilic fungi can generate certain value added and environmentally friendly chemicals.

Interactions affect hyphal growth and enzyme profiles in combinations of coniferous wood-decaying fungi of Agaricomycetes

Fomitopsis pinicola is a species of Polyporales frequently encountered in Nordic temperate and boreal forests. In nature, the fungus causes destructive brown rot in wood, colonizing tree trunks often occupied by other Basidiomycota species. We mimicked these species-species interactions by introducing F. pinicola to five white rot species, all common saprotrophs of Norway spruce. Hyphal interactions and mycelial growth in various combinations were recorded, while activities of lignocellulose-acting CAZymes and oxidoreductases were followed in co-cultures on two different carbon-source media. Of the species, Phlebia radiata and Trichaptum abietinum were the strongest producers of lignin-modifying oxidoreductases (laccase, manganese peroxidase) when evaluated alone, as well as in co-cultures, on the two different growth media (low-nitrogen liquid medium containing ground coniferous wood, and malt extract broth). F. pinicola was an outstanding producer of oxalic acid (up to 61 mM), whereas presence of P. radiata prevented acidification of the growth environment in the liquid malt-extract cultures. When enzyme profiles of the species combinations were clustered, time-dependent changes were observed on wood-supplemented medium during the eight weeks of growth. End-point acidity and production of mycelium, oxalic acid and oxidoreductase activities, in turn clustered the fungal combinations into three distinct functional groups, determined by the presence of F. pinicola and P. radiata, by principal component analysis. Our findings indicate that combinations of wood-decay fungi have dramatic dynamic effects on the production of lignocellulose-active enzymes, which may lead to divergent degradative processes of dead wood and forest litter.

Experimental and computational investigation of the substituent effects on the reduction of Fe3+by 1,2-dihydroxybenzenes

This study reports on the kinetics of the early steps of the reactions between substituted 1,2-dihydroxybenzenes (1,2-DHB) and Fe³⁺.

Fungal variegatic acid and extracellular polysaccharides promote the site-specific generation of reactive oxygen species

This study aims to clarify the role of variegatic acid (VA) in fungal attack by Serpula lacrymans, and also the generation and scavenging of reactive oxygen species (ROS) by the fungus. VA promotes a mediated Fenton reaction to generated ROS after oxalate solubilizes oxidized forms of iron. The fungal extracellular matrix (ECM) β-glucan scavenged ROS, and we propose this as a mechanism to protect the fungal hyphae while ROS generation is promoted to deconstruct the lignocellulose cell wall. A relatively high pH (4.4) also favored Fe(III) transfer from oxalate to VA as opposed to a lower pH (2.2) conditions, suggesting a pH-dependent Fe(III) transfer to VA employed by S. lacrymans. This permits ROS generation within the higher pH of the cell wall, while limiting ROS production near the fungal hyphae, while β-glucan from the fungal ECM scavenges ROS in the more acidic environments surrounding the fungal hyphae.

Oxidative stress in fungi: its function in signal transduction, interaction with plant hosts, and lignocellulose degradation

In this review article, we want to present an overview of oxidative stress in fungal cells in relation to signal transduction, interaction of fungi with plant hosts, and lignocellulose degradation. We will discuss external oxidative stress which may occur through the interaction with other microorganisms or plant hosts as well as internally generated oxidative stress, which can for instance originate from NADPH oxidases or “leaky” mitochondria and may be modulated by the peroxiredoxin system or by protein disulfide isomerases thus contributing to redox signaling. Analyzing redox signaling in fungi with the tools of molecular genetics is presently only in its beginning. However, it is already clear that redox signaling in fungal cells often is linked to cell differentiation (like the formation of perithecia), virulence (in plant pathogens), hyphal growth and the successful passage through the stationary phase.

RNA-sequencing reveals the complexities of the transcriptional response to lignocellulosic biofuel substrates in Aspergillus niger

Background

Filamentous fungi are well known for their ability to degrade lignocellulosic biomass and have a natural ability to convert certain products of biomass degradation, for example glucose, into various organic acids. Organic acids are suggested to give a competitive advantage to filamentous fungi over other organisms by decreasing the ambient pH. They also have an impact on the ecosystem by enhancing weathering and metal detoxification. Commercially, organic acids can be used as chemical intermediates or as synthons for the production of biodegradable polymers which could replace petroleum-based or synthetic chemicals. One of the advantages of filamentous fungi as biotechnological production platforms for synthetic biology is their ability to degrade vegetal biomass, which is a promising feedstock for the biotechnological production of organic acids. The Fungal Culture Collection of the International Centre of Microbial Resources (CIRM-CF), curated by our laboratory, contains more than 1600 strains of filamentous fungi, mainly Basidiomycetes and Ascomycetes. The natural biodiversity found in this collection is wide, with strains collected from around the world in different climatic conditions. This collection is mainly studied to unravel the arsenal of secreted lignocellulolytic enzymes available to the fungi in order to enhance biomass degradation. While the fungal biodiversity is a tremendous reservoir for “green” molecules production, its potentiality for organic acids production is not completely known.

Results

In this study, we screened 40 strains of Ascomycota and 26 strains of Basidiomycota, representing the distribution of fungal diversity of the CIRM-CF collection, in order to evaluate their potential for organic acid and ethanol production, in a glucose liquid medium. We observed that most of the filamentous fungi are able to grow and acidify the medium. We were also able to discriminate two groups of filamentous fungi considering their organic acid production at day 6 of incubation. This first group represented fungi co-producing a wide variety of organic acids and ethanol at concentrations up to 4 g.L⁻¹ and was composed of all the Aspergilli and only 3 other Ascomycota. The second group was composed of the remaining Ascomycota and all the Basidiomycota which produced mainly ethanol. Among the Basidiomycota, two strains produced oxalic acid and one strain produced gluconic and formic acid. Six strains of Aspergillus producing high concentrations of oxalic, citric and gluconic acids, and ethanol were selected for metabolism analysis.

Conclusion

These results illustrate the versatility in metabolites production among the fungal kingdom. Moreover, we found that some of the studied strains have good predispositions to produce valuable molecules. These strains could be of great interest in the study of metabolism and may represent new models for synthetic biology or consolidated bioprocessing of biomass.

Electronic supplementary material

The online version of this article (doi:10.1186/s40694-014-0001-z) contains supplementary material, which is available to authorized users.

Induction of polygalacturonase and the formation of oxalic acid by pectin in brown-rot fungi

Extracellular polygalacturonase (PG) production was estimated in vitro, using liquid cultures of three species of brown-rot decay fungi (Postia placenta, Gloeophyllum trabeum and Serpula incrassata), by cup-plate assay, assay of reducing sugars, and decrease in viscosity. Although all three experimental assays demonstrated that PG was induced by pectin in all three fungi, decrease in viscosity gave the best correlation with decay capacity in soil block tests. PG activity, determined as an increase in reducing sugar activity, was greatest in G. trabeum and weakest in S. incrassata. The optimum pH for PG activity was between pH 2.5 and 4.5. Oxalic acid production was also enhanced by pectin and functioned synergistically with PG activity. We conclude that these fungi produce PG that is best induced by pectin and that PG activity exceeds production of xylanase and endoglucanase activity in vitro. Polygalacturonase is likely to act synergistically with oxalic acid to solubilize and hydrolyse the pectin in pit membranes and middle lamellae. Thus, production of PG and oxalic acid should facilitate early spread of hyphae and enhance the lateral flow of wood-decay enzymes and agents into adjacent tracheids and the wood cell wall, thus initiating the diffuse decay caused by brown-rot fungi.

Oxalic acid, versatile peroxidase secretion and chelating ability of Bjerkandera fumosa in rich and limited culture conditions

Efficient ligninolytic systems of wood-degrading fungi include not only oxidizing enzymes, but also low-molecular-weight effectors. The ability of Bjerkandera fumosa to secrete oxalic acid and versatile peroxidase (VP) in nitrogen-rich and nitrogen-limited media was studied. Higher activity of VP was determined in the nitrogen-limited media but greater concentration of oxalic acid was observed in the cultures of B. fumosa without nitrogen limitation. Ferric ions chelating ability of Bjerkandera fumosa studied in ferric ions limited media was correlated with the increased level of oxalic acid. The presence of hydroxamate-type siderophores in B. fumosa media were also detected. Oxalate decarboxylase was found to be responsible for regulation of oxalic acid concentration in the tested B. fumosa cultures.

Oxalate efflux transporter from the brown rot fungus Fomitopsis palustris

An oxalate-fermenting brown rot fungus, Fomitopsis palustris, secretes large amounts of oxalic acid during wood decay. Secretion of oxalic acid is indispensable for the degradation of wood cell walls, but almost nothing is known about the transport mechanism by which oxalic acid is secreted from F. palustris hyphal cells. We characterized the mechanism for oxalate transport using membrane vesicles of F. palustris. Oxalate transport in F. palustris was ATP dependent and was strongly inhibited by several inhibitors, such as valinomycin and NH(4)(+), suggesting the presence of a secondary oxalate transporter in this fungus. We then isolated a cDNA, FpOAR (Fomitopsis palustris oxalic acid resistance), from F. palustris by functional screening of yeast transformants with cDNAs grown on oxalic acid-containing plates. FpOAR is predicted to be a membrane protein that possesses six transmembrane domains but shows no similarity with known oxalate transporters. The yeast transformant possessing FpOAR (FpOAR-transformant) acquired resistance to oxalic acid and contained less oxalate than the control transformant. Biochemical analyses using membrane vesicles of the FpOAR-transformant showed that the oxalate transport property of FpOAR was consistent with that observed in membrane vesicles of F. palustris. The quantity of FpOAR transcripts was correlated with increasing oxalic acid accumulation in the culture medium and was induced when exogenous oxalate was added to the medium. These results strongly suggest that FpOAR plays an important role in wood decay by acting as a secondary transporter responsible for secretion of oxalate by F. palustris.

Oxalate decarboxylase: biotechnological update and prevalence of the enzyme in filamentous fungi

Oxalate decarboxylase (ODC) is a manganese-containing, multimeric enzyme of the cupin protein superfamily. ODC is one of the three enzymes identified to decompose oxalic acid and oxalate, and within ODC catalysis, oxalate is split into formate and CO(2). This primarily intracellular enzyme is found in fungi and bacteria, and currently the best characterized enzyme is the Bacillus subtilis OxdC. Although the physiological role of ODC is yet unidentified, the feasibility of this enzyme in diverse biotechnological applications has been recognized for a long time. ODC could be exploited, e.g., in diagnostics, therapeutics, process industry, and agriculture. So far, the sources of ODC enzyme have been limited including only a few fungal and bacterial species. Thus, there is potential for identification and cloning of new ODC variants with diverse biochemical properties allowing e.g. more enzyme fitness to process applications. This review gives an insight to current knowledge on the biochemical characteristics of ODC, and the relevance of oxalate-converting enzymes in biotechnological applications. Particular emphasis is given to fungal enzymes and the inter-connection of ODC to fungal metabolism of oxalic acid.

Assessment of saccharification efficacy in the cellulase system of the brown rot fungus Gloeophyllum trabeum

Brown rot fungi uniquely degrade wood by creating modifications thought to aid in the selective removal of polysaccharides by an incomplete cellulase suite. This naturally successful mechanism offers potential for current bioprocessing applications. To test the efficacy of brown rot cellulases, southern yellow pine wood blocks were first degraded by the brown rot fungus Gloeophyllum trabeum for 0, 2, 4, and 6 weeks. Characterization of the pine constituents revealed brown rot decay patterns, with selective polysaccharide removal as lignin compositions increased. G. trabeum liquid and solid state cellulase extracts, as well as a commercial Trichoderma reesei extract (Celluclast 1.5 L), were used to saccharify this pretreated material, using beta-glucosidase amendment to remove limitation of cellobiose-to-glucose conversion. Conditions varied according to source and concentration of cellulase extract and to pH (3.0 vs. 4.8). Hydrolysis yields were maximized using solid state G. trabeum extracts at a pH of 4.8. However, the extent of glucose release was low and was not significantly altered when cellulase loading levels were increased threefold. Furthermore, Celluclast 1.5 L continually outperformed G. trabeum cellulase extracts, although extent of glucose release never exceeded 22.0%. Results suggest methodological advances for utilizing crude G. trabeum cellulases and imply that the suboptimal hydrolysis levels obtained with G. trabeum and Celluclast 1.5 L cellulases, even at high loading levels, may be due to brown rot modifications insufficiently distributed throughout the pretreated material.

Fenton (H2O2/Fe) reaction involved in Penicillium sp. culture for DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane)] degradation

The purpose of this work was to demonstrate that a Fenton (H(2)O(2)/Fe) reaction was involved in DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane)] degradation in a culture of Penicillium sp. spiked with FeSO(4). A commercial DDT mixture (10% DDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene], 30% o,p-DDT and 60% of p,p' -DDT) of 10 mg L(-1) was used. Hydrogen peroxide (H(2)O(2)), tartaric acid and oxalic acid were identified at 18 h in culture media, with and without added DDT; this correlated positively with lowering of pH from 5.8 to 2.7. Lower concentrations of oxalic acid and H(2)O(2) (7.9 and 52.6 mg L(-1), respectively) occurred in media with DDT at 30 h, in comparison to that one without DDT mixture (27.9 and 65.3 mg L(-1), respectively), at this time there was maximum degradation (87.7, 91.7 and 94.2%) for DDE, o,p-DDT and p,p'-DDT, respectively. We propose that the degradation of the DDT mixture by Penicillium sp. was through a Fenton reaction (H(2)O(2)/Fe) under acidic conditions produced in situ during the fungal culture amended with FeSO(4).

Oxalate decarboxylase of the white-rot fungus Dichomitus squalens demonstrates a novel enzyme primary structure and non-induced expression on wood and in liquid cultures

Oxalate decarboxylase (ODC) catalyses the conversion of oxalic acid to formic acid and CO2 in bacteria and fungi. In wood-decaying fungi the enzyme has been linked to the regulation of intra- and extracellular quantities of oxalic acid, which is one of the key components in biological decomposition of wood. ODC enzymes are biotechnologically interesting for their potential in diagnostics, agriculture and environmental applications, e.g. removal of oxalic acid from industrial wastewaters. We identified a novel ODC in mycelial extracts of two wild-type isolates of Dichomitus squalens, and cloned the corresponding Ds-odc gene. The primary structure of the Ds-ODC protein contains two conserved Mn-binding cupin motifs, but at the N-terminus, a unique, approximately 60 aa alanine-serine-rich region is found. Real-time quantitative RT-PCR analysis confirmed gene expression when the fungus was cultivated on wood and in liquid medium. However, addition of oxalic acid in liquid cultures caused no increase in transcript amounts, thereby indicating a constitutive rather than inducible expression of Ds-odc. The detected stimulation of ODC activity by oxalic acid is more likely due to enzyme activation than to transcriptional upregulation of the Ds-odc gene. Our results support involvement of ODC in primary rather than secondary metabolism in fungi.

Fungal hydroquinones contribute to brown rot of wood

The fungi that cause brown rot of wood initiate lignocellulose breakdown with an extracellular Fenton system in which Fe(2+) and H(2)O(2) react to produce hydroxyl radicals (.OH), which then oxidize and cleave the wood holocellulose. One such fungus, Gloeophyllum trabeum, drives Fenton chemistry on defined media by reducing Fe(3+) and O(2) with two extracellular hydroquinones, 2,5-dimethoxyhydroquinone (2,5-DMHQ) and 4,5-dimethoxycatechol (4,5-DMC). However, it has never been shown that the hydroquinones contribute to brown rot of wood. We grew G. trabeum on spruce blocks and found that 2,5-DMHQ and 4,5-DMC were each present in the aqueous phase at concentrations near 20 microM after 1 week. We determined rate constants for the reactions of 2,5-DMHQ and 4,5-DMC with the Fe(3+)-oxalate complexes that predominate in wood undergoing brown rot, finding them to be 43 l mol(-1) s(-1) and 65 l mol(-1) s(-1) respectively. Using these values, we estimated that the average amount of hydroquinone-driven .OH production during the first week of decay was 11.5 micromol g(-1) dry weight of wood. Viscometry of the degraded wood holocellulose coupled with computer modelling showed that a number of the same general magnitude, 41.2 micromol oxidations per gram, was required to account for the depolymerization that occurred in the first week. Moreover, the decrease in holocellulose viscosity was correlated with the measured concentrations of hydroquinones. Therefore, hydroquinone-driven Fenton chemistry is one component of the biodegradative arsenal that G. trabeum expresses on wood.

Evaluation of the mechanical, physical properties and decay resistance of particleboard made from particles impregnated with Pinus brutia bark extractives

The mechanical, physical properties and decay resistances of particleboard made from particles impregnated with Pinus brutia bark extractives were examined. Properties included were modulus of rupture, modulus of elasticity, internal bond, thickness swelling, and weight loss according to European standards. The results showed that particleboards made from particles impregnated with bark extractives had significantly lower mechanical values than those made from unimpregnated particles. Impregnating wood particles with bark extractives improved the decay resistance and thickness swelling of particleboard. Increasing concentration of the extractives decreased the mechanical properties and improved the thickness swelling and decay resistance of the panels. Particleboards made from 1% P. brutia bark extractives met the specifications for modulus of rupture and internal bond strength for general purposes.

Metal accumulation without enhanced oxalate secretion in wood degraded by brown rot fungi

Brown rot fungi were incubated in agar and agar-wood microcosms containing metallic or hydroxide forms of Al, Cu, and Fe. Metal dissolution was associated with elevated oxalate concentrations in agar, but metals translocated into wood did not affect oxalate accumulation, crystal production, or decay rate, demonstrating a substrate-dependent oxalate dynamic.

Indoor Cultivation and Cultural Characteristics of Wolfiporia cocos Sclerotia Using Mushroom Culture Bottles

We newly developed an indoor cultivation technique for Wolfiporia cocos (Wolf) Ryvarden et Gilbertson (Syn. Poria cocos Wolf), not with soil, but using mushroom culture bottles with pine logs, and clarified some cultural characteristics of sclerotia in the laboratory. To determine the optimum conditions for sclerotia growth, the weight of sclerotia and concentration of CO2 in three different air filters; cloth, paper and urethane resin, and closed bottles were tested. When the cloth air filter was used, the growth rate was the fastest and the yield was maximal. These results suggested that the aeration was an important environmental factor for cultivation. To clarify the characteristics of culture in the cloth air filtered and closed bottles, the weight of sclerotia, the compositions of pine logs and the contents of pachymic acid and dehydropachymic acid were examined during 24 weeks. The growth of scleroia and the wood decaying efficiency in the cloth air filtered bottles were better than those in the closed bottles. Also, it was found that W. cocos was a brown rot fungus due to the alkaline solubility of pine logs in the wood decay process. In addition, the contents of pachymic acid and dehydropachymic acid and the TLC pattern between the cultivated and commercial sclerotia did not differ remarkably.

Effect of pH and oxalate on hydroquinone-derived hydroxyl radical formation during brown rot wood degradation

The redox cycle of 2,5-dimethoxybenzoquinone (2,5-DMBQ) is proposed as a source of reducing equivalent for the regeneration of Fe2+ and H2O2 in brown rot fungal decay of wood. Oxalate has also been proposed to be the physiological iron reductant. We characterized the effect of pH and oxalate on the 2,5-DMBQ-driven Fenton chemistry and on Fe3+ reduction and oxidation. Hydroxyl radical formation was assessed by lipid peroxidation. We found that hydroquinone (2,5-DMHQ) is very stable in the absence of iron at pH 2 to 4, the pH of degraded wood. 2,5-DMHQ readily reduces Fe3+ at a rate constant of 4.5 x 10(3) M(-1)s(-1) at pH 4.0. Fe2+ is also very stable at a low pH. H2O2 generation results from the autoxidation of the semiquinone radical and was observed only when 2,5-DMHQ was incubated with Fe3+. Consistent with this conclusion, lipid peroxidation occurred only in incubation mixtures containing both 2,5-DMHQ and Fe3+. Catalase and hydroxyl radical scavengers were effective inhibitors of lipid peroxidation, whereas superoxide dismutase caused no inhibition. At a low concentration of oxalate (50 micro M), ferric ion reduction and lipid peroxidation are enhanced. Thus, the enhancement of both ferric ion reduction and lipid peroxidation may be due to oxalate increasing the solubility of the ferric ion. Increasing the oxalate concentration such that the oxalate/ferric ion ratio favored formation of the 2:1 and 3:1 complexes resulted in inhibition of iron reduction and lipid peroxidation. Our results confirm that hydroxyl radical formation occurs via the 2,5-DMBQ redox cycle.

Pathways for extracellular Fenton chemistry in the brown rot basidiomycete Gloeophyllum trabeum

The brown rot fungus Gloeophyllum trabeum uses an extracellular hydroquinone-quinone redox cycle to reduce Fe(3+) and produce H(2)O(2). These reactions generate extracellular Fenton reagent, which enables G. trabeum to degrade a wide variety of organic compounds. We found that G. trabeum secreted two quinones, 2,5-dimethoxy-1,4-benzoquinone (2,5-DMBQ) and 4,5-dimethoxy-1,2-benzoquinone (4,5-DMBQ), that underwent iron-dependent redox cycling. Experiments that monitored the iron- and quinone-dependent cleavage of polyethylene glycol by G. trabeum showed that 2,5-DMBQ was more effective than 4,5-DMBQ in supporting extracellular Fenton chemistry. Two factors contributed to this result. First, G. trabeum reduced 2,5-DMBQ to 2,5-dimethoxyhydroquinone (2,5-DMHQ) much more rapidly than it reduced 4,5-DMBQ to 4,5-dimethoxycatechol (4,5-DMC). Second, although both hydroquinones reduced ferric oxalate complexes, the predominant form of Fe(3+) in G. trabeum cultures, the 2,5-DMHQ-dependent reaction reduced O(2) more rapidly than the 4,5-DMC-dependent reaction. Nevertheless, both hydroquinones probably contribute to the extracellular Fenton chemistry of G. trabeum, because 2,5-DMHQ by itself is an efficient reductant of 4,5-DMBQ.

Mechanisms of wood degradation by brown-rot fungi: chelator-mediated cellulose degradation and binding of iron by cellulose

Iron, hydrogen peroxide, biochelators and oxalate are believed to play important roles in cellulose degradation by brown-rot fungi. The effect of these compounds in an 'enhanced' Fenton system on alpha-cellulose degradation was investigated specifically in regard to molecular weight distribution and cellulose-iron affinity. This study shows that the degradative ability of an ultrafiltered low molecular weight preparation of chelating compounds isolated from the brown-rot fungus Gloeophyllum trabeum (termed 'Gt chelator') increased with increasing Gt chelator concentration when the FeIII to Gt chelator ratio was greater than about 30:1. When this ratio was less than 30:1, increasing Gt chelator concentration did not accelerate cellulose degradation. In excess hydrogen peroxide, cellulose degradation increased and then decreased with increasing iron concentration when FeIII was present in excess of the Gt chelator. The critical ratio of FeIII to Gt chelator varied depending on the concentration of hydrogen peroxide in the system. Increasing iron concentration above a critical iron:chelator ratio inhibited cellulose degradation. The optimum pH for cellulose degradation mediated by Gt chelator was around 4.0. A comparison of the effects of 2,3-DHBA (a chelator that reduces iron similarly to Gt chelator) and Gt chelator with respect to cellulose degradation demonstrated the same pattern of cellulose degradation. Cellulose-iron affinity studies were conducted at three pH levels (3.6, 3.8, 4.1), and the binding constants for cellulose-FeIII, cellulose-FeII and cellulose-FeIII in the presence of Gt chelator were calculated. The binding constants for cellulose-FeIII at all three pH levels were much higher than those for cellulose-FeII, and the binding constants for cellulose-FeIII in the presence of Gt chelator were very close to those for cellulose-FeII. This is probably the result of FeIII reduction to FeII by Gt chelator and suggests that chelators from the fungus may be able to sequester iron from cellulose and reduce it in near proximity to the cellulose and thereby better promote depolymerization. The free radical generating system described has potential for use in a variety of industrial processing and pollution control applications.

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

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 semi-arid 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 oxido-reduction 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.

Translocation and incorporation of strontium carbonate derived strontium into calcium oxalate crystals by the wood decay fungus Resinicium bicolor

The white-rot wood decay fungus Resinicium bicolor (Abertini & Schwein.: Fr.) Parmasto was studied for its ability to solubilize and translocate ions from the naturally occurring mineral strontianite. Resinicium bicolor colonized a soil mixture culture medium containing strontianite sand, solubilized strontium ions from this mineral phase, translocated the ions vertically, and reprecipitated the strontium into strontium-containing calcium oxalate crystals. Storage of the Sr in crystals was highest in mycelial cords and was dynamic in character. These results suggest that non-mycorrhizal saprotrophic fungi should be evaluated for their potential participation in forest nutrient cycling via biologically weathering parent material and translocating the mobilized mineral nutrients vertically within soils.Key words: fungi, strontium, calcium oxalate, translocation, soil, minerals nutrient cycling.

Biodegradative mechanism of the brown rot basidiomycete Gloeophyllum trabeum: evidence for an extracellular hydroquinone-driven fenton reaction

We have identified key components of the extracellular oxidative system that the brown rot fungus Gloeophyllum trabeum uses to degrade a recalcitrant polymer, polyethylene glycol, via hydrogen abstraction reactions. G. trabeum produced an extracellular metabolite, 2,5-dimethoxy-1,4-benzoquinone, and reduced it to 2,5-dimethoxyhydroquinone. In the presence of 2,5-dimethoxy-1,4-benzoquinone, the fungus also reduced extracellular Fe3+ to Fe2+ and produced extracellular H2O2. Fe3+ reduction and H2O2 formation both resulted from a direct, non-enzymatic reaction between 2,5-dimethoxyhydroquinone and Fe3+. Polyethylene glycol depolymerization by G. trabeum required both 2,5-dimethoxy-1,4-benzoquinone and Fe3+ and was completely inhibited by catalase. These results provide evidence that G. trabeum uses a hydroquinone-driven Fenton reaction to cleave polyethylene glycol. We propose that similar reactions account for the ability of G. trabeum to attack lignocellulose.

Rapid polyether cleavage via extracellular one-electron oxidation by a brown-rot basidiomycete

Fungi that cause brown rot of wood are essential biomass recyclers and also the principal agents of decay in wooden structures, but the extracellular mechanisms by which they degrade lignocellulose remain unknown. To test the hypothesis that brown-rot fungi use extracellular free radical oxidants as biodegradative tools, Gloeophyllum trabeum was examined for its ability to depolymerize an environmentally recalcitrant polyether, poly(ethylene oxide) (PEO), that cannot penetrate cell membranes. Analyses of degraded PEOs by gel permeation chromatography showed that the fungus cleaved PEO rapidly by an endo route. 13C NMR analyses of unlabeled and perdeuterated PEOs recovered from G. trabeum cultures showed that a major route for depolymerization was oxidative C---C bond cleavage, a reaction diagnostic for hydrogen abstraction from a PEO methylene group by a radical oxidant. Fenton reagent (Fe(II)/H2O2) oxidized PEO by the same route in vitro and therefore might account for PEO biodegradation if it is produced by the fungus, but the data do not rule out involvement of less reactive radicals. The reactivity and extrahyphal location of this PEO-degrading system suggest that its natural function is to participate in the brown rot of wood and that it may enable brown-rot fungi to degrade recalcitrant organopollutants.

The biodegradation of recalcitrant effluents from an olive mill by a white-rot fungus

Biodegradation of olive-mill wastewater (OMW) was performed by the polyurethane-immobilized mycelium of Lentinula edodes. Throughout three consecutive treatment cycles of the effluent significant abatements of its polluting characteristics were observed. In fact, its contents in total organic carbon, total phenols, total ortho-diphenol were dramatically reduced. In addition, a significant effluent decolorization was evident.

A kinetic and ESR investigation of iron(II) oxalate oxidation by hydrogen peroxide and dioxygen as a source of hydroxyl radicals

The reaction of Fe(II) oxalate with hydrogen peroxide and dioxygen was studied for oxalate concentrations up to 20 mM and pH 2-5, under which conditions mono- and bis-oxalate complexes (Fe[II](ox) and Fe[II](ox)2[2-]) and uncomplexed Fe2+ must be considered. The reaction of Fe(II) oxalate with hydrogen peroxide (Fe2+ + H2O2 --> Fe3+ + .OH + OH-) was monitored in continuous flow by ESR with t-butanol as a radical trap. The reaction is much faster than for uncomplexed Fe2+ and a rate constant, k = 1 x 10(4) M(-1) s(-1) is deduced for Fe(II)(ox). The reaction of Fe(II) oxalate with dioxygen is strongly pH dependent in a manner which indicates that the reactive species is Fe(II)(ox)2(2-), for which an apparent second order rate constant, k = 3.6 M(-1) s(-1), is deduced. Taken together, these results provide a mechanism for hydroxyl radical production in aqueous systems containing Fe(II) complexed by oxalate. Further ESR studies with DMPO as spin trap reveal that reaction of Fe(II) oxalate with hydrogen peroxide can also lead to formation of the carboxylate radical anion (CO2-), an assignment confirmed by photolysis of Fe(II) oxalate in the presence of DMPO.

A Mechanism for Production of Hydroxyl Radicals by the Brown-Rot Fungus Coniophora Puteana: Fe(III) Reduction by Cellobiose Dehydrogenase and Fe(II) Oxidation at a Distance from the Hyphae

In timber infested by brown-rot fungi, a rapid loss of strength is attributed to production of hydroxyl radicals (HO.). The hydroxyl radicals are produced by the Fenton reaction [Fe(II)/H2O2], but the pathways leading to Fe(II) and H2O2 have remained unclear. Cellobiose dehydrogenase, purified from cultures of Coniophora puteana, has been shown to couple oxidation of cellodextrins to conversion of Fe(III) to Fe(II). Two characteristics of brown rot are release of oxalic acid and lowering of the local pH, often to about pH 2. Modelling of Fe(II) speciation in the presence of oxalate has revealed that Fe(II)-oxalate complexes are important at pH 4-5, but at pH 2 almost all Fe(II) is in an uncomplexed state which reacts very slowly with dioxygen. Diffusion of Fe(II) away from the hyphae will promote conversion to Fe(II)-oxalate and autoxidation with H2O2 as product. Thus the critical Fe(II)/H2O2 combination will be generated at a distance, enabling hydroxyl radicals to be formed without damage to the hyphae.

Oxalate production by fungi: its role in pathogenicity and ecology in the soil environment

Oxalate secretion by fungi provides many advantages for their growth and colonization of substrates. The role of oxalic acid in pathogenesis is through acidification of host tissues and sequestration of calcium from host cell walls. The formation of calcium oxalate crystals weakens the cell walls, thereby allowing polygalacturonase to effect degradation more rapidly in a synergistic response. There is good correlation between pathogenesis, virulence, and oxalic acid secretion. Solubility of soil nutrients is achieved by soil-living species, when cations freed by oxalate diffusing in clay layers increases the effective solubility of Al and Fe. Oxalate retained in hyphal mats of mycorrhizal species increases phosphate and sulphate availability. The formation of calcium oxalate crystals provides a reservoir of calcium in the ecosystem. The ability of oxalate to bind divalent cations permits detoxification of copper, particularly evident in wood preserved with copper salts. Oxalate plays a unique role in lignocellulose degradation by wood-rotting basidiomycetes, acting as a low molecular mass agent initiating decay. In addition, in white-rot fungi oxalate acts as a potential electron donor for lignin-peroxidase catalysed reduction and chelates manganese, allowing the dissolution of Mn3+from the manganese–enzyme complex and thus stimulating extracellular manganese peroxidase activity. The biosynthesis and degradation of oxalate are discussed.Key words: oxalic acid, calcium oxalate, pathogenicity, fungi.

Degradation of the lignocellulose complex in wood

Degradation of the lignocellulose complex in wood varies depending on the microorganism causing decay. The degradative processes of white-, brown-, and soft-rot fungi as well as different forms of bacterial degradation are presented. Ultrastructural methods were used to elucidate cell-wall alterations that occurred during the various stages of decay. In wood inoculated with the white-rot fungus Ceriporiopsis subvermispora, changes in the cell wall, such as electron-dense zones after staining with uranyl acetate, were evident during incipient stages of decay. The ratio of syringyl:guaiacyl lignin of different woods, different cell types, and even the different layers within a cell wall influenced the type and extent of decay by white-rot fungi. Soft rots caused unique changes in the lignocellulose matrix. The type of wood substrate governed the form (type I or type II) of soft rot that occurred. Brown-rot fungi depolymerized cellulose early in the decay process and degraded cellulose without prior removal of lignin. Bacterial degradation was common in waterlogged woods and three forms, tunneling, erosion and cavitation, are discussed. In addition to an improved understanding of decay processes in living trees and forest products, knowledge of decomposition mechanisms is important to utilize effectively these microorganisms for new industrial bioprocessing technologies. Key words: biodegradation, white rot, brown rot, soft rot, bacterial degradation.

High-yield production of oxalic acid for metal leaching processes by Aspergillus niger

The complex-forming compound oxalic acid can effectively solubilise metals such as aluminium, iron, lithium and manganese. In order to produce high amounts of oxalic acid for biohydrometallurgical processes, it was the aim of this work to optimise oxalic acid production by Aspergillus niger, a fungus well known for its ability to produce oxalic acid. A. niger excreted 427 mmol oxalic acid l-1 if it was cultivated in a pH-controlled (pH 6.0) fed-batch run in a 2-l stirred tank reactor. Sucrose and lactose permeate were suitable carbon sources for oxalic acid production. In sucrose medium, A. niger produced high amounts of gluconic and oxalic acids, whereas in lactose permeate medium only oxalic acid was produced. Cultivation in green syrup and molasses media lead to high yields of biomass, but low oxalic acid production (< 20 mmol l-1).