Friedemann Bublitz

Degradation of lignite (low-rank coal) by ligninolytic basidiomycetes and their manganese peroxidase system

Abstract Ligninolytic basidiomycetes (wood and leaf-litter-decaying fungi) have the ability to degrade low-rank coal (lignite). Extracellular manganese peroxidase is the crucial enzyme in the depolymerization process of both coal-derived humic substances and native coal. The depolymerization of coal by Mn peroxidase is catalysed via chelated Mn(III) acting as a diffusible mediator with a high redox potential and can be enhanced in the presence of additional mediating agents (e.g. glutathione). The depolymerization process results in the formation of a complex mixture of lower-molecular-mass fulvic-acid-like compounds. Experiments using a synthetic 14C-labeled humic acid demonstrated that the Mn peroxidase-catalyzed depolymerization of humic substances was accompanied by a substantial release of carbon dioxide (17%–50% of the initially added radioactivity was released as 14CO2). Mn peroxidase was found to be a highly stable enzyme that remained active for several weeks under reaction conditions in a liquid reaction mixture and even persisted in sterile and native soil from an opencast mining area for some days.

Fungal attack on coal II. Solubilization of low-rank coal by filamentous fungi

Abstract The solubilization of low-rank coal by fungi is not accompanied by its depolymerization, which was proven by several screening programs. A total number of 728 fungal strains were yet tested for solubilizing a German low-rank coal (lignite). The coal were used oxidatively pretreated (3% H 2 O 2 ) and natively. During the first screening, the nitrogen content of the media were reduced to select fungi, which solubilize coal under nitrogen-limited conditions. Among 480 micromycetous fungi tested, at least ten strains were able to solubilize native low-rank coal pieces on a nitrogen-reduced agar-medium. They all belong to the conidiospores forming deuteromycetes (molds). Under ‘solubilizing’ conditions, only two of these ten strains showed weak extracellular activities (oxidases, peroxidases) toward 2,2′-azinobis(3-ethylbenzthiazoline-6-sulphonate) [ABTS]. In a second screening, 256 wood and litter decaying basidiomycetes, above all white-rot fungi, were tested for their ability to solubilize low-rank coal under ligninolytic conditions: not any strain was able to form black coal droplets or conspicuous diffusion areas. By using a nitrogen-rich medium, a few basidiomycetes showed also coal solubilizing activities, however, their ligninolytic enzymes were inhibited. The ‘typical’ solubilization of low-rank coal (formation of black liquids from solid coal particles) by molds depends mainly on the nitrogen content of the medium (resulting in higher pH values) as well as on the oxidation grade of coal (e.g. H 2 O 2 -pretreatment). Extracellular oxidases and peroxidases, both ligninolytic and nonligninolytic ones, seem to play a minor, (or not any), role for this process.

Fungal attack on coal: I. Modification of hard coal by fungi

Abstract Within a screening program more than 750 fungal strains were tested for their ability to attack a German hard coal (Westerholt Mine). Six strains were selected, which modified the physico-chemical properties of hard coal pieces placed on the overgrown surface of Petri dishes (loss of the compact coal structure, ‘erosion,’ increase in wettability). One of these strains, Coprinus sclerotigenis C142-1, liberated 2-hydroxybiphenyl, alkylated benzenes and polycyclic aromatic hydrocarbons (PAH) from powdered hard coal. It is presumed that most of these compounds were liberated from micropores inside of the hard coal macromolecule. Investigations using hard coal derived asphaltenes indicate that the liberation of hydroxylated biphenyls by C. sclerotigenis is due to a real cleavage of chemical bonds. The cultivation of the white-rot fungus Panus tigrinus on wood shavings coated with asphaltenes led to a decrease of the average molecular weights of these hard coal-derived hydrogenation products.

Depolymerization of Straw Lignin by Manganese Peroxidase from Nematoloma frowardii is Accompanied by Release of Carbon Dioxide

Summary Manganese peroxidase preparations (MnP) from the white-rot fungus Nematoloma frowardii were able to release 14CO2 directly from 14C-labeled milled wheat straw (MWS; total lignin fraction) and milled straw lignin (MSL; dioxane soluble part of MWS). Apart from the formation of 14CO2 (4–10 %) the treatment of insoluble MWS and MSL with MnP resulted in the formation of water-soluble 14C-lignin fragments (lignin solubilization, 14–25%). Analyses with gel permeation chromatography (GPC) demonstrated the formation of lignin fragments with predominant molecular masses around 1 kDa. The extent of MWS mineralization and solubilization was enhanced in the presence of reduced glutathione (GSH) acting as thiol mediator, whereas MSL mineralization was not stimulated by GSH. The principle of direct extracellular mineralization of lignin catalyzed by the MnP system may make a significant contribution to the formation of carbon dioxide in lignincellulose containing habitats.