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 2O 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 2O 2-pretreatment). Extracellular oxidases and peroxidases, both ligninolytic and nonligninolytic ones, seem to play a minor, (or not any), role for this process.