Extracellular oxidative enzymes of coal-attacking fungi

Abstract

The participation of extracellular oxidative enzymes, as known from lignin degraders, in fungal coal degradation is demonstrated and discussed in this paper. Two basidiomycete strains that are able to solubilize German lignite (brown coal) were isolated and characterized according to their enzymology. One of them (strain RBS 1k) produces extracellular Mn-peroxidase, the other (strain RBS 1b) excretes two oxidative enzymes with laccase and peroxidase activity. These enzymes were excreted shortly before the beginning of coal solubilization. Different coal substances can affect the production of these exoenzymes. The production of extracellular peroxidase by strain RBS 1k can be induced by the addition of native lignite powder or different lignite fractions (bitumen, matrix and humic acids) to the culture medium. Fungal growth was enhanced under these culture conditions, too. On the other hand, production of extracellular laccase activity by strain RBS 1b can be stimulated by the addition of water-soluble humic acids, too, but is inhibited by the addition of other coal fractions (bitumen and matrix) or by native lignite powder. In the direct neighbourhood of lignite pieces the fungal surface cultures of the basidiomycete RBS 1k show guttation. The drops are dark brown colored and contain strong peroxidase activity (up to 300 U/ml). But this cellfree solution is not able to solubilize coal, alone. The fungus RBS 1k possesses enzymatic capacities for modifying the structure of coal-derived humic acids. The strain can metabolize these alkali-soluble coal macromolecules under cometabolic conditions by simultaneous excretion of peroxidase, which probably is involved in the degradation process. In vitro-tests with the cellfree peroxidase (RBS 1k) or laccase (RBS 1b), purified by affinity chromatography, could not demonstrate the ability of these enzymes to promote water solubility of low rank coals, alone.