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Synthesis and Transformation of Phenolic Compounds by Epicoccum nigrum in Relation to Humic Acid Formation¹

ABSTRACT

During the growth of the fungus Epicocium nigrum in glucose-asparagine medium, 2,4-dihydroxy-6-methylbenzoic acid(orsellinic acid) and 2-methyl-3,5-dihydroxybenzoic acid (cresorsellinic acid) are synthesized. These are altered by decarboxylation, by introduction of additional hydroxyl groups, by oxidation of methyl groups, and by further decarboxylation to form over 20 phenols. Some of the phenols are trihydroxy benzene compounds such as 2,4,5-trihydroxy toluene, which are autoxidizable at pH values of 6 or above. These compounds, after autoxidation to quinones, couple with resorcinol-type phenols or with o-diphenols. Nonresorcinol type phenols added to the culture solution are altered by introduction of hydroxyl groups, by demethoxylation, and by oxidative splitting of the side chain. The benzene ring is essentially resistant to degradation by the fungus. Amino acid compounds present in the culture medium or formed by the organism are either linked into oxidative polymers of the phenols or are deaminated by action of some of the oxidized phenols. "Humic acid" formation commences at about pH 6 in the nutrient solution and is associated with the disappearance of the phenols. Carbon-14 studies indicated that the phenol ring of added compounds takes part in polymer formation. With time, mixtures of the same phenols synthesized by the fungus and peptides or amino acids will react to give "humic acid"-type compounds, However, evidence for weak phenoloxidase activity in the fungus mycelium was found. During autolysis this phenolase may be released into the medium and could further an oxidative polymerization of the phenols and their autoxidation products.

¹ Joint contribution of the Inst. für Biochem. Des Bodens der Forschungsanstalt für Landwirtschaft Braunschweig-Völkenrode, Germany, and Univ. of California, Citrus Res. Center and Agr. Exp. Sta., Riverside, Calif.

² Biochemist, Institute für Biochemie des Bodens and Professor of Soil Science, respectively. The authors give thanks to Miss E. Pleiss and Miss H. Lemke for laboratory assistance. Dr. J. Chr. Salfeld and E. Baume furnished some of the phenols used for comparison. Prof. Dr. W. Flaig made helpful suggestions.

Received for publication April 6, 1967. Accepted for publication July 13, 1967.