Degradation of humic acids by the litter-decomposing basidiomycete Collybia dryophila

The basidiomycete Collybia diyophila K209, which colonizes forest soil, was found to decompose a natural humic acid isolated from pine-forest litter (LHA) and a synthetic C-14-Iabeled humic acid (14 C-HA) prepared from [U-C-14] catechol in liquid culture. Degradation resulted in the formation of polar, lower-molecular-mass fulvic acid (FA) and carbon dioxide. RA decomposition was considerably enhanced in the presence of Mn 21 (200 muM), leading to 75% conversion of LHA and 50% mineralization of C-14-HA (compared to 60% and 20%, respectively, in the absence of Mn 21). There was a strong indication that manganese peroxidase (MnP), the production of which was noticeably increased in Mn2+-supplemented cultures, was responsible for this effect. The enzyme was produced as a single protein with a pI of 4.7 and a molecular mass of 44 kDa. During solid-state cultivation, C diyophila released substantial amounts of water-soluble FA (predominantly of 0.9 kDa molecular mass) from insoluble litter material. The results indicate that basidiomycetes such as C diyophila which colonize forest litter and soil are involved in humus turnover by their recycling of highmolecular-mass humic substances. Extracellular MnP seems to be a key enzyme in the conversion process.