Most basidiomycetous fungi produce oxalic acid as a result of their metabolic activities and nutrient procurement. There is currently a renewed interest in the role that oxalic acid may play in the decomposition of wood by basidiomycete fungi. It has been observed that although most wood degrading fungi have the capacity to produce oxalic acid, not all of these organisms express this capacity equally in the wood environment. In addition, not all of the fungi which produce oxalic acid will accumulate this metabolite. Very often the production of oxalic acid is coincident with the precipitation of oxalate salts such as calcium oxalate. At this time it is unclear as to what controls the differential production and accumulation of oxalate by wood degrading fungi. An investigative series of experiments was established using the brown rot fungus Gloeophyllum trabeum to examine the conditions which favor oxalate production and accumulation as manifested through the production of metastable calcium oxalate dihydrate crystals. Batch cultures which were buffered by base cation exchange sites of mineral amendments stimulated the production of calcium oxalate crystals. The results of these buffering experiments indicate that at pH values below approximately 6.0 there is a diminution of oxalate accumulation, and that a pH environment of approximately 3.0 is consistently attained in weakly buffered cultures inoculated with this fungus. These pH values correspond to the pH optima for oxaloacetase (EC 3.7.1.1) and oxalate decarboxylase (EC 4.1.1.2) respectively, and thereby suggest a mechanism for both pH control and oxalate production and accumulation.

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