The mechanism by which brown-rot fungi depolymerize wood and cellulose remains a perplexing problem. Current evidence favors oxidation by low-molecular weight, non-enzymatic fungal metabolites. During attack of cotton cellulose by Postia placenta MAD-698, the degree of polymerization (DP) decreases to limit of degree of polymerization (LODP) over a period of 5-6 weeks with relatively low weight loss (circa 20%). Attack by MAD-698 can be accelerated by glucose and gluconic acid. At or below a DP of 350, dispersion of macrofibers was observed in aqueous Triton X-100 (0.1%). Similar dispersion was observed when cotton cellulose was pretreated with 1.0 N oxalic acid prior to colonization with Postia placenta MAD-698 and strain ME-20. The latter is not able to depolymerize cotton cellulose or cause weight loss of wood in soil block tests. The purpose of this paper is to describe the fragmentation of cotton cellulose, with or without pretreatment with oxalic acid, by Postia placenta. We conclude that short fiber formation correlates with depolymerization near to LODP, and that oxalic acid likely contributes to the process without effecting a measurable reduction in percent crystallinity of the cellulosic structure.

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