Reevaluation of the manganese requirement for the biobleaching of kraft pulp by white rot fungi

Manganese dependent peroxidase (MnP) is the main enzyme implicated in the b iobleaching of kraft pulps by white rot fungi. The goal of this study was t o evaluate the Mn requirement for biobleaching of eucalyptus oxygen deligni fied kraft pulp (OKP) by various white rot fungi: Trametes versicolor, Phan erochaete sordida, Phlebia radiata, Stereum hirsutum and Bjerkandera sp. st rain BOS55. All of the strains tested produced MnP and provided extensive b leaching of OKP when 33 mu M Mn was included in the medium. Bjerkandera sp. strain BOS55 was the only strain that also displayed MnP production and bi obleaching activity of EDTA-extracted OKP in the complete absence of Mn. Ho wever, MnP and biobleaching activity in the absence of Mn was dependent on the presence of organic acids in the medium. The fact the biobleaching was correlated to MnP activity irrespective of whether Mn was present or absent suggests that there may be roles for MnP in Bjerkandera under Mn-deficient conditions. Although manganese-independent peroxidase (MIP) and lignin per oxidase (LiP) were also detected, the titres were much smaller in compariso n with those of MnP, so their relative role in biobleaching can be predicte d to have a minor importance in comparison with MnP. Only in the case of Bj erkandera, was the expression of LiP stimulated in the presence of oxalate but final brightness was not substantially affected. (C) 1999 Elsevier Ltd. All rights reserved.