Synthetic Lignin Mineralization by Ceriporiopsis subvermispora Is Inhibited by an Increase in the pH of the Cultures Resulting from Fungal Growth.
(sup14)C-synthetic lignin mineralization by the basidiomycete Ceriporiopsis subvermispora occurs at the highest rate (about 30% after 29 days) in liquid cultures containing 1% glucose and a growth-limiting amount (1 mM) of ammonium tartrate. The titers of manganese peroxidase (MnP) and laccase are lower in these cultures than in cultures containing 1% glucose and 10 mM ammonium tartrate, where the extent of lignin mineralization in the same period is only about 15%. The inverse correlation between enzyme activity and lignin mineralization is also observed when ammonium tartrate is replaced by ammonium chloride or Casamino Acids as the source of nitrogen. This phenomenon can be explained by a gradual increase in the pH of the medium that takes place only in the cultures with high nitrogen concentrations. Supporting this finding, when cultures with 1 mM ammonium tartrate were grown at different pHs, (sup14)CO(inf2) evolved more rapidly from those with pH values near the optimum for MnP activity. On the other hand, (sup14)CO(inf2) evolution from cultures containing 1% glucose supplemented with 1 mM ammonium tartrate plus 9 mM sodium tartrate was as low as that from cultures with a high ammonium tartrate concentration. Since the changes in the pH of these cultures were not as pronounced as those in cultures containing high nitrogen concentrations, tartrate itself may also be contributing to limit the extent of lignin mineralization. Considering that pH instability seems to constitute a common feature of fungal cultures, precautions must be taken to avoid underestimation of their ligninolytic efficiencies.
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