Two enzymes, oxaloacetase and glyoxylate oxidase that catalyze the hydrolysis of oxaloacetate and the oxidation of glyoxylate, respectively, are responsible for the biosynthesis of oxalate. An important aspect is that LiPs and MnPs are capable of decomposing oxalate in the presence of VA or Mn2+ [98,125]. The breakage of ox­alate results in the formation of carbon dioxide and formate anion radical (R-CO+-), which is further oxidized by O2 to give CO2 and superoxide (O+- or HOO+) under aerobic conditions. The active oxygen species are suggested to directly participate in the oxidation of lignin. This reaction can be observed in oxidation of phenol red and kojic acid by MnP in the presence of Mn2+ and oxalate without exogenous addition of H2O2. This suggests that oxalate maybe regarded as a passive sink for H2O2 pro­duction [98]. If the oxalate reduces the VA+- and Mn3+ ions, the mineralization rate of lignin will be affected adversely (Figs. 1.2 and 1.3). As mentioned earlier, VA+- and Mn3+ both should be reduced by phenolic and/or non-phenolic compounds of lignin for the effective degradation of lignin. For better biological treatment, it is important to conquer the excessive action of oxalate on VA+- and Mn3+.