Oxygen requirement for denitrification by the fungus Fusarium oxysporum

The effects of dioxygen (O-2) on the denitrification activity of the fungus Fusarium oxysporum MT-811 in fed-batch culture in a stirred jar fermentor were examined. The results revealed that fungal denitrifying activity requi res a minimal amount of O(2)or induction, which is repressed by excess O-2 The optimal O-2 supply differed between the denitrification substrates : 69 0 mu mol O-2 h(-1) (g dry cell wt.)(-1) for nitrate (NO3-) and about 250 mu mol O-2 h(-1) (g dry cell wt.)(-1) for nitrite (NO2-). The reduction of NO 3- required more O-2 than that of NO2-. With an optimal O-2 supply, 80% and 52% of nitrogen atoms in NO3- and NO2-, respectively, were recovered as th e denitrification product N2O. These features of F. oxysporum differ from t hose of bacterial denitrifiers that work exclusively under anoxic condition s. The denitrification activity of F. oxysporum MT-811 mutants with impaire d NO3- assimilation was about double that of the wild-type strain, suggesti ng competition for the substrate between assimilatory and dissimilatory typ es of NO3- reduction. These results showed that denitrification by F. oxysp orum has unique features, namely, a minimal O-2 requirement and competi tio n with assimilatory NO3-.