Oxygen requirement for denitrification by the fungus Fusarium oxysporum

Abstract

The effects of dioxygen (O2) 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 requires a minimal amount of O2 for induction, which is repressed by excess O2. The optimal O2 supply differed between the denitrification substrates : 690 micromol O2 x h(-1) (g dry cell wt.)(-1) for nitrate (NO3-) and about 250 micromol O2 x h(-1) (g dry cell wt.)(-1) for nitrite (NO2-). The reduction of NO3- required more O2 than that of NO2- . With an optimal O2 supply, 80% and 52% of nitrogen atoms in NO3- and NO2-, respectively, were recovered as the denitrification product N2O. These features of F. oxysporum differ from those of bacterial denitrifiers that work exclusively under anoxic conditions. The denitrification activity of F. oxysporum MT-811 mutants with impaired NO3- assimilation was about double that of the wild-type strain, suggesting competition for the substrate between assimilatory and dissimilatory types of NO3- reduction. These results showed that denitrification by F. oxysporum has unique features, namely, a minimal O2 requirement and competition with assimilatory NO3-.

Multi-Energy Metabolic Mechanisms of the FungusFusarium oxysporumin Low Oxygen Environments

“…7) F. oxysporum catalyzes the conversion of NO 3 À or NO 2 À to N 2 O under O 2 -limited conditions. 2,8) The cellfree dissimilatory NO 3 À reductase (dNar) and NO 2 À reductase (dNir) activities of F. oxysporum can be reconstituted using artificial electron donors that are also effective in reconstituting the activity of their bacterial counterparts. 2,9,10) The fungal denitrifying system is located in the respiring organelle mitochondrion, and is coupled to the mitochondrial electron transport chain to produce ATP.…”

“…14) We further found that expression of the fungal denitrifying system is regulated by the oxygen (O 2 ) supply, and that denitrification by F. oxysporum requires a minimal amount of O 2 for induction, and is repressed by excess O 2 . 8) It has been reported that an optimal O 2 supply is dependent on denitrified substrates, and that the values are 690 mmol O 2 h À1 g À1 dry cells for NO 3 À and 250 mmol O 2 h À1 g À1 dry cells for NO 2 À . 8) The final denitrified product of NO 3 À or NO 2 À by F. oxysporum is N 2 O, because this fungus does not contain an N 2 O reductase enzyme.…”

“…8) It has been reported that an optimal O 2 supply is dependent on denitrified substrates, and that the values are 690 mmol O 2 h À1 g À1 dry cells for NO 3 À and 250 mmol O 2 h À1 g À1 dry cells for NO 2 À . 8) The final denitrified product of NO 3 À or NO 2 À by F. oxysporum is N 2 O, because this fungus does not contain an N 2 O reductase enzyme. 2) We have found another metabolic system, by which F. oxysporum reduces NO 3 À to ammonium (NH 4 þ ) under anoxic conditions.…”

“…One of the key enzymes, acetaldehyde dehydrogenase (acylating) (AddA), catalyzes acetyl-CoA production, indicating that substratelevel phosphorylation contributes to anoxic cell growth under ammonia fermentation culture conditions. 8) The genes encoding the enzymes involved in ammonia fermentation have been identified using another fungus, Aspergillus nidulans. 18) aNar and aNir are essential for reducing nitrate and for anaerobic cell growth during ammonia fermentation.…”

“…20) In addition to these recently discovered metabolic mechanisms of this fungus, alcohol/lactic acid fermentation is a well-known type of metabolism among fungi. [21][22][23][24] Although all forms of denitrification, ammonia fermentation, and alcohol/lactic acid fermentation can support cell growth under hypoxic and anoxic conditions, 8,17,21) the predominant sequence and relationship between these types of metabolism have not been elucidated. Here we found that the energy metabolism of F. oxysporum is dependent on environmental O 2 tension as well as the carbon source.…”