Ammonia fermentation, a novel anoxic metabolism of nitrate by fungi

The induction of fungal denitrification by Fusarium oxysporum requires a minimal amount of O(2), although excess O(2) completely represses this process (Zhou, Z., Takaya, N., Sakairi, M. A. C., and Shoun, H. (2001) Arch. Microbiol. 175, 19-25). Here we describe another metabolic mechanism of nitrate in fungal cells, termed ammonia fermentation, that supports growth under conditions more anoxic than those of denitrification. The novel nitrate metabolism of eukaryotes consists of the reduction of nitrate to ammonium coupled with the catabolic oxidation of electron donors to acetate and substrate-level phosphorylation. F. oxysporum thus has two pathways of dissimilatory nitrate reduction that are alternatively expressed in response to environmental O(2) tension. F. oxysporum prefers O(2) respiration when the O(2) supply is sufficient. We discovered that this fungus is the first eukaryotic, facultative anaerobe known to express one of three distinct metabolic energy mechanisms closely depending on environmental O(2) tension. We also showed that ammonia fermentation occurs in many other fungi that are common in soil, suggesting that facultative anaerobes are widely distributed among fungi that have been considered aerobic organisms.