Fungal denitrification revisited – Recent advancements and future opportunities

Fungi play a key role in the nitrogen cycle. Diverse fungi are known to reduce nitrate or nitrite to gaseous nitrogen oxides such as nitric oxide, nitrous oxide (N₂O), and dinitrogen via denitrification or co-denitrification (microbially mediated nitrosation), and to ammonium via ammonia fermentation (fungal dissimilatory nitrate reduction to ammonium). These processes could significantly contribute to the emission of N₂O from soils and the removal of nitrogen from nitrate and nitrite-contaminated environments. However, fungal N₂O production may not be necessarily related to their denitrification activity sensu stricto (i.e., reduction of nitrate or nitrite to gaseous N oxides for respiration): N₂O can be produced by partially abiotic processes. Therefore, fungi that can reduce nitrate or nitrite to N₂O should not be called denitrifying fungi instantaneously. Experiments should be carefully conducted to better discriminate fungal denitrification, co-denitrification, and chemo-denitrification. Various analytical tools have been developed and applied to clarify fungal denitrification and other nitrate/nitrite reduction processes, including the substrate-induced respiration-inhibition method, stable isotope analyses, and culture-dependent and -independent molecular and genomic approaches. In this mini-review, we overview fungal denitrification and other nitrate/nitrite reduction processes, discuss their environmental impacts, summarize recent advancements in the methods to study fungal denitrification, and provide insights on future research opportunities.