NO and N2O transformations of diverse fungi in hypoxia: evidence for anaerobic respiration only in Fusarium strains

Fungal denitrification is claimed to produce non-negligible amounts of N2 O in soils, but few tested species have shown significant activity. We hypothesized that denitrifying fungi would be found among those with assimilatory nitrate reductase, and tested 20 such batch cultures for their respiratory metabolism, including two positive controls, Fusarium oxysporum and Fusarium lichenicola, throughout the transition from oxic to anoxic conditions in media supplemented with NO 2 - . Enzymatic reduction of NO 2 - (NIR) and NO (NOR) was assessed by correcting measured NO- and N2 O-kinetics for abiotic NO- and N2 O-production (sterile controls). Significant anaerobic respiration was only confirmed for the positive controls and for two of three Fusarium solani cultures. The NO kinetics in six cultures showed NIR but not NOR activity, observed through the accumulation of NO. Others had NOR but not NIR activity, thus reducing abiotically produced NO to N2 O. The presence of candidate genes (nirK and p450nor) was confirmed in the positive controls, but not in some of the NO or N2 O accumulating cultures. Based on our results, we conclude that only the Fusarium cultures were able to sustain anaerobic respiration and produced low amounts of N2 O as a response to an abiotic NO production from the medium.