Denitrification of Nitrate by the FungusCylindrocarpon tonkinense

The denitrifying fungus Cylindrocarpon tonkinense was thought to be able to denitrify only nitrite (NO2-) but not nitrate (NO3-) to form nitrous oxide (N2O). Here we found, however, that C. tonkinense can denitrify NO3- under certain conditions. Presence of ammonium (NH3+) in addition to NO3- and the use of a fermentable sugar as an electron donor were key conditions for inducing the denitrifying activity. Such induction accompanied a remarkable increase in the intracellular level of the enzyme activities related to NO3- metabolism. These activities contained assimilatory type NADPH (or NADH)-dependent NO3- reductase (aNar), dissimilatory nitrite reductase (dNir), and nitric oxide reductase (P450nor), but did not contain ubiquinol-dependent, dissimilatory NO3- reductase (dNar). The denitrification was inhibited by tungstate, an inhibitor of Nar. These results demonstrated occurrence of a novel type of denitrification in C. tonkinense, in which assimilatory type Nar is possibly involved.