Role of nitrite reductase in the ammonia-oxidizing pathway of Nitrosomonas europaea
Metabolism of ammonia (NH3) and hydroxylamine (NH2OH) by wild-type and a nitrite reductase (nirK) deficient mutant of Nitrosomonas europaea was investigated to clarify the role of NirK in the NH3 oxidation pathway. NirK-deficient N. europaea grew more slowly, consumed less NH3, had a lower rate of nitrite (NO2−) production, and a significantly higher rate of nitrous oxide (N2O) production than the wild-type when incubated with NH3 under high O2 tension. In incubations with NH3 under low O2 tension, NirK-deficient N. europaea grew more slowly, but had only modest differences in NH3 oxidation and product formation rates relative to the wild-type. In contrast, the nirK mutant oxidized NH2OH to NO2− at consistently slower rates than the wild-type, especially under low O2 tension, and lost a significant pool of NH2OH–N to products other than NO2− and N2O. The rate of N2O production by the nirK mutant was ca. three times higher than the wild-type during hydrazine-dependent NO2− reduction under both high and low O2 tension. Together, the results indicate that NirK activity supports growth of N. europaea by supporting the oxidation of NH3 to NO2− via NH2OH, and stimulation of hydrazine-dependent NO2− reduction by NirK-deficient N. europaea indicated the presence of an alternative, enzymatic pathway for N2O production.
KeywordsAmmonia oxidation Hydroxylamine oxidation Nitrifier denitrification Nitrite reductase Nitrosomonas europaea Nitrous oxide
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