Production of NO 2 - and N 2 O by Nitrifying Bacteria at Reduced Concentrations of Oxygen

Pure cultures of the marine ammonium-oxidizing bacteriumNitrosomonas sp. were grown in the laboratory at oxygen partial pressures between 0.005 and 0.2 atm (0.18 to 7 mg/liter). Low oxygen conditions induced a marked decrease in the rate for production of NO2 - , from 3.6 × 10−10 to 0.5 × 10−10 mmol of NO2 - per cell per day. In contrast, evolution of N2 O increased from 1 × 10−12 to 4.3 × 10−12 mmol of N per cell per day. The yield of N2 O relative to NO2 - increased from 0.3% to nearly 10% (moles of N in N2 O per mole of NO2 - ) as the oxygen level was reduced, although bacterial growth rates changed by less than 30%. Nitrifying bacteria from the generaNitrosomonas, Nitrosolobus, Nitrosospira , andNitrosococcus exhibited similar yields of N2 O at atmospheric oxygen levels. Nitrite-oxidizing bacteria (Nitrobacter sp.) and the dinoflagellateExuviaella sp. did not produce detectable quantities of N2 O during growth. The results support the view that nitrification is an important source of N2 O in the environment.