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

Abstract

Pure cultures of the marine ammonium-oxidizing bacterium Nitrosomonas 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 NO 2 - , from 3.6 × 10 −10 to 0.5 × 10 −10 mmol of NO 2 - per cell per day. In contrast, evolution of N 2 O increased from 1 × 10 −12 to 4.3 × 10 −12 mmol of N per cell per day. The yield of N 2 O relative to NO 2 - increased from 0.3% to nearly 10% (moles of N in N 2 O per mole of NO 2 - ) as the oxygen level was reduced, although bacterial growth rates changed by less than 30%. Nitrifying bacteria from the genera Nitrosomonas, Nitrosolobus, Nitrosospira , and Nitrosococcus exhibited similar yields of N 2 O at atmospheric oxygen levels. Nitrite-oxidizing bacteria ( Nitrobacter sp.) and the dinoflagellate Exuviaella sp. did not produce detectable quantities of N 2 O during growth. The results support the view that nitrification is an important source of N 2 O in the environment.