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.