Aerobic denitrification as an N2O source from microbial communities.

Area of Science:

• Environmental Microbiology
• Biogeochemistry
• Greenhouse Gas Emissions

Background:

• Nitrous oxide (N2O) is a potent greenhouse gas primarily produced by microbial processes.
• Traditionally, aerobic N2O emissions are linked to nitrification and anoxic emissions to denitrification.
• Frequent oxygen fluctuations are common in natural and engineered ecosystems, influencing microbial activity.

Purpose of the Study:

• To quantitatively assess the contribution of heterotrophic denitrification to aerobic nitrogen turnover and N2O emissions.
• To investigate N2O production under cyclic oxygen availability in nitrification-inhibited cultures.
• To understand the ecological implications of aerobic denitrification in dynamic environments.

Main Methods:

• Established two planktonic, nitrification-inhibited enrichment cultures.
• Fed cultures with continuous organic carbon and nitrate under cyclic oxygen availability.
• Quantified substrate respiration, N2O production, and enzyme activity under varying oxygen concentrations.

Main Results:

• Over a third of organic substrate was respired using nitrate as an electron acceptor even at high oxygen levels (>6.5 mg/L).
• Nitrous oxide (N2O) accounted for up to 25% of nitrate reduced under oxic conditions.
• Microorganisms maintained denitrifying enzymes due to frequent oxic/anoxic transitions, enabling residual aerobic denitrification.

Conclusions:

• Heterotrophic denitrification significantly contributes to aerobic nitrogen turnover and N2O emissions, challenging the established dichotomy.
• Organisms capable of aerobic denitrification possess a competitive advantage in dynamic oxygen environments.
• The role of heterotrophic denitrification in N2O emissions within fluctuating environments is likely underestimated.