Nitrous oxide reduction in nodules: denitrification or n(2) fixation?

Area of Science:

• Microbiology
• Environmental Science
• Biochemistry

Background:

• Rhizobium bacteria in cowpea nodules play a role in nitrogen fixation.
• Nitrous oxide (N2O) is a potent greenhouse gas, and its reduction is crucial for mitigating climate change.
• Understanding the microbial pathways of N2O reduction is essential for agricultural and environmental management.

Purpose of the Study:

• To investigate the N2O reduction pathways in two different cowpea rhizobium strains.
• To compare the efficiency of nitrous oxide reductase versus nitrogenase in N2O reduction under varying atmospheric conditions.
• To determine the primary mechanism of N2O reduction in a nitrous oxide reductase-positive rhizobium strain.

Main Methods:

• Incubation of detached cowpea nodules containing specific rhizobium strains (8A55 Nor+, 32H1 Nor-) with 15N-labeled N2O.
• Exposure to three different atmospheric conditions: aerobic with acetylene, aerobic without acetylene, and anaerobic.
• Quantification of N2 production and acetylene reduction activity.

Main Results:

• Strain 8A55 (Nor+) exhibited significantly higher N2 production under anaerobic conditions compared to strain 32H1 (Nor-).
• Aerobic N2 production by strain 8A55 was approximately tenfold greater than by strain 32H1 in the absence of acetylene.
• Acetylene reduction activity was slightly higher in strain 32H1, but N2 production was dominated by strain 8A55.

Conclusions:

• The primary pathway for N2O reduction in the denitrifying rhizobium strain 8A55 is through nitrous oxide reductase, not nitrogenase.
• Environmental conditions, particularly anaerobiosis, favor N2O reduction by nitrous oxide reductase in rhizobium.
• These findings have implications for understanding greenhouse gas dynamics in legume-based agricultural systems.