Nitric oxide-dependent anaerobic ammonium oxidation

Area of Science:

• Microbiology
• Biogeochemistry
• Astrobiology

Background:

• Nitric oxide (NO) plays diverse roles in biology and atmospheric chemistry, including as a toxin and signaling molecule.
• The ability of microorganisms to utilize NO for growth, especially in early Earth conditions, remains largely unexplored.
• Anaerobic ammonium-oxidizing (anammox) bacteria are known to involve NO as an intermediate in their metabolism.

Purpose of the Study:

• To investigate whether the anammox bacterium Kuenenia stuttgartiensis can grow by coupling ammonium oxidation to nitric oxide reduction.
• To elucidate the metabolic capabilities of anammox bacteria beyond their canonical nitrite-dependent pathway.
• To explore the potential implications of microbial NO metabolism for early Earth environments and current biogeochemical cycles.

Main Methods:

• Culturing Kuenenia stuttgartiensis under anaerobic conditions with ammonium as the electron donor and nitric oxide as the electron acceptor.
• Monitoring bacterial growth and gas production (N₂, N₂O).
• Analyzing gene transcription to assess the expression of proteins involved in NO metabolism under novel growth conditions.

Main Results:

• Kuenenia stuttgartiensis demonstrated growth by coupling ammonium oxidation to nitric oxide reduction, producing solely N₂.
• Under these conditions, genes associated with NO production were transcriptionally downregulated, indicating a shift in metabolic strategy.
• This represents a novel form of microbial metabolism previously uncharacterized in anammox bacteria.

Conclusions:

• The anammox bacterium Kuenenia stuttgartiensis possesses a metabolic capability to grow using nitric oxide reduction, expanding the known functions of this genus.
• This discovery has implications for understanding nitrogen cycling in various ecosystems and controlling emissions of NO and nitrous oxide (N₂O).
• The findings support the hypothesis that microbial nitric oxide-dependent ammonium oxidation could have been a significant metabolic process on early Earth.