Competition between Ammonia-Oxidizing Archaea and Bacteria from Freshwater Environments

Area of Science:

• Environmental microbiology
• Biogeochemical cycles
• Microbial ecology

Background:

• Ammonia oxidation is a key step in the global nitrogen cycle, performed by ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB).
• These microorganisms coexist in various natural environments, competing for ammonium as their energy and nitrogen source.
• Understanding their competition dynamics is crucial for predicting microbial community shifts in response to changing nutrient availability.

Purpose of the Study:

• To investigate the competitive interactions between enriched freshwater cultures of AOA (AOA-AC1) and AOB (AOB-G5-7) for ammonium.
• To determine how varying ammonium concentrations influence the dominance of AOA versus AOB.
• To correlate laboratory findings with environmental observations regarding AOA and AOB distribution.

Main Methods:

• Enrichment of AOA (related to *Nitrosotenuis* spp.) and AOB (related to *Nitrosomonas* spp.) from freshwater systems.
• Growth experiments in batch cultures to assess growth rates at different ammonium concentrations.
• Chemostat competition experiments under ammonium-limiting conditions to evaluate dominance.

Main Results:

• AOB-G5-7 exhibited higher growth rates than AOA-AC1 at elevated ammonium concentrations.
• Under ammonium-limiting conditions in chemostats, AOA-AC1 dominated, while AOB-G5-7 declined.
• In batch cultures, AOA-AC1 was dominant at 50 μM ammonium, whereas AOB-G5-7 dominated at 500 μM ammonium.

Conclusions:

• AOA-AC1 can utilize ammonium at concentrations too low for AOB-G5-7, conferring a competitive advantage in oligotrophic conditions.
• AOB-G5-7 thrives at higher ammonium concentrations, indicating dominance in eutrophic environments.
• These findings align with environmental data, suggesting AOA prevail in oligotrophic and AOB in eutrophic freshwater systems.