Affinity informs environmental cooperation between ammonia-oxidizing archaea (AOA) and anaerobic ammonia-oxidizing (Anammox) bacteria.

Anaerobic ammonia-oxidizing (Anammox) bacteria (AnAOB) rely on nitrite supplied by ammonia-oxidizing bacteria (AOB) and archaea (AOA). Affinities for ammonia and oxygen play a crucial role in AOA/AOB competition and their association with AnAOB. In this work we measured the affinity constants for ammonia and oxygen (half-saturation; k) of two freshwater AOA enrichments, an AOA soil isolate (N. viennensis), and a freshwater AnAOB enrichment. The AOA enrichments had similar kinetics (μ ≈ 0.36 d, k ≈ 0.78 µM, and k ≈ 2.9 µM), whereas N. viennensis had similar k values but lower μ (0.23 d). In agreement with the current paradigm, these AOA strains showed a higher affinity for ammonia (lower k; 0.34-1.27 µM) than published AOB measurements (>20 µM). The slower growing AnAOB (μ ≈ 0.16 d) had much higher k values (k ≈ 132 µM, k ≈ 48 µM) and were inhibited by oxygen at low levels (half-oxygen inhibition; k ≈ 0.092 µM). The higher affinity of AOA for ammonia relative to AnAOB, suggests AOA/AnAOB cooperation is only possible where AOA do not outcompete AnAOB for ammonia. Using a biofilm model, we show that environments of ammonia/oxygen counter diffusion, such as stratified lakes, favors this cooperation.