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

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_m) of two freshwater AOA enrichments, an AOA soil isolate (N. viennensis), and a freshwater AnAOB enrichment. The AOA enrichments had similar kinetics (μ_max ≈ 0.36 d^-1, k_m,NH4 ≈ 0.78 µM, and k_m,O2 ≈ 2.9 µM), whereas N. viennensis had similar k_m values but lower μ_max (0.23 d^-1). In agreement with the current paradigm, these AOA strains showed a higher affinity for ammonia (lower k_m,NH4; 0.34-1.27 µM) than published AOB measurements (>20 µM). The slower growing AnAOB (μ_max ≈ 0.16 d^-1) had much higher k_m values (k_m,NH4 ≈ 132 µM, k_m,NO2 ≈ 48 µM) and were inhibited by oxygen at low levels (half-oxygen inhibition; k_i,O2 ≈ 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.