Ammonia as the driving factor for aerobic ammonia oxidizers

Ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and complete ammonia oxidizers (comammox) comprise aerobic ammonia oxidizers (AO). Here, we present the ammonium concentration as a determinant of the distribution of ammonia-oxidizers. The ecophysiology of comammox enrichment Cmx-BO4 was characterized by growing it in different ammonium concentrations, pH, light and nitrite concentrations. Cmx-BO4 oxidized up to 3 mM ammonium, with the highest growth rate at 10 M ammonium. The comammox enrichment oxidized ammonium through pH 6 to 8 and was inhibited by blue and white light. Cmx-BO4 oxidized nitrite in presence of ammonium but did not oxidize nitrite only. The genomic characterization of comammox strain Nitrospira sp. BO4 in the comammox enrichment Cmx-BO4 was performed through metagenomic sequencing. The metagenome assembled genome (MAG) of Nitrospira sp. BO4 possessed genes for ammonia and nitrite oxidation. The MAG, however, lacked genes for assimilatory nitrite reduction and nitrite transporters, corresponding to the inability of the culture to consume nitrite as the nitrogen source.Comammox Nitrospira sp. BO4 was competed for ammonium against AOA Nitrosoarchaeum sp. BO1 to investigate if the two oligotrophic AO had niche differentiation or if they co-existed in nature. Cmx-BO4 and AOA-BO1 have Nitrosopira sp. BO4 and Nitrosoarchaeum sp. BO1 as the AO respectively. Nitrospira sp. BO4 outcompeted Nitrospira sp. BO1 at all three tested ammonium concentrations (