Displayed are the 450 most abundant proteins from Ca. N. inopinata in the metaproteome from culture ENR4 after incubation with 1 mM ammonium for 48 h. Red arrows and labels highlight key proteins for ammonia and nitrite oxidation. Columns show the mean normalized spectral abundance factor (NSAF), error bars show 1 s.d. of n = 4 biological replicates. In total 1,083 proteins in the metaproteome were unambiguously assigned to Ca. N. inopinata. Only one of the four putative NXR gamma subunits (NxrC) was among the top 450 expressed proteins. The other three NxrC candidates ranked at positions 561, 605 and 931. The AmoE1 protein was ranked at position 520, and HaoB at position 653.