a, Maximum likelihood phylogeny based on the concatenated alignment of single copy housekeeping (left) and 16S rRNA (right) genes in SAGs from this study, SAR11 and representative alphaproteobacterial genomes. Values in parentheses denote the number of housekeeping genes used per genome. For the 16S-based tree, only full-length sequences from the genomes in the left tree were included. Star symbols of the same color represent closely related narG genes (>97% aa identity), encoding the catalytic subunit of the respiratory nitrate reductase of the DMSO family. b, Abundance of SAR11 subclades (left) in selected oceanic metagenomes. Note that the major nar-encoding clade IIa.A peaks in abundance at oxygen-depleted OMZ depths. Dataset descriptions are available in Supplementary Table 1. c, Normalized average coverage of SAR11 subclades in ETNP metatranscriptomes. Transcription by nar-encoding lineages increases from the base of the oxycline (85 m) to spike at the OMZ core (300 m), but is negligible in the overlying oxic zone (30 m).