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Bacteria of the SAR11 clade constitute up to one half of all microbial cells in the oxygen-rich surface ocean. SAR11 bacteria are also abundant in oxygen minimum zones (OMZs), where oxygen falls below detection and anaerobic microbes have vital roles in converting bioavailable nitrogen to [N.sub.2] gas. Anaerobic metabolism has not yet been observed in SAR11, and it remains unknown how these bacteria contribute to OMZ biogeochemical cycling. Here, genomic analysis of single cells from the world's largest OMZ revealed previously uncharacterized SAR11 lineages with adaptations for life without oxygen, including genes for respiratory nitrate reductases (Nar). SAR11 nar genes were experimentally verified to encode proteins catalysing the nitrite-producing first step of denitrification and constituted ~40% of OMZ nar transcripts, with transcription peaking in the anoxic zone of maximum nitrate reduction activity. These results link SAR11 to pathways of ocean nitrogen loss, redefining the ecological niche of Earth's most abundant organismal group., Alphaproteobacteria of the SAR11 clade form one of the most ecologically dominant organism groups on the planet, representing up to half of the total microbial community in the oxygen-rich surface [...]

Nitrous oxide (N2O) emitted from upwelled waters in the Arabian Sea is only slightly enriched in O and moderately depleted in N, compared with air, even though there are very large denitrification-related enrichments of N and O in N2O at mid-depths. Research indicates that the intense N2O accumulation in the upper and lower parts of the oxygen minimum zone could be caused by different mechanisms. There seems to be evidence to support the view that denitrification could lead to a net N2O accumulation under certain conditions. N2O production in the upper layer could be the result of a nitrification-denitrification coupling.

On page 543 of this issue, Ettwig et at. (1) report their discovery of a new type of methane-oxidizing (methanotrophic) bacterium, provisionally dubbed Methytomirabitis oxyfera. As its name suggests, this [...]