Diversity and relative abundance of ammonia- and nitrite-oxidizing microorganisms in the offshore Namibian hypoxic zone.

Nitrification, the microbial oxidation of ammonia (NH3) to nitrite (NO2-) and NO2- to nitrate (NO3-), plays a vital role in ocean nitrogen cycling. Characterizing the distribution of nitrifying organisms over environmental gradients can help predict how nitrogen availability may change with shifting ocean conditions, for example, due to loss of dissolved oxygen (O2). We characterized the distribution of nitrifiers at 5 depths spanning the oxic to hypoxic zone of the offshore Benguela upwelling system above the continental slope off Namibia. Based on 16S rRNA gene amplicon sequencing, the proportional abundance of nitrifiers (ammonia and nitrite oxidizers) increased with depth, driven by an increase in ammonia-oxidizing archaea (AOA; Thaumarchaeota) to up to 33% of the community at hypoxic depths where O2 concentrations fell to ~25 µM. The AOA community transitioned from being dominated by a few members at oxic depths to a more even representation of taxa in the hypoxic zone. In comparison, the community of NO2--oxidizing bacteria (NOB), composed primarily of Nitrospinae, was far less abundant and exhibited higher evenness at all depths. The AOA:NOB ratio declined with depth from 41:1 in the oxic zone to 27:1 under hypoxia, suggesting potential variation in the balance between NO2- production and consumption via nitrification. Indeed, in contrast to prior observations from more O2-depleted sites closer to shore, NO2- did not accumulate at hypoxic depths near this offshore site, potentially due in part to a tightened coupling between AOA and NOB.