A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O 2 ) sensitivity of these processes down to the O 2 concentrations present in the OMZ core (<10 nmol·L −1 ) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O 2 levels between 5 nmol·L −1 and 20 μmol·L −1 . Rates of both processes were detectable in the low nanomolar range (5-33 nmol·L −1 O 2 ), but demonstrated a strong dependence on O 2 concentrations with apparent half-saturation constants (K m s) of 333 ± 130 nmol·L −1 O 2 for ammonium oxidation and 778 ± 168 nmol·L −1 O 2 for nitrite oxidation assuming one-component Michaelis-Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis-Menten model, indicating a high-affinity component with a K m of just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O 2 affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O 2 is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.is a key factor regulating biogeochemical cycling in the marine environment (1). Although the vast majority of the ocean remains well oxygenated, subsurface regions of extreme oxygen depletion can persist along eastern boundaries of the world's ocean basins. These regions are known as oxygen minimum zones (OMZs) and are located within the eastern tropical North and South Pacific, the Arabian Sea, and off the coast of Namibia, where oxygen depletion results from poor ventilation and a high export of organic matter from productive surface waters, generating high rates of subsurface oxygen consumption (2, 3).Oceanic waters characterized by oxygen-deficient conditions (<4.5 μmol·L −1 O 2 ) account for <0.1% of total ocean volume but for >30% of fixed nitrogen (N) loss (3-6) due to the onset of anaerobic processes, including denitrification and anammox (7-10). Both field and modeling observations point to the expansion of low oxygen regions as a result of global warming (11). Thus, to evaluate the biogeochemical impact of these regions, it is imperative to understand fully how oxygen controls the cycling of substrates involved in nitrogen loss pathways.Recent studies have quantified the oxygen sensitivity of anaerobic OMZ nitrogen transformations, finding that denitrification has relatively low oxygen tolerance with a half-inhibition concentration (IC 50 ) of 0.3 μmol·L −1 , compared with higher values for...

The genetic composition of marine microbial communities varies at the microscale between particleassociated (PA; 41.6 μm) and free-living (FL; 0.2-1.6 μm) niches. It remains unclear, however, how metabolic activities differ between PA and FL fractions. We combined rate measurements with metatranscriptomics to quantify PA and FL microbial activity in the oxygen minimum zone (OMZ) of the Eastern Tropical North Pacific, focusing on dissimilatory processes of the nitrogen (N) cycle. Bacterial gene counts were 8-to 15-fold higher in the FL compared with the PA fraction. However, rates of all measured N cycle processes, excluding ammonia oxidation, declined significantly following particle (41.6 μm) removal. Without particles, rates of nitrate reduction to nitrite (1.5-9.4 nM N d − 1 ) fell to zero and N 2 production by denitrification (0.5-1.7 nM N d − 1 ) and anammox (0.3-1.9 nM N d − 1 ) declined by 53-85%. The proportional representation of major microbial taxa and N cycle gene transcripts in metatranscriptomes followed fraction-specific trends. Transcripts encoding nitrate reductase were uniform among PA and FL fractions, whereas anammox-associated transcripts were proportionately enriched up to 15-fold in the FL fraction. In contrast, transcripts encoding enzymes for N 2 O and N 2 production by denitrification were enriched up to 28-fold in PA samples. These patterns suggest that the majority of N cycle activity, excluding N 2 O and N 2 production by denitrification, is confined to a FL majority that is critically dependent on access to particles, likely as a source of organic carbon and inorganic N. Variable particle distributions may drive heterogeneity in N cycle activity and gene expression in OMZs.

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Bacteria of the NC10 phylum link anaerobic methane oxidation to nitrite denitrification through a unique O 2 -producing intra-aerobic methanotrophy pathway. A niche for NC10 in the pelagic ocean has not been confirmed. We show that NC10 bacteria are present and transcriptionally active in oceanic oxygen minimum zones (OMZs) off northern Mexico and Costa Rica. NC10 16S rRNA genes were detected at all sites, peaking in abundance in the anoxic zone with elevated nitrite and methane concentrations. Phylogenetic analysis of particulate methane monooxygenase genes further confirmed the presence of NC10. rRNA and mRNA transcripts assignable to NC10 peaked within the OMZ and included genes of the putative nitrite-dependent intra-aerobic pathway, with high representation of transcripts containing the unique motif structure of the nitric oxide (NO) reductase of NC10 bacteria, hypothesized to participate in O 2 -producing NO dismutation. These findings confirm pelagic OMZs as a niche for NC10, suggesting a role for this group in OMZ nitrogen, methane and oxygen cycling.