Oceanic oxygen minimum zones (OMZs) are globally significant sites of biogeochemical cycling where microorganisms deplete dissolved oxygen (DO) to concentrations <20 µM. Amid intense competition for DO in these metabolically challenging environments, aerobic nitrite oxidation may consume significant amounts of DO and help maintain low DO concentrations, but this remains unquantified. Using parallel measurements of oxygen consumption rates and 15N-nitrite oxidation rates applied to both water column profiles and oxygen manipulation experiments, we show that the contribution of nitrite oxidation to overall DO consumption systematically increases as DO declines below 2 µM. Nitrite oxidation can account for all DO consumption only under DO concentrations <393 nM found in and below the secondary chlorophyll maximum. These patterns are consistent across sampling stations and experiments, reflecting coupling between nitrate reduction and nitrite-oxidizing Nitrospina with high oxygen affinity (based on isotopic and omic data). Collectively our results demonstrate that nitrite oxidation plays a pivotal role in the maintenance and biogeochemical dynamics of OMZs.

Oceanic oxygen minimum zones (OMZs) play a pivotal role in biogeochemical cycles due to extensive microbial activity. How OMZ microbial communities assemble and respond to environmental variation is therefore essential to understanding OMZ functioning and ocean biogeochemistry. Sampling along depth profiles at five stations in the eastern tropical North Pacific Ocean (ETNP), we captured systematic variations in dissolved oxygen (DO) and associated variables (nitrite, chlorophyll, and ammonium) with depth and between stations. We quantitatively analysed relationships between oceanographic gradients and microbial community assembly and activity based on paired 16S rDNA and 16S rRNA sequencing. Overall microbial community composition and diversity were strongly related to regional variations in density, DO, and other variables (regression and redundancy analysis r 2 = 0.68-0.82), displaying predictable patterns with depth and between stations. Although similar factors influenced the active community, diversity was substantially lower within the OMZ. We also identified multiple active microbiological networks that tracked specific gradients or featuresparticularly subsurface ammonium and nitrite maxima. Our findings indicate that overall microbial community assembly is consistently shaped by hydrography and biogeochemistry, while active segments of the community form discrete networks inhabiting distinct portions of the water column, and that both are tightly tuned to environmental conditions in the ETNP.

The genome of “Candidatus Sulcia muelleri” strain KPTW1 from Kolla paulula, a vector of Xylella fastidiosa that causes Pierce’s disease (PD) of grapevine in Taiwan, was sequenced. The strain has a genome size of 253,942 bp, GC content of 22.7%, 237 predicted protein-coding genes, and 34 RNA genes.

29Oceanic oxygen minimum zones (OMZs) occur where microorganisms deplete dissolved oxygen 30 (DO) to exceptionally low levels, and are globally significant sites of biogeochemical cycling. 31 Amid the intense competition for DO and other substrates occurring in these metabolically 32 challenging environments, aerobic nitrite oxidation may consume significant amounts of DO, but 33 this has not been examined comprehensively. Using parallel measurements of oxygen 34 consumption rates and 15 N-nitrite oxidation rates applied to water column profiles and to oxygen 35 manipulation experiments, we show that nitrite oxidation is a substantial sink for DO in the 36 ocean's largest OMZ. The contribution of nitrite oxidation to overall DO consumption increased 37 at low DO concentrations, tracking gradients and variations within and across multiple stations in 38 the eastern tropical North Pacific Ocean. Oxygen manipulation experiments produced highly 39 consistent effects, with nitrite oxidation responsible for progressively more DO consumption (up 40 to 97%) as DO was experimentally decreased. Natural abundance stable isotope data indicated 41 coupling of nitrite oxidation and nitrate reduction, while 16S rRNA and metagenome sequencing 42 revealed that Nitrospina ecotypes possessing high-affinity cytochrome oxidase genes were 43 prevalent and active within the OMZ. Collectively, our results demonstrate that nitrite oxidation 44 consumes significant amounts of DO, and that this proportion increases as DO declines-45 indicating that nitrite oxidation is critically important to the formation and maintenance of 46 OMZs. 47 48 49 Significance 50 Oceanic oxygen minimum zones (OMZs) are naturally-occuring regions of low oxygen found in 51 select areas of the ocean. Lack of dissolved oxygen has important implications for both the 52 distribution of marine organisms and global biogeochemical cycles, yet we have a limited 53 understanding of how oxygen is depleted to such low levels. Here we comprehensively quantify 54 the contribution of nitrite oxidation to oxygen depletion in the ocean's largest OMZ. We 55 observed highly consistent patterns across depth profiles, and in multiple experiments where we 56 manipulated oxygen concentrations, finding that nitrite oxidation consumes progressively more 57 oxygen at lower oxygen concentrations. Our findings demonstrate that nitrite oxidation plays a 58 pivotal role in exhausting oxygen to the low levels found in OMZs.59 60 Aerobic nitrite oxidation is pervasive throughout much of the oceanic water column, playing a 61 central role in deep ocean chemoautotrophy and carbon cycling (1, 2), as well as in the oceanic 62 nitrogen (N) cycle (3, 4). Nitrite oxidation rates are typically undetectable using 15 NO 2 isotopic 63 tracer in the sunlit euphotic zone, peak at the base of the euphotic zone, and subsequently decline 64 with depth (5). However, nitrite oxidation rate profiles deviate from this pattern in oceanic 65 oxygen minimum zones (OMZs) that are depleted in dissolved oxygen...