Eastern Tropical North Pacific Oxygen Research Articles

Anoxic waters constrain the vertical distribution of fish developmental stages in an oxygen minimum zone

AbstractIn the Eastern Tropical North Pacific Oxygen Minimum Zone (ETNP‐OMZ), fish larvae undergo development amidst highly variable dissolved oxygen environments. As OMZs expand, understanding the implications of low‐oxygen environments on fish development becomes increasingly relevant for fisheries management and ecosystem modeling. Using horizontal zooplankton tows to track five oxygen levels (oxic [200 μmol/kg], hypoxic [100 μmol/kg] suboxic [10 μmol/kg], anoxic [<1 μmol/kg], and deep [10 μmol/kg at ~ 1000 m depth]), this study analyzed the three‐dimensional distribution of fish larvae and adults across the ETNP‐OMZ. Results revealed a wide midwater anoxic core, extending from Costa Rica to Baja California, that was almost devoid of fish larvae (< 1 larvae/1000 m3). Early larval stages primarily inhabited the oxic and hypoxic levels above the core, while postflexion and transformation stages occurred across a wider oxygen gradient, including the deep level below the anoxic core. Epipelagic species (e.g., Auxis sp.) were predominantly found in the surface oxic level, whereas coastal‐demersal species (e.g., Bregmaceros bathymaster, Ophidion spp.) were prevalent in the hypoxic level above the core. Meso‐bathypelagic species (e.g., Diogenichthys laternatus, Cyclothone spp.) were present throughout the study area, including below the anoxic core as transformation larvae and juveniles. These findings indicate that a vertical expansion of anoxic waters in OMZs could further constrain the habitat of epipelagic species, while also affecting the ontogenic vertical movements of meso‐bathypelagic species.

Regional Fluctuations in the Eastern Tropical North Pacific Oxygen Minimum Zone during the Late Holocene

This study presents a high-resolution record of δ15Nsed, which serves as a proxy for water column denitrification and oxygen minimum zone (OMZ) intensity, from the Soledad Basin in the Eastern Tropical North Pacific OMZ. The Soledad Basin δ15Nsed record is compared to the Pescadero Slope and Santa Barbara Basin (SBB) δ15Nsed records to gain insight into regional variations in the ETNP OMZ. During the Medieval Climate Anomaly (MCA; 950–1250 CE), Soledad Basin, Pescadero Slope, and SBB records exhibit coherent trends suggesting that there was general water column oxygenation stability. During the Little Ice Age (LIA; 1350–1850 CE), Soledad Basin and SBB showed a similar decreasing trend in δ15Nsed values while the Pescadero Slope δ15Nsed exhibited an increasing trend until values abruptly declined between 1740 and 1840 CE. We suggest that increased δ15Nsed variability and the different trends at the Pescadero Slope during the LIA are due to the influence of the North American monsoon (NAM), which can suppress upwelling when enhanced and result in OMZ contraction. The decoupling between the Soledad Basin, SBB, and the Pescadero Slope could also be due to the increased influence of enriched 15NO3− subarctic waters in the California Current System. Since each site is influenced by local productivity, basin morphology, and regional atmospheric and ocean circulation patterns, we suggest that assessing OMZ fluctuations from multiple sites provides a more comprehensive view of regional OMZ dynamics in response to climate variations.

All about nitrite: exploring nitrite sources and sinks in the eastern tropical North Pacific oxygen minimum zone

Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here, we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2-) as a product, reactant, or intermediate, in the eastern tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruises present additional evidence that NO3- reduction is the predominant OMZ N flux, followed by NO2- oxidation back to NO3-. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200 times) than the combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic–oxic interface. We also show that NO2- oxidation can occur when O2 is maintained near 1 nM by a continuous-purge system, NO2- oxidation and O2 measurements that further strengthen the case for truly anaerobic NO2- oxidation. We also evaluate the possibility that NO2- dismutation provides the oxidative power for anaerobic NO2- oxidation. The partitioning of N loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact, N loss rates at many depths were entirely due to anammox. Our new NO3- reduction, NO2- oxidation, dismutation, and N loss data shed light on many open questions in OMZ N cycling research, especially the possibility of truly anaerobic NO2- oxidation.

All about Nitrite: Exploring Nitrite Sources and Sinks in the Eastern Tropical North Pacific Oxygen Minimum Zone

All about Nitrite: Exploring Nitrite Sources and Sinks in the Eastern Tropical North Pacific Oxygen Minimum Zone

Associations between picocyanobacterial ecotypes and cyanophage host genes across ocean basins and depth.

Cyanophages, viruses that infect cyanobacteria, are globally abundant in the ocean's euphotic zone and are a potentially important cause of mortality for marine picocyanobacteria. Viral host genes are thought to increase viral fitness by either increasing numbers of genes for synthesizing nucleotides for virus replication, or by mitigating direct stresses imposed by the environment. The encoding of host genes in viral genomes through horizontal gene transfer is a form of evolution that links viruses, hosts, and the environment. We previously examined depth profiles of the proportion of cyanophage containing various host genes in the Eastern Tropical North Pacific Oxygen Deficient Zone (ODZ) and at the subtropical North Atlantic (BATS). However, cyanophage host genes have not been previously examined in environmental depth profiles across the oceans. We examined geographical and depth distributions of picocyanobacterial ecotypes, cyanophage, and their viral-host genes across ocean basins including the North Atlantic, Mediterranean Sea, North Pacific, South Pacific, and Eastern Tropical North and South Pacific ODZs using phylogenetic metagenomic read placement. We determined the proportion of myo and podo-cyanophage containing a range of host genes by comparing to cyanophage single copy core gene terminase (terL). With this large dataset (22 stations), network analysis identified statistical links between 12 of the 14 cyanophage host genes examined here with their picocyanobacteria host ecotypes. Picyanobacterial ecotypes, and the composition and proportion of cyanophage host genes, shifted dramatically and predictably with depth. For most of the cyanophage host genes examined here, we found that the composition of host ecotypes predicted the proportion of viral host genes harbored by the cyanophage community. Terminase is too conserved to illuminate the myo-cyanophage community structure. Cyanophage cobS was present in almost all myo-cyanophage and did not vary in proportion with depth. We used the composition of cobS phylotypes to track changes in myo-cyanophage composition. Picocyanobacteria ecotypes shift with changes in light, temperature, and oxygen and many common cyanophage host genes shift concomitantly. However, cyanophage phosphate transporter gene pstS appeared to instead vary with ocean basin and was most abundant in low phosphate regions. Abundances of cyanophage host genes related to nutrient acquisition may diverge from host ecotype constraints as the same host can live in varying nutrient concentrations. Myo-cyanophage community in the anoxic ODZ had reduced diversity. By comparison to the oxic ocean, we can see which cyanophage host genes are especially abundant (nirA, nirC, and purS) or not abundant (myo psbA) in ODZs, highlighting both the stability of conditions in the ODZ and the importance of nitrite as an N source to ODZ endemic LLV Prochlorococcus.

Iron and manganese accumulation within the Eastern Tropical North Pacific oxygen deficient zone

The Eastern Tropical North Pacific (ETNP) contains the largest oxygen deficient zone (ODZ) in the modern ocean. We determined dissolved concentrations of Fe, Fe(II), and Mn from three cruises in the region. Similar to other reported ODZs, Fe(II) was highest in the depth range associated with the secondary nitrite maximum. The main source of this feature is likely lateral advection of water overlying reducing shelf sediments within a narrow density range centered on the potential density anomaly of 26.5 kg/m3. This density horizon is similar to the Eastern Tropical South Pacific (ETSP) and reflects the intersection of the same density range with a large fraction of the continental shelf bottom waters. We also observed subsurface maxima of dissolved Mn in this density range, in contrast to the ETSP. Deep waters were enriched in Fe within the ETNP, analogous to other eastern boundary upwelling systems as well as the Arabian Sea. We argue that in these systems, reducing conditions on the shelf and overlying water column facilitate a robust shelf to basin shuttle of Fe, moving Fe from the coastal margin to deep plumes. Mn is also transported offshore in the core of the ODZ, and the relationship between Fe(II), Mn, and nitrite is remarkably similar between the ETNP, ETSP, and Arabian Sea. The exception is that Mn supply from the Peruvian shelf is less pronounced than in the other two ODZs, potentially reflecting the absence of large rivers in the Peruvian system.

Slow Particle Remineralization, Rather Than Suppressed Disaggregation, Drives Efficient Flux Transfer Through the Eastern Tropical North Pacific Oxygen Deficient Zone

AbstractModels and observations suggest that particle flux attenuation is lower across the mesopelagic zone of anoxic environments compared to oxic environments. Flux attenuation is controlled by microbial metabolism as well as aggregation and disaggregation by zooplankton, all of which shape the relative abundance of differently sized particles. Observing and modeling particle spectra can provide information about the contributions of these processes. We measured particle size spectrum profiles at one station in the oligotrophic Eastern Tropical North Pacific Oxygen Deficient Zone (ETNP ODZ) using an underwater vision profiler (UVP), a high‐resolution camera that counts and sizes particles. Measurements were taken at different times of day, over the course of a week. Comparing these data to particle flux measurements from sediment traps collected over the same time‐period allowed us to constrain the particle size to flux relationship, and to generate highly resolved depth and time estimates of particle flux rates. We found that particle flux attenuated very little throughout the anoxic water column, and at some time points appeared to increase. Comparing our observations to model predictions suggested that particles of all sizes remineralize more slowly in the ODZ than in oxic waters, and that large particles disaggregate into smaller particles, primarily between the base of the photic zone and 500 m. Acoustic measurements of multiple size classes of organisms suggested that many organisms migrated, during the day, to the region with high particle disaggregation. Our data suggest that diel‐migrating organisms both actively transport biomass and disaggregate particles in the ODZ core.

Organic complexation of iron by strong ligands and siderophores in the eastern tropical North Pacific oxygen deficient zone

Continental margins are an important external source of dissolved iron to the marine environment. However, the mechanisms responsible for the offshore transport of dissolved iron is impacted by the resulting iron speciation. We characterized the iron speciation in the Eastern Tropical North Pacific (ETNP) oxygen deficient zone (ODZ), including dissolved iron, organic iron-binding ligands, and reduced iron. Organic iron-binding ligands were measured using both competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) and liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS) in order to explore the impact of organic ligands on dissolved iron (dFeT) and iron(II) biogeochemistry in the region. Organic ligands were present in high concentrations (1.06–5.30 nmol L−1) and exceeded dissolved iron concentrations (0.36–4.52 nmol L−1) at all locations. Iron-binding strengths (logKFeL,Fe′cond) ranged 11.22 to 12.75 and were elevated in the ODZ layer relative to the oxygenated water column. LC-ESI-MS revealed the presence of siderophores, or bacterially-produced organic ligands with high Fe-affinity, in all samples analyzed, suggesting these compounds may be produced by microbes in the ODZ despite high ambient dFeT concentrations. This study is the first to characterize siderophores in an ODZ environment to date, and the three siderophores found (amphibactin B, synechobactin c9, synechobactin c10) could contribute to the observed elevated logKFeL,Fe′cond of ligands in the ODZ. Comparative analysis of organic ligand logKFeL,Fe′cond values in other low oxygen environments suggests that strong ligands, including siderophores, could be present in other low oxygen regions. In a simple model of the shelf-to-offshore iron transport mechanism, strong organic iron-binding ligands had a large impact on the longevity and transport of iron in the ODZ. These results suggest that organic ligand composition can have an impact on iron distributions in the ETNP ODZ and regulate the offshore transport of iron to the open ocean.

Lack of redox cycling for nickel in the water column of the Eastern tropical north pacific oxygen deficient zone: Insight from dissolved and particulate nickel isotopes

Marine oxygen deficient zones (ODZs) promote unique plankton communities and redox environments which impact the cycling of biologically essential trace metals in the ocean. Here we use measurements of dissolved and particulate Ni concentrations and isotopes to investigate the biotic and abiotic processes controlling Ni cycling in the world’s largest ODZ, located in the Eastern Tropical North Pacific (ETNP). We observed a negative correlation between dissolved Ni concentrations and isotopic composition (δ60Ni) throughout the water column, such that Ni concentrations increased from roughly 3 nmol kg−1 to 8 nmol kg−1 over the upper 1000 m, while δ60Ni values decreased by 0.2‰ from about +1.6‰ to +1.4‰. These vertical patterns are characteristic of both the subtropical North and South Pacific, and can be explained by a combination of physical mixing of water masses and biological uptake and export, either with all of the Ni being bioavailable or with separate bioavailable and non-bioavailable Ni pools. Although evidence for additional Ni cycling processes such as sulfide precipitation or Ni sorption/desorption through Fe/Mn redox chemistry have been observed in other ODZs and euxinic waters, we found no clear evidence for these in either the redoxcline or low oxygen waters of the ETNP. Indeed, the relationship between dissolved [Ni] and δ60Ni observed in the ETNP is similar to results reported elsewhere in the subtropical North and South Pacific, falling generally on a mixing line between a surface water endmember (dissolved [Ni] = 2 nmol kg−1 and δ60Ni = +1.7‰) and a deep-water endmember (dissolved [Ni] = 6–10 nmol kg−1 and δ60Ni = ~+1.4‰). While this surface water endmember is similar to that of the Atlantic, the deep endmember in the Pacific is approximately 0.1‰ heavier than deep Atlantic Ni. This subtle isotopic difference suggests gradual accumulation of isotopically heavy Ni isotopes in the deep ocean, consistent with recent evidence of heavy Ni remobilization during early diagenesis. Lastly, in the ETNP, particulate δ60Ni is generally ~0.5‰ lighter than the dissolved Ni pool, and this pattern is consistent across both the euphotic zone and redoxcline, suggesting that biological export from the euphotic zone is the primary source of particulate Ni to the deep ocean.

Diversity and N2O Production Potential of Fungi in an Oceanic Oxygen Minimum Zone.

Fungi in terrestrial environments are known to play a key role in carbon and nitrogen biogeochemistry and exhibit high diversity. In contrast, the diversity and function of fungi in the ocean has remained underexplored and largely neglected. In the eastern tropical North Pacific oxygen minimum zone, we examined the fungal diversity by sequencing the internal transcribed spacer region 2 (ITS2) and mining a metagenome dataset collected from the same region. Additionally, we coupled 15N-tracer experiments with a selective inhibition method to determine the potential contribution of marine fungi to nitrous oxide (N2O) production. Fungal communities evaluated by ITS2 sequencing were dominated by the phyla Basidiomycota and Ascomycota at most depths. However, the metagenome dataset showed that about one third of the fungal community belong to early-diverging phyla. Fungal N2O production rates peaked at the oxic–anoxic interface of the water column, and when integrated from the oxycline to the top of the anoxic depths, fungi accounted for 18–22% of total N2O production. Our findings highlight the limitation of ITS-based methods typically used to investigate terrestrial fungal diversity and indicate that fungi may play an active role in marine nitrogen cycling.

Limited iodate reduction in shipboard seawater incubations from the Eastern Tropical North Pacific oxygen deficient zone

The relative abundance of the inorganic iodine species, iodide and iodate, are applied to characterize both modern and ancient marine oxygen deficient zones (ODZs). However, the rates and mechanisms responsible for in situ iodine redox transformations are poorly characterized, rendering iodine-based redox reconstructions uncertain. Here, we provide constraints on the rates and mechanisms of iodate reduction in the Eastern Tropical North Pacific (ETNP) offshore ODZ using a shipboard tracer–incubation method. Observations of iodate reduction from incubations were limited to the top of the oxycline (σθ∼25.2kgm−3) where native oxygen concentrations were low, but detectable (≈11 μM). Incubations from additional depths below the oxycline—where O2 was <2 μM—yielded no detectable evidence of iodate reduction despite hosting the lowest iodate concentrations. These experiments place an upper limit of iodate reduction rates of generally <15 nM day−1 but as low as <2.3 nM day−1, which are based on variable precision of individually incubated replicates between experiments. Experimental inferences of limited or slow iodate reduction in the ODZ core relative to that observed in the oxycline are generally consistent with iodate persistence of up to 70 nM and low biological productivity in this zone. We also compare dissolved iodine and oxygen concentrations between variable water masses of the ETNP and globally distributed open ocean ODZs. Consistent with sluggish reduction rates, comparison of iodate concentrations with previously published water mass analyses within the ETNP ODZ (σθ=26-27kgm−3; O<27μM) demonstrate iodate as a semi-conservative tracer at least partially reflecting regional water mass mixing. A compilation of iodate and dissolved oxygen concentrations from global ODZs generally supports that at least some iodate variations in both vertical and lateral transects largely reflect variable combinations of relatively slow reduction and mixing of iodate reduction signals generated in adjacent regions—as opposed to solely rapid in situ processes. In this context, the variations in iodine speciation inferred for the ancient and modern ocean represent a combination of in situ processes and regional mixing between water masses that retain variable spatially and temporally integrated redox histories.

Cyanophage host-derived genes reflect contrasting selective pressures with depth in the oxic and anoxic water column of the Eastern Tropical North Pacific.

Cyanophages encode host-derived genes that may increase their fitness. We examined the relative abundance of 18 host-derived cyanophages genes in metagenomes and viromes along depth profiles from the Eastern Tropical North Pacific Oxygen Deficient Zone (ETNP ODZ) where Prochlorococcus dominates a secondary chlorophyll maximum within the ODZ. Cyanophages at the oxic primary chlorophyll maximum encoded genes related to light and phosphate stress (psbA, psbD and pstS in T4-like and psbA in T7-like), but the proportion of cyanophage with these genes decreased with depth. The proportion of cyanophage with purine biosynthesis genes increased with depth in T4-like, but not T7-like cyanophages. No additional host-derived genes were found in deep T7-like cyanophages, suggesting that T4-like and T7-like cyanophages have different host-derived gene acquisition strategies, possibly linked to their different genome packaging mechanisms. In contrast to the ETNP, in the oxic North Atlantic T4-like cyanophages encoded psbA and pstS throughout the euphotic zone. Differences in pstS between the ETNP and the North Atlantic stations were consistent with differences in phosphate concentrations in those regimes. We suggest that the low proportion of cyanophage with psbA within the ODZ reflects the stably stratified low-light conditions occupied by their hosts, a Prochlorococcus ecotype endemic to ODZs.

The role of water masses in shaping the distribution of redox active compounds in the Eastern Tropical North Pacific oxygen deficient zone and influencing low oxygen concentrations in the eastern Pacific Ocean

AbstractOceanic oxygen deficient zones (ODZs) influence global biogeochemical cycles in a variety of ways, most notably by acting as a sink for fixed nitrogen (Codispoti et al. 2001). Optimum multiparameter analysis of data from two cruises in the Eastern Tropical North Pacific (ETNP) was implemented to develop a water mass analysis for the large ODZ in this region. This analysis reveals that the most pronounced oxygen deficient conditions are within the 13°C water (13CW) mass, which is distributed via subsurface mesoscale features such as eddies branching from the California Undercurrent. Nitrite accumulates within these eddies and slightly below the core of the 13CW. This water mass analysis also reveals that the 13CW and deeper Northern Equatorial Pacific Intermediate Water (NEPIW) act as the two Pacific Equatorial source waters to the California Current System. The Equatorial Subsurface Water and Subtropical Subsurface Water are synonymous with the 13CW and this study refers to this water mass as the 13CW based on its history. Since the 13CW has been found to dominate the most pronounced oxygen deficient conditions within the Eastern Tropical South Pacific ODZ and the Peru‐Chile Undercurrent, the 13CW and the NEPIW define boundaries for oxygen minimum conditions across the entire eastern Pacific Ocean.

Dinitrogen Fixation Across Physico‐Chemical Gradients of the Eastern Tropical North Pacific Oxygen Deficient Zone

AbstractThe Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (&lt;20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (&gt;9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

Determination of iron(II) by chemiluminescence using masking ligands to distinguish interferences

AbstractIron(II) can catalyze the oxidation of luminol in seawater and this chemiluminescent reaction has been widely used for iron(II) determination. The method is vulnerable to interferences from other analytes that catalyze luminol oxidation. We have shown that addition of diethylenetriamine pentaacetic acid (DTPA) to a sample inhibits the reaction of iron(II) with luminol, while not affecting other substances that also catalyze luminol oxidation under our experimental conditions. DTPA‐treated samples can therefore be used as sample blanks, with the difference between an untreated seawater sample and a DTPA‐treated seawater sample related to the concentration of dissolved iron(II). The DTPA correction has been applied to measure diel variability of iron(II) due to photoreduction in a coastal environment, and to measure vertical distributions of iron(II) in the eastern tropical north Pacific oxygen deficient zone.

Utilization of urea and cyanate in waters overlying and within the eastern tropical north Pacific oxygen deficient zone.

In marine oxygen deficient zones (ODZs), which contribute up to half of marine N loss, microbes use nitrogen (N) for assimilatory and dissimilatory processes. Here, we examine N utilization above and within the ODZ of the Eastern Tropical North Pacific Ocean, focusing on distribution, uptake and genes for the utilization of two simple organic N compounds, urea and cyanate. Ammonium, urea and cyanate concentrations generally peaked in the oxycline while uptake rates were highest in the surface. Within the ODZ, concentrations were lower, but urea N and C and cyanate C were taken up. All identified autotrophs had an N assimilation pathway that did not require external ammonium: ODZ Prochlorococcus possessed genes to assimilate nitrate, nitrite and urea; nitrite oxidizers (Nitrospina) possessed genes to assimilate nitrite, urea and cyanate; anammox bacteria (Scalindua) possessed genes to utilize cyanate; and ammonia-oxidizing Thaumarchaeota possessed genes to utilize urea. Urease genes were present in 20% of microbes, including SAR11, suggesting the urea utilization capacity was widespread. In the ODZ core, cyanate genes were largely (∼95%) associated with Scalindua, suggesting that, within this ODZ, cyanate N is primarily used for N loss via anammox (cyanammox), and that anammox does not require ammonium for N loss.

An N isotopic mass balance of the Eastern Tropical North Pacific oxygen deficient zone

Oxygen deficient zones host up to 50% of marine N2 production and the Eastern Tropical North Pacific (ETNP) is the largest marine oxygen deficient zone. We measured δ15N-NO3-, δ15N-NO2-, and δ15N-N2 at 7 stations along a transect normal to the coast in the heart of the ETNP oxygen deficient zone in 2012. The δ15N-N2 minimum was 0.34‰ at 300 m, which corresponded with the N2:Ar maximum. When the atmospheric N2 background was removed, the biological δ15N-N2 for the ODZ ranged from −7‰ to −22‰. In the ODZ, δ15N-NO3- ranged from 15 to 24‰ while δ15N-NO2- was generally between − 11 and −18‰, generating differences up to 40‰ between δ15N-NO3- and δ15N-NO2-. The isotopic separation between nitrite and nitrogen gas (Δ15N NO2-N2) changed sign from ~5‰ at the top of the oxygen deficient zone to ~−10‰ at 300 m, indicating an important shift in nitrogen cycling with depth. We calculated the closed system Rayleigh isotope effect (ε) for N2 production from both the δ15N-DIN (εDIN = 26 ± 11‰) and δ15N-N2 (εN2 = 27 ± 6‰) data. When examined individually by depth, both εDIN and εN2 matched closely and both ε depth profiles showed maximal fractionation at 300 m. Additionally, closed system isotope effects were calculated for one offshore station from the Arabian Sea in 2007 using δ15N-N2 ( εN2 = 24 ± 4‰) and δ15N-DIN (εDIN = 26 ± 3‰). The relatively large isotope effects for N2 production appear to be found in both major offshore oxygen deficient zones, which implies a large denitrification term in the marine N budget.

Origin and fate of methane in the Eastern Tropical North Pacific oxygen minimum zone

Oxygen minimum zones (OMZs) contain the largest pools of oceanic methane but its origin and fate are poorly understood. High-resolution (<15 m) water column profiles revealed a 300 m thick layer of elevated methane (20–105 nM) in the anoxic core of the largest OMZ, the Eastern Tropical North Pacific. Sediment core incubations identified a clear benthic methane source where the OMZ meets the continental shelf, between 350 and 650 m, with the flux reflecting the concentration of methane in the overlying anoxic water. Further incubations characterised a methanogenic potential in the presence of both porewater sulphate and nitrate of up to 88 nmol g−1day−1 in the sediment surface layer. In these methane-producing sediments, the majority (85%) of methyl coenzyme M reductase alpha subunit (mcrA) gene sequences clustered with Methanosarcinaceae (⩾96% similarity to Methanococcoides sp.), a family capable of performing non-competitive methanogenesis. Incubations with 13C-CH4 showed potential for both aerobic and anaerobic methane oxidation in the waters within and above the OMZ. Both aerobic and anaerobic methane oxidation is corroborated by the presence of particulate methane monooxygenase (pmoA) gene sequences, related to type I methanotrophs and the lineage of Candidatus Methylomirabilis oxyfera, known to perform nitrite-dependent anaerobic methane oxidation (N-DAMO), respectively.

Decadal to centennial fluctuations in the intensity of the eastern tropical North Pacific oxygen minimum zone during the last 1200 years

AbstractOxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuates naturally; however, the degree of these fluctuations and a comprehensive understanding of the factors that drive these fluctuations on decadal to centennial time scales is lacking. Our high‐resolution (near‐annual) record of δ15Nsed from laminated sediments at the Pescadero Slope in the Gulf of California (eastern tropical North Pacific) fluctuates between maximum values of 10.5‰ and minimum values of 8.0‰ over the past 1200 years. An analysis of the relationship between δ15NO3− and [O2] in the water column suggests that the observed range of δ15Nsed values is equivalent to an approximately 8 µM fluctuation in O2 content and that these changes can occur in less than 25 years. Our findings show that the OMZ typically intensifies quickly and contracts gradually; the average rate of OMZ intensification (−0.24 µM O2/yr) is twice as fast as the rate of OMZ reoxygenation. Spectral analyses of the δ15Nsed record and Br/Cl counts, with the latter are used as a proxy for organic carbon preservation, suggest that the Pacific Decadal Oscillation and the Suess (deVries) solar cycle (solar irradiance) may influence the intensity of the OMZ and carbon production/export during the late Holocene. Coherence between δ15Nsed and weight percent organic carbon also suggests that similar mechanisms influence both OMZ fluctuations and variation in organic carbon production/export.

Meta-omic signatures of microbial metal and nitrogen cycling in marine oxygen minimum zones.

Iron (Fe) and copper (Cu) are essential cofactors for microbial metalloenzymes, but little is known about the metalloenyzme inventory of anaerobic marine microbial communities despite their importance to the nitrogen cycle. We compared dissolved O2, NO, NO, Fe and Cu concentrations with nucleic acid sequences encoding Fe and Cu-binding proteins in 21 metagenomes and 9 metatranscriptomes from Eastern Tropical North and South Pacific oxygen minimum zones and 7 metagenomes from the Bermuda Atlantic Time-series Station. Dissolved Fe concentrations increased sharply at upper oxic-anoxic transition zones, with the highest Fe:Cu molar ratio (1.8) occurring at the anoxic core of the Eastern Tropical North Pacific oxygen minimum zone and matching the predicted maximum ratio based on data from diverse ocean sites. The relative abundance of genes encoding Fe-binding proteins was negatively correlated with O2, driven by significant increases in genes encoding Fe-proteins involved in dissimilatory nitrogen metabolisms under anoxia. Transcripts encoding cytochrome c oxidase, the Fe- and Cu-containing terminal reductase in aerobic respiration, were positively correlated with O2 content. A comparison of the taxonomy of genes encoding Fe- and Cu-binding vs. bulk proteins in OMZs revealed that Planctomycetes represented a higher percentage of Fe genes while Thaumarchaeota represented a higher percentage of Cu genes, particularly at oxyclines. These results are broadly consistent with higher relative abundance of genes encoding Fe-proteins in the genome of a marine planctomycete vs. higher relative abundance of genes encoding Cu-proteins in the genome of a marine thaumarchaeote. These findings highlight the importance of metalloenzymes for microbial processes in oxygen minimum zones and suggest preferential Cu use in oxic habitats with Cu > Fe vs. preferential Fe use in anoxic niches with Fe > Cu.