Dissolved organic nitrogen (DON) is likely comprised of multiple compound classes with varying reactivities and turnover times, resulting in numerous roles in ocean biogeochemistry. Here, we present measurements of total DON and solid-phase extracted DON (SPE-DON) concentrations and delta 15N values from sampling sites across the global ocean. An optimized SPE protocol was developed to maximize total DON recovery, with a recovery of 41.0 +/- 9.4% for surface DON across the global ocean and 56.6 +/- 11.2% for deep water DON from the Sargasso Sea. SPE-DON concentrations were 1.8-1.9 mu M across most sampling sites, in contrast to greater variation in total DON concentration (4.0-5.9 mu M). However, in the equatorial upwelling zones, SPE-DON concentrations were slightly (0.3-0.4 mu M) higher than in other regions. The delta 15N of total DON in surface waters correlated well with the delta 15N of nitrate supplied to the euphotic zone from the subsurface. SPE-DON delta 15N was also correlated with nitrate delta 15N, but SPE-DON delta 15N values were confined to a narrower range compared to those of total DON. The combined concentration and delta 15N data indicate that while SPE-DON is biased toward long-lived DON, it still retains some reactive components introduced through regional inputs in the upper ocean, and even some of its longer-lived components may be labile on the timescales of deep ocean circulation. The longer average turnover time of SPE-DON suggests that greater molecular polarity and/or charge directly increase or are otherwise correlated with the biogeochemical lability of different DON pools.

Caribbean reefs have experienced major human-driven changes to their coral and fish communities1-4, yet how these changes have affected trophic dynamics remains poorly understood owing to challenges in reconstructing the trophic structure of pre-human-impact reefs. Advances in fossil-bound protein nitrogen isotope (15N/14N) analysis now enable the reconstruction of ancient trophic dynamics5,6, as the 15N to 14N ratio reflects an animal's trophic position7. Here we apply this method to modern and prehistoric (7,000-year-old) fish otoliths (ear stones) and corals from Caribbean Panama and the Dominican Republic, focusing on fishes occupying low to middle trophic levels. We find that although the trophic level typically declined in high-trophic-level fishes over time, it increased or remained unchanged in low-trophic-level fishes, indicating that modern food chains are 60-70% shorter than on the prehistoric reefs in both Panama and the Dominican Republic. Furthermore, across all trophic groups, we observed a marked reduction in dietary variation, with a 20-70% lower trophic range on the modern reefs compared to the prehistoric reefs. This pattern is best explained by less dietary specialization in modern reefs, consistent with less ecological complexity than in prehistoric reefs. These differences document and quantify the trophic simplification that has occurred on modern Caribbean reefs, a change that may increase their vulnerability to ecosystem collapse.

While the ocean's photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean's microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

The nitrogen (N) isotopic composition of coral tissue provides insight into N sources and cycling on reefs, and coral skeleton-bound organic matter (CS-δ15N) can extend these insights into the past. Across the Bermuda platform, we measured the δ15N of four coral species and their potential N sources, as well as an asymbiotic filter feeder as a comparative heterotroph and benthic macroalgae as a comparative autotroph. Organisms and organic N pools from the coral reefs exhibit a δ15N increase toward the Bermuda coast, likely due to anthropogenic N inputs. At all sites, the δ15N of bulk coral tissue is consistent with corals feeding dominantly on zooplankton-sized organic matter and some smaller suspended particulate N. The corals lack the trophic δ15N elevation that characterizes serpulids; this is consistent with internal recycling and retention of low-δ15N metabolic N by symbiont-bearing corals. The data are inconsistent with corals’ reliance on the dissolved inorganic N used by macroalgae at the same sites. Among coral species, two species with smaller polyps (1-2 mm) have ~1‰ lower bulk tissue δ15N than two counterparts with larger polyps (5-10 mm), perhaps due to differences in food source. Taxon-specific δ15N differences are also observed between coral tissue and skeleton-bound N, with larger differences in the two small-polyp species. In net, however, CS-δ15N mean values and spatial gradients were similar in the four species studied.

The biogeochemical processes shaping the fluxes of nitrogen (N) in the tropical to subarctic Pacific are illuminated by nitrate isotope ratios (δ 15 N and δ 18 O) along the GEOTRACES GP15 section. In the equatorial and tropical Pacific, nitrate δ 15 N and δ 18 O are 2–4‰ higher in the thermocline than in deeper waters. This widespread elevation is driven by both nitrate assimilation in Southern Ocean surface waters and denitrification in the eastern tropical Pacific. In addition to this, a poleward increase in the δ 15 N of surface nitrate from the Equator to 10°S is generated by the progressive consumption of nitrate upwelled at the Equator and transported southward. This process leads to increases in the δ 15 N of phytoplankton biomass and a strong meridional nitrate isotopic gradient in the thermocline due to regeneration of sinking N. North of the Equator, an analogous gradient is barely detectable due to weaker northward flow and a compressed spatial scale for nitrate drawdown. By contrast, between 5°N and 40°N, the nitrate isotope gradients are dominated by the processes driving the oceanic fixed N budget: denitrification and N 2 fixation. High‐δ 15 N and ‐δ 18 O nitrate in the tropics is advected from the oxygen deficient zone where denitrification occurs, whereas in the adjacent subtropics, low‐δ 15 N thermocline nitrate suggests a response by N 2 fixation that is not observed in the south. The asymmetry of the north and south tropical Pacific gradients in thermocline nitrate isotopes has implications for efforts to reconstruct the N cycle in the past.

Hypoxia is observed and projected to expand in many aquatic environments, largely due to excess anthropogenic nutrient inputs and climate change, thus influencing biogeochemical processes. Denitrification, generally an anaerobic process, removes bioavailable nitrogen and produces nitrous oxide (N2O). However, limited observations of the effect of oxygen on denitrification restrict our ability to estimate changes in the amount of bioavailable nitrogen and N2O emissions under anthropogenic perturbations and climate change. Here, we show that all denitrification steps increased, while the N2O production yield from denitrification decreased with decreasing oxygen in Chesapeake Bay - the largest estuary in the United States. The different steps of denitrification responded similarly to oxygen changes in Chesapeake Bay, unlike open ocean oxygen minimum zones, with implications for the accumulation or depletion of denitrification intermediates such as nitrite and N2O. Our observations also suggest that current model parametrizations of denitrification in Chesapeake Bay likely overestimate denitrification and nitrogen removal in the presence of oxygen, which would bias the evaluation of nutrient cycling, ecosystem productivity, and the extent of hypoxia. Overall, our newly derived oxygen sensitivities of denitrification could be used to improve model parametrizations of denitrification and constrain the nitrogen budget and N2O emissions in estuarine and coastal environments experiencing hypoxia.

The nitrogen isotopes of the organic matter preserved in fossil fish otoliths (ear stones) are a promising tool for reconstructing past environmental changes. We analyzed the 15 N/ 14 N ratio (δ 15 N) of fossil otolith-bound organic matter in Late Cretaceous fish otoliths (of Eutawichthys maastrichtiensis , Eutawichthys zideki and Pterothrissus sp.) from three deposits along the US east coast, with two of Campanian (83.6 to 77.9 Ma) and one Maastrichtian (72.1 to 66 Ma) age. δ 15 N and N content were insensitive to cleaning protocol and the preservation state of otolith morphological features, and N content differences among taxa were consistent across deposits, pointing to a fossil-native origin for the organic matter. All three species showed an increase in otolith-bound organic matter δ 15 N of ~4‰ from Campanian to Maastrichtian. As to its cause, the similar change in distinct genera argues against changing trophic level, and modern field data argue against the different locations of the sedimentary deposits. Rather, the lower δ 15 N in the Campanian is best interpreted as an environmental signal at the regional scale or greater, and it may be a consequence of the warmer global climate. A similar decrease has been observed in foraminifera-bound δ 15 N during warm periods of the Cenozoic, reflecting decreased water column denitrification and thus contraction of the ocean’s oxygen deficient zones (ODZs) under warm conditions. The same δ 15 N-climate correlation in Cretaceous otoliths raises the prospect of an ODZ-to-climate relationship that has been consistent over the last ~80 My, applying before and after the end-Cretaceous mass extinction and spanning changes in continental configuration.

The carbon isotope composition (δ13C) of ancient shallow-water carbonates frequently is used to reconstruct changes to Earth's global carbon cycle and to perform chemostratigraphic correlation. However, previous work demonstrates that local banktop processes also exert an important control on shallow carbonate δ13C as well as other isotope systems like δ18O. To effectively interpret ancient δ13C records, we must understand how both global carbon cycle perturbations and changes to local conditions are translated to the stratigraphic record. Modern environments, while imperfect analogues, can serve as a guide for interpreting physical and geochemical records of more ancient environments. Shallow carbonate strata from the most recent Pleistocene glacial cycles, which drove high-amplitude perturbations to sea level, temperature, and pCO2 without significantly altering the δ13C of global-mean seawater DIC, present an opportunity to begin untangling signals of global and local processes. However, the geochemistry of Pleistocene platform carbonates largely was overprinted by dissolution and meteoric diagenesis during glacial sea level lowstands. To understand how shallow carbonate geochemistry has changed during the Pleistocene, we instead look to the periplatformal slope and proximal basins. These deep environments serve as a refuge for carbonate produced on the shelf and exported to the slope, and contain a record of shallow carbonate geochemistry that persists across glacial cycles. We study 21 short piston cores from around Bahamian platforms to quantify differences in banktop production and geochemistry between the Holocene interglacial, the last glacial period, and the last interglacial (LIG) period. We show that mud production persists on the periplatformal slopes during the last glacial period, but differences in geochemistry between glacial and interglacial carbonates are a complex function of surface conditions and diagenesis. In contrast, Holocene and LIG carbonates show no evidence of post-depositional alteration, and offer the chance to study differences in δ13C and δ18O between interglacials. We find that while the δ13C of aragonite mud is the same during the Holocene and LIG, the LIG carbonate factory may have delivered more aragonite mud to the periplatform. In addition, the mean δ18O of this mud is elevated compared to the Holocene. We posit that these differences are caused by changes to regional climate and LIG surface conditions.

The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-δ15 N) using the oxidation-denitrifier (O-D) method provides a novel tool to study past changes in N cycling processes.METHODS:We report a set of methodological improvements to the O-D method, including (a) a method for sealing the reaction vials in which the oxidation of organic N to NO3 - takes place, (b) a recipe for bypassing the pH adjustment step before the bacterial conversion of NO3 - to N2 O, and (c) a method for storing recrystallized dipotassium peroxodisulfate (K2 S2 O8 ) under Ar atmosphere.RESULTS:The new sealing method eliminates the occasional contamination and vial breakage that occurred previously while increasing sample throughput. The protocol for bypassing pH adjustment does not affect BB-δ15 N, and it significantly reduces the processing time. Storage of K2 S2 O8 reagent under Ar atmosphere produces stable oxidation blanks over more than 3.5 years. We report analytical blanks, accuracy, and precision for this methodology from eight users over the course of ~3.5 years of analyses at the Max Planck Institute for Chemistry. Our method produces analytical blanks characterized by low N content (0.30 ± 0.13 nmol N, 1σ, n = 195) and stable δ15 N (-2.20 ± 3.13‰, n = 195). The analysis of reference amino acid standards USGS 40 and USGS 65 indicates an overall accuracy of -0.23 ± 0.35‰ (1σ, n = 891). The analysis of in-house fossil standards gives similar analytical precision (1σ) across a range of BB-δ15 N values and biominerals: zooxanthellate coral standard PO-1 (6.08 ± 0.21‰, n = 267), azooxanthellate coral standard LO-1 (10.20 ± 0.28‰, n = 258), foraminifera standard MF-1 (5.92 ± 0.28‰, n = 243), and tooth enamel AG-Lox (4.06 ± 0.49‰, n = 78).CONCLUSIONS:The methodological improvements significantly increase sample throughput without compromising analytical precision or accuracy down to 1 nmol of N.

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 O-2 is maintained near 1 nM by a continuous-purge system, NO2- oxidation and O-2 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.

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (delta N-15, delta O-18, delta(15-18) = delta N-15-delta O-18) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate delta N-15 is lower (4.9-5.8 parts per thousand) than the sub-thermocline source, Subantarctic Mode Water (6.9 parts per thousand); we attribute this difference to local N-2 fixation. Using a one-box model to simulate the newly fixed nitrate flux, we estimate a local N-2 fixation rate of 7-25 Tg N.a(-1), with the upper limit likely biased high. In the mixed layer, nitrate delta N-15 and delta O-18 rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N-2 fixation can be used to approximate carbon export. Thermocline and mixed-layer nitrate delta(15-18) is low, due to both N-2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low-delta(15-18) nitrate in Agulhas rings indicates leakage of low-delta N-15 nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

Biological nitrogen fixation (BNF) by canonical molybdenum and complementary vanadium and iron-only nitrogenase isoforms is the primary natural source of newly fixed nitrogen. Understanding controls on global nitrogen cycling requires knowledge of the isoform responsible for environmental BNF. The isotopic acetylene reduction assay (ISARA), which measures carbon stable isotope ( 13 C/ 12 C) fractionation between ethylene and acetylene in acetylene reduction assays, is one of the few methods that can quantify isoform-specific BNF fluxes. Application of classical ISARA has been challenging because environmental BNF activity is often too low to generate sufficient ethylene for isotopic analyses. Here we describe a high sensitivity method to measure ethylene δ 13 C by in-line coupling of ethylene preconcentration to gas chromatography-combustion-isotope ratio mass spectrometry (EPCon-GC-C-IRMS). Ethylene requirements in samples with 10% v/v acetylene are reduced from > 500 to ~ 20 ppmv (~ 2 ppmv with prior offline acetylene removal). To increase robustness by reducing calibration error, single nitrogenase-isoform Azotobacter vinelandii mutants and environmental sample assays rely on a common acetylene source for ethylene production. Application of the Low BNF activity ISARA (LISARA) method to low nitrogen-fixing activity soils, leaf litter, decayed wood, cryptogams, and termites indicates complementary BNF in most sample types, calling for additional studies of isoform-specific BNF.

Background Cancerous cells can recycle metabolic ammonium for their growth. As this ammonium has a low nitrogen isotope ratio ( 15 N/ 14 N), its recycling may cause cancer tissue to have lower 15 N/ 14 N than surrounding healthy tissue. We investigated whether, within a given tissue type in individual mice, tumoral and healthy tissues could be distinguished based on their 15 N/ 14 N. Methods Micro-biopsies of murine tumors and adjacent tissues were analyzed for 15 N/ 14 N using novel high-sensitivity methods. Isotopic analysis was pursued in Nude and C57BL/6 mice models with mature orthotopic brain and head&neck tumors generated by implantation of H454 and MEERL95 murine cells, respectively. Results In the 7 mice analyzed, the brain tumors had distinctly lower 15 N/ 14 N than healthy neural tissue. In the 5 mice with head&neck tumors, the difference was smaller and more variable. This was at least partly due to infiltration of healthy head&neck tissue by tumor cells. However, it may also indicate that the 15 N/ 14 N difference between tumoral and healthy tissue depends on the nitrogen metabolism of the healthy organ in question. Conclusions The findings, coupled with the high sensitivity of the 15 N/ 14 N measurement method used here, suggest a new approach for micro-biopsy-based diagnosis of malignancy as well as an avenue for investigation of cancer metabolism.

Bulk sediment δ 15 N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ 15 N, then a deglacial δ 15 N maximum, followed by a gradual decline to a late Holocene δ 15 N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ 15 N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ 15 N) measurements for the two species Neogloboquadrina dutertrei and Neogloboquadrina incompta over the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ 15 N. FB‐δ 15 N contrasts with bulk sediment δ 15 N by not indicating a lower δ 15 N during the LGM. Instead, the FB‐δ 15 N records are dominated by a deglacial δ 15 N maximum, with comparable LGM and Holocene values. The lower LGM δ 15 N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.

For times prior to those represented by the air trapped in Antarctic ice core records, the concentration of CO2 in the atmosphere must be reconstructed using geochemical proxies. The delta C-13 of particulate organic carbon (POC) produced in ocean surface waters has previously been observed to covary with the concentration of CO2 in the water. Relative to bulk sedimentary organic carbon, the minute quantity of organic matter trapped within the shell walls of planktonic foraminifera, "foraminifera-bound organic matter" (FBOM), has the potential advantage of being protected from diagenetic alteration and contamination by allochthonous organic matter. Here, with new protocols and instrumentation, FBOM-delta C-13 is investigated as a potential proxy for the aqueous CO2 concentration ([CO2(aq)]) and thus the partial pressure of CO2 (pCO(2)) in past surface waters. We achieve a full method precision for FBOM-delta C-13 of 0.4 parts per thousand (1SD) on sample sizes of 20 nanomole C by adding a cryofocus step, a helium sheath flow to the oxidation and reduction reactors, and other modifications to an elemental analyzer and by introducing new approaches to reduce contamination during sample preparation. FBOM-delta C-13 was analyzed in core-top sediments from the eastern tropical Pacific that span the equatorial maximum in surface water [CO2(aq)]. FBOM-delta C-13 is lower than predicted for marine phytoplankton, consistent with a substantial lipid component in FBOM. Anticorrelation is observed between FBOM-delta C-13 and climatological [CO2(aq)]; the relationship is statistically significant (P < 0.05) in mixed-species samples and in 4 out of 5 picked species. The slope of the FBOM-delta C-13:[CO2(aq)] anticorrelation is equivalent to or greater than has been measured for the delta C-13 of suspended POC in the surface ocean. Based on the few species analyzed in this study, endosymbiont-bearing and -barren species do not clearly differ from each other either in terms of their average value of FBOM-delta C-13 or the strength of the FBOM-delta C-13:[CO2(aq)] anticorrelation, despite the recognized importance of the photosynthetic endosymbionts as a source of organic carbon for the symbiotic species. Correcting for variations in the delta C-13 of CO2(aq) and phytoplankton growth rate did not improve the significance of the FBOM-delta C-13:[CO2(aq)] anticorrelation. The findings support the possibility that FBOM-delta C-13 can be used as a paleoceanographic proxy for surface water [CO2(aq)] and thus atmospheric pCO(2). (C) 2021 Elsevier Ltd. All rights reserved.

The15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may allow for reconstruction of fish trophic history and changes in food webs. To support this application, ground-truthing data are needed on the relationships among the δ15N of diet, of fish tissue (e.g., white muscle tissue, δ15Nwmt), and δ15Noto. Using a highly sensitive method for N isotope analysis, δ15Notowas compared with δ15Nwmtin 24 teleost species. Within a species, the difference between δ15Notoand δ15Nwmt(Δδ15No-w) varied little across individuals, confirming the utility of δ15Nototo reconstruct δ15Nwmtchanges for a given species. Across species, δ15Notoand δ15Nwmtwere highly correlated. However, Δδ15No-wvaried systematically across species. Phylogeny, the concentrations of total N and amino acids, and life history were ruled out as the main cause for the observed variation in Δδ15No-w. δ15Notowas lowest relative to δ15Nwmtin species producing larger otoliths. We propose that δ15Notois elevated by isotopically fractionating metabolism of the organic matrix, which is less important when otolith growth is fast and thus when the otolith is large.

Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O production from ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH4+ and the other from other substrates such as NO2-) was the dominant species, comprising 70 %–85 % of total produced N2O from NH4+, regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.