151 research outputs found
Effect of chloride on the chemical conversion of nitrate to nitrous oxide for δ15N analysis
We investigate the influence of chloride concentration on the performance of the chemical reduction method for measurement of the nitrogen isotopic ratio (δ15N) in NO3â in natural waters (McIlvin and Altabet, 2005). In this method, NO3â is first reduced to NO2â using activated cadmium metal, with further reduction to N2O using sodium azide in an acetic acid buffer. N2O is introduced into an isotope ratio mass spectrometer (IRMS) for isotopic measurement. Previously, it was recognized that the presence of halides was necessary for the speed and efficiency of the second step but not thought to be important for the first step. Whereas quantitative Cd reduction of NO3â to NO2â had been noted for seawater samples, here we report, for freshwater and lowâsalinity (S 99%) reduction of NO3â to NO2â as well as stable δ15N values that closely matched expected values for standards (within 0.3â° of standard value). The positive effect of NaCl is likely due to a decrease in free Cd2+ produced over the course of the reaction due to formation of CdCl2
N-loss stoichiometry in a Peru ODZ eddy
Assuming heterotrophic denitrification as the dominant microbial process, Richards (1965) formulated a stoichiometry governing nitrogen loss in open-ocean oxygen deficient zones (ODZs). It prescribes the quantitative coupling between the oxidation of organic matter by NOâ3 in the absence of O2 and the corresponding production of CO2, N2, and POâ34. Applied globally, this relationship defines key linkages between the C, N, and P cycles. However, the validity of Richards\u27s stoichiometry is challenged by recognition of complex microbial N processing in ODZs including anammox as an important pathway and nitrite reoxidation. Whereas Richards\u27s stoichiometry would result in N2-N production to NOâ3 removal rates of 1.17, dominance by anammox with respect to biogenic N2 production could in theory result in a ratio as high as 2. Ratios with POâ34 production provide an additional constraint on the quantity and composition of respired organic matter. Here we use a mesoscale eddy with extreme N-loss in the Peru ODZ as a natural laboratory to examine N-loss stoichiometry. Its intense biogeochemical signatures, relatively well-defined timescales, and simplified hydrography allowed for the development of strong co-occurring gradients in NOâ3, NOâ2, biogenic N2, and POâ34. The production of biogenic N2 as compared with the removal of NOâ3 (analyzed either directly or as N deficits) was slightly less than predicted by Richards\u27s stoichiometry and did not at all support any excess biogenic N2. POâ34 production, however, was twice the expectation from Richards\u27s stoichiometry suggesting that respired organic matter was P-rich as compared with C:N:P Redfield composition. These results suggest major gaps remain between current understanding of microbial N pathways in ODZs and their net biogeochemical output
Vertical patterns in 15N natural abundance in PON from the surface waters of warm-core rings
The natural abundance of 15N in PON from the upper 200 m of 4 warm-core rings and the Sargasso Sea was measured. Minima in the δ15N of PON often occurred near the depth at which NO3â was first detectable. Frequently, maxima in PON concentration and minima in C/N ratio also co-occurred in this region. The average value for the δ15N of PON below the top of the nitracline was almost always greater than that above the top of the nitracline. The observed vertical pattern for the δ15N of PON is most likely the result of isotopic fractionation in the processes of NO3â uptake by phytoplankton at the base of the euphotic zone and the degradation of PON below the euphotic zone. Variations in the strength and coherence of this vertical pattern appear to occur in response to both rapid physical modification of the water column and the trophic status of the euphotic zone
A model for the vertical flux of nitrogen in the upper ocean: Simulating the alteration of isotopic ratios
An idealized, one-dimensional, constant diffusivity mathematical model for the study of the vertical flux of nitrogen in the upper-ocean is presented. We attempt to simulate observed patterns in vertical profiles for the natural abundance of 15N in particulate organic nitrogen (PON) and the concentrations of PON and NO3â. The concentration of phytoplankton nitrogen (II) increased as a result of either increasing the upward flux of NO3â(N) or by increasing the residence time of II. A minimum in the δ15N of phytoplankton nitrogen (δ2) appeared near a maximum in II at the inflection point of the N profile. Increasing the residence time or the vertical eddy diffusivity, reduced the amplitude of the δ2 profile. The model was able to produce reasonably good simulations of observed profiles from two warm-core rings, Rings 82-E and 82-H, using the most appropriate values for the light extinction coefficient and the residence time of PON. These results lend general support to current views regarding the nature and significance of the vertical fluxes of nitrogen in the upper-ocean and hypotheses presented previously concerning the factors which affect the δ15N of PON
Advancing science from plankton to whalesâCelebrating the contributions of James J. McCarthy
Hailing from Sweet Home, Oregon, where his father introduced him to the fascinations of pondwater (McCarthy 2018), Jim McCarthy graduated from Gonzaga University, and in the late 1960s joined the Food Chain Research Group at the Scripps Institution of Oceanography, where he received his doctorate in 1971. The Food Chain Research Group, which was becoming recognized as the premier research group on plankton, was at that time directed by such distinguished scientists as John Strickland and Dick Eppley, among others. The goal of the Food Chain Group was to understand plankton dynamics and trophodynamics, âto a degree that will enable man to exercise satisfactory control of the environment and make useful predictionsâ (Institute of Marine Resources annual report, 1968, cited in Shor 1978:143) and âto predict the formation and transfer of nutrients through the full cycle of life in the oceanâ (Shor 1978:140). It was there that Jim became immersed in all aspects of nutrients, plankton, and the marine food web
N-loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment
Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent ânatural tracer experimentsâ with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (Îľ; the â° deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3â), nitrite (NO2â) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3â was nearly exhausted, we measured the highest δ15N values for both NO3â and NO2â (up to ~70â° and 50â°) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ~40 Âľmol Lâ1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2â reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable Îľ for NO3â reduction (up to ~30â° in the presence of NO2â). However, the overall Îľ for N-loss was calculated to be only ~13-14â° (as compared to canonical values of ~20-30â°) assuming a closed system and only slightly higher assuming an open system (16-19â°). Our results were similar whether calculated from the disappearance of DIN (NO3â + NO2â) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate Îľ for NO3â reduction to NO2â and NO2â reduction to N2 of ~16-21â° and ~12â°, respectively, when the effect of NO2â oxidation could be removed. These results, together with the relationship between N and O of NO3â isotopes and the difference in δ15N between NO3â and NO2-, confirm a role for NO2â oxidation in increasing the apparent Îľ associated with NO3â reduction. The lower Îľ for NO3â and NO2â reduction as well as N-loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N-loss
Simulating the global distribution of nitrogen isotopes in the ocean
We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth system climate model designed for millennial timescale simulations. The model includes prognostic tracers for the two stable nitrogen isotopes, 14N and 15N, in the nitrate (NO3â), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3â uptake, nitrogen fixation, water column denitrification, and zooplankton excretion are considered as well as the removal of NO3â by sedimentary denitrification. A global database of δ15NO3â observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce many of the observed patterns such as high subsurface values in water column denitrification zones and the meridional and vertical gradients in the Southern Ocean. The observed pronounced subsurface minimum in the Atlantic is underestimated by the model presumably owing to too little simulated nitrogen fixation there. Sensitivity experiments reveal that algal NO3â uptake, nitrogen fixation, and water column denitrification have the strongest effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotope distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for nitrogen fixers and, thus, stimulates additional N2 fixation in the model. Important model deficiencies are identified, and strategies for future improvement and possibilities for model application are outlined
Microbial associations with macrobiota in coastal ecosystems : patterns and implications for nitrogen cycling
Author Posting. Š Ecological Society of America, 2016. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Frontiers in Ecology and the Environment 14 (2016): 200-208, doi:10.1002/fee.1262.In addition to their important effects on nitrogen (N) cycling via excretion and assimilation (by macrofauna and macroflora, respectively), many macrobiota also host or facilitate microbial taxa responsible for N transformations. Interest in this topic is expanding, especially as it applies to coastal marine systems where N is a limiting nutrient. Our understanding of the diversity of microbes associated with coastal marine macrofauna (invertebrate and vertebrate animals) and macrophytes (seaweeds and marine plants) is improving, and recent studies indicate that the collection of microbes living in direct association with macrobiota (the microbiome) may directly contribute to N cycling. Here, we review the roles that macrobiota play in coastal N cycling, review current knowledge of macrobialâmicrobial associations in terms of N processing, and suggest implications for coastal ecosystem function as animals are harvested and as foundational habitat is lost or degraded. Given the biodiversity of microbial associates of macrobiota, we advocate for more research into the functional consequences of these associations for the coastal N cycle.University of Chicago-Marine Biological Laboratories (MBL
A review of nitrogen isotopic alteration in marine sediments
Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth
Abstract:
Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle
Copepod-Associated Gammaproteobacteria Respire Nitrate in the Open Ocean Surface Layers
Microbial dissimilatory nitrate reduction to nitrite, or nitrate respiration, was detected in association with copepods in the oxygenated water column of the North Atlantic subtropical waters. These unexpected rates correspond to up to 0.09 nmol N copepodâ1 dâ1 and demonstrate a previously unaccounted nitrogen transformation in the oceanic pelagic surface layers. Genes and transcripts for both the periplasmic and membrane associated dissimilatory nitrate reduction pathways (Nap and Nar, respectively) were detected. The napA genes and transcripts were closely related with sequences from several clades of Vibrio sp., while the closest relatives of the narG sequences were Pseudoalteromonas spp. and Alteromonas spp., many of them representing clades only distantly related to previously described cultivated bacteria. The discovered activity demonstrates a novel Gammaproteobacterial respiratory role in copepod association, presumably providing energy for these facultatively anaerobic bacteria, while supporting a reductive path of nitrogen in the oxygenated water column of the open ocean
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