Kelp forests are known as key habitats for species diversity and macroalgal productivity; however, we know little about how these biogenic habitats interact with seawater chemistry and phototroph ...productivity in the water column. We examined kelp forest functions at three locales along the Olympic Peninsula of Washington state by quantifying carbonate chemistry, nutrient concentrations, phytoplankton productivity, and seawater microbial communities inside and outside of kelp beds dominated by the canopy kelp species Nereocystis luetkeana and Macrocystis pyrifera. Kelp beds locally increased the pH, oxygen, and aragonite saturation state of the seawater, but lowered seawater inorganic carbon content and total alkalinity. Although kelp beds depleted nitrate and phosphorus concentrations, ammonium and dissolved organic carbon (DOC) concentrations were enhanced. Kelp beds also decreased chlorophyll concentrations and carbon fixed by phytoplankton, although kelp carbon fixation more than compensated for any difference in phytoplankton production. Kelp beds entrained distinct microbial communities, with higher taxonomic and phylogenetic diversity compared to seawater outside of the kelp bed. Kelp forests thus had significant effects on seawater chemistry, productivity and the microbial assemblages in their proximity. Thereby, the diversity of pathways for carbon and nitrogen cycling was also enhanced. Overall, these observations suggest that the contribution of kelp forests to nearshore carbon and nitrogen cycling is greater than previously documented.
We present a novel method for nitrogen and oxygen natural isotopic abundance analysis of nitrate and nitrite of seawater and freshwater at environmental concentrations. The method involves the ...reduction of nitrate to nitrite using spongy cadmium with further reduction to nitrous oxide using sodium azide in an acetic acid buffer. For separate nitrite analysis, the cadmium reduction step is simply bypassed. Nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed on a continuous flow isotope ratio mass spectrometer via an open split. This paper describes the basic protocol and reaction conditions required to obtain reproducible natural abundance level nitrogen and oxygen isotopic ratios from nitrate, nitrite, or both, and the results obtained to support these conclusions. A standard deviation less than 0.2‰ for nitrogen and 0.5‰ for oxygen was found for nitrate samples ranging in concentration from 40 to 0.5 μM (better for nitrite), with a blank of 2 nmol for 50-mL samples. Nitrogen and oxygen isotopic fractionation and oxygen atom exchange were consistent within each batch of analysis. There was no interference from any seawater matrixes. Only one other method published to date can measure the nitrate oxygen isotopic abundance in seawater and none that do so for nitrite alone in the presence of nitrate. This method may prove to be simpler, faster, and obtain isotopic information for lower concentrations of nitrate and nitrite than other methods.
The ocean is an important source of nitrous oxide (N2O) to the atmosphere, yet the factors controlling N2O production and consumption in oceanic environments are still not understood nor constrained. ...We measured N2O concentrations and isotopomer ratios, as well as O2, nutrient and biogenic N2 concentrations, and the isotopic compositions of nitrate and nitrite at several coastal stations during two cruises off the Peru coast (~5–16°S, 75–81°W) in December 2012 and January 2013. N2O concentrations varied from below equilibrium values in the oxygen deficient zone (ODZ) to up to 190 nmol L−1 in surface waters. We used a 3‐D‐reaction‐advection‐diffusion model to evaluate the rates and modes of N2O production in oxic waters and rates of N2O consumption versus production by denitrification in the ODZ. Intramolecular site preference in N2O isotopomer was relatively low in surface waters (generally −3 to 14‰) and together with modeling results, confirmed the dominance of nitrifier‐denitrification or incomplete denitrifier‐denitrification, corresponding to an efflux of up to 0.6 Tg N yr−1 off the Peru coast. Other evidence, e.g., the absence of a relationship between ΔN2O and apparent O2 utilization and significant relationships between nitrate, a substrate during denitrification, and N2O isotopes, suggest that N2O production by incomplete denitrification or nitrifier‐denitrification decoupled from aerobic organic matter remineralization are likely pathways for extreme N2O accumulation in newly upwelled surface waters. We observed imbalances between N2O production and consumption in the ODZ, with the modeled proportion of N2O consumption relative to production generally increasing with biogenic N2. However, N2O production appeared to occur even where there was high N loss at the shallowest stations.
Key Points
High N2O concentrations were observed in coastal waters off Peru
Incomplete denitrifier‐denitrification was an important N2O production pathway
N2O production occurred at high extent of N loss in the shallow ODZ
Microbial nitrogen processing in direct association with marine animals and
seaweeds is poorly understood. Microbes can both attach to the surfaces of
macrobiota and make use of their excreted ...nitrogen and dissolved organic
carbon (DOC). We tested the role of an intertidal mussel (Mytilus californianus) and red alga (Prionitis sternbergii), as well as
inert substrates for microbial activity using enclosed chambers with seawater
labeled with 15N-enriched ammonium and nitrate. Chambers with only
seawater from the same environment served as a control. We found that 3.21 nmol
of ammonium per gram of dry mass of mussel, on average, was oxidized per
hour, while 1.60 nmol of nitrate was reduced per hour. Prionitis
was associated with the oxidation of 1.50 nmol of ammonium per gram of wet mass per
hour, while 1.56 nmol of nitrate was reduced per hour. Inert substrates
produced relatively little change compared to seawater alone. Extrapolating
to a square meter of shoreline, microbial activity associated with mussels
could oxidize 2.5 mmol of ammonium and reduce per 1.2 mmol of nitrate per
day. A square meter of seaweed could oxidize 0.13 mmol ammonium per day and
reduce the same amount of nitrate. Seawater collected proximal to the shore
versus 2–5 km offshore showed no difference in ammonium oxidation or nitrate
reduction. Microbial nitrogen metabolism associated with mussels was not
influenced by the time of day. When we experimentally added DOC (glucose) as
a carbon source to chambers with the red alga and inert substrates, no change
in nitrification rates was observed. Added DOC did increase dissolved inorganic
nitrogen (DIN) and phosphorus uptake, indicating that DOC addition stimulated heterotrophic
microbial activity, and suggests potential competition for DIN between
heterotrophic and chemolithotrophic microbes and their seaweed hosts. Our
results demonstrate that microbes in direct association with coastal animals
and seaweeds greatly enhance nitrogen processing and likely provide a
template for a diversity of ecological interactions.
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 (NO3−), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3− uptake, nitrogen fixation, water column denitrification, and zooplankton excretion are considered as well as the removal of NO3− by sedimentary denitrification. A global database of δ15NO3− 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 NO3− 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.
Mesoscale eddies in Oxygen Minimum Zones (OMZs) 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 “natural tracer experiments” with intensified biogeochemical signals that can be exploited to understand the large‐scale processes that control N‐loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−), and biogenic N2 associated with an anticyclonic mode‐water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) 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 µmol L−1). δ15N‐N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N‐loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N‐loss was calculated to be only ~13–14‰ (as compared to canonical values of ~20–30‰) assuming a closed system and only slightly higher assuming an open system (16–19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε values for NO3− reduction to NO2− and NO2− reduction to N2 of ~16–21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2−, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for N‐loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N‐loss.
Key Points
Large N‐loss signals were observed in a mesoscale eddy
Nitrite oxidation increases the overall isotope effect of nitrate reduction
Low isotope effects for net N-loss were estimated
Most global biogeochemical processes are known to respond to climate change, some of which have the capacity to produce feedbacks through the regulation of atmospheric greenhouse gases. Marine ...denitrification-the reduction of nitrate to gaseous nitrogen-is an important process in this regard, affecting greenhouse gas concentrations directly through the incidental production of nitrous oxide, and indirectly through modification of the marine nitrogen inventory and hence the biological pump for CO2. Although denitrification has been shown to vary with glacial-interglacial cycles, its response to more rapid climate change has not yet been well characterized. Here we present nitrogen isotope ratio, nitrogen content and chlorin abundance data from sediment cores with high accumulation rates on the Oman continental margin that reveal substantial millennial-scale variability in Arabian Sea denitrification and productivity during the last glacial period. The detailed correspondence of these changes with Dansgaard-Oeschger events recorded in Greenland ice cores indicates rapid, century-scale reorganization of the Arabian Sea ecosystem in response to climate excursions, mediated through the intensity of summer monsoonal upwelling. Considering the several-thousand-year residence time of fixed nitrogen in the ocean, the response of global marine productivity to changes in denitrification would have occurred at lower frequency and appears to be related to climatic and atmospheric CO2 oscillations observed in Antarctic ice cores between 20 and 60 kyr ago.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Anaerobic ammonium oxidation (anammox) contributes substantially to ocean nitrogen loss, particularly in anoxic marine zones (AMZs). Ammonium is scarce in AMZs, raising the hypothesis that organic ...nitrogen compounds may be ammonium sources for anammox. Biochemical measurements suggest that the organic compounds urea and cyanate can support anammox in AMZs. However, it is unclear if anammox bacteria degrade these compounds to ammonium themselves, or rely on other organisms for this process. Genes for urea degradation have not been found in anammox bacteria, and genomic evidence for cyanate use for anammox is limited to a cyanase gene recovered from the sediment bacterium Candidatus Scalindua profunda. Here, analysis of Ca. Scalindua single amplified genomes from the Eastern Tropical North Pacific AMZ revealed genes for urea degradation and transport, as well as for cyanate degradation. Urease and cyanase genes were transcribed, along with anammox genes, in the AMZ core where anammox rates peaked. Homologs of these genes were also detected in meta-omic datasets from major AMZs in the Eastern Tropical South Pacific and Arabian Sea. These results suggest that anammox bacteria from different ocean regions can directly access organic nitrogen substrates. Future studies should assess if and under what environmental conditions these substrates contribute to the ammonium budget for anammox.
Rationale
The nitrogen and oxygen (δ15N, δ18O, δ17O) isotopic compositions of NO3− and NO2− are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd‐azide method was ...used to convert NO3− or NO2− to N2O for N and triple‐O isotopic analyses by N2O laser spectrometry. A protocol for laser‐based headspace isotope analyses was compared with isotope ratio mass spectrometry. Lasers provide the ability to directly measure 17O anomalies which can help discern atmospheric N sources.
Methods
δ15N, δ18O and δ17O values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N2O using the Cd‐azide method and headspace N2O laser spectrometry. A 15N tracer test assessed the position‐specific routing of N to the α or β positions in the N2O molecule. A data processing algorithm was used to correct for isotopic dependencies on N2O concentration, cavity pressure and water content.
Results
NO3−/NO2− nitrogen is routed to the 15Nα position of N2O in the azide reaction; hence the δ15Nα value should be used for N2O laser spectrometry results. With corrections for cavity pressure, N2O concentration and water content, the δ15NαAIR, δ18OVSMOW and δ17OVSMOW values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO3), −1.7 ± 0.1, −26.8 ± 0.8, −14.4 ± 1.1 (USGS34) and +2.6 ± 0.1, +57.6 ± 1.2, +51.2 ± 2.0 (USGS35), in agreement with their values and with the isotope ratio mass spectrometry results. The 17O excess for USGS35 was +21.2 ± 9‰, in good agreement with previous results.
Conclusions
The Cd‐azide method yielded excellent results for routine determination of δ15N, δ18O and δ17O values (and the 17O excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd‐azide chemicals and the lack of automated sampling devices for N2O laser spectrometers. The 15N‐enriched tracer test revealed potential for position‐specific experimentation of aqueous nutrient dynamics at high 15N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of 17O.
Animal contributions to ecosystem productivity via nutrient regeneration are increasingly recognized as significant for ecosystem productivity. The paradigm in coastal upwelling systems, in contrast, ...remains centered around the idea that nutrient supply is extrinsic and sourced from deep water. Here we tested for both animal contributions to nitrogen regeneration and microbial retention of nitrogen along a gradient of animal abundance in Washington State, USA. Using inferences from the concentration and isotopic composition of nitrogen forms, as well as the stable nitrogen isotopic composition of particulate organic matter, and plant and animal tissue, we found increased ammonium as well as greater stable nitrogen isotope values in these organisms in areas of increased animal abundance. We further tested the effect of the nearshore biota by comparing it with an immediately adjacent offshore area, and found an enriched δ
15
N value for nitrate and particulate organic matter. Further, an analysis of the dual isotopes of δ
15
N and the δ
18
O of nitrate indicated increasing microbial processing in the nearshore. Isoscapes, or the spatial pattern of stable isotopes, revealed that animal and microbial processing of nitrogen leave a consistent signature and are thus a previously overlooked and essential component of nearshore productivity.
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