Buchwald
Carolyn
Buchwald
Carolyn
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ArticleNitrogen cycling in the secondary nitrite maximum of the eastern tropical North Pacific off Costa Rica(John Wiley & Sons, 2015-12-15) Buchwald, Carolyn ; Santoro, Alyson E. ; Stanley, Rachel H. R. ; Casciotti, Karen L.Nitrite is a central intermediate in the marine nitrogen cycle and represents a critical juncture where nitrogen can be reduced to the less bioavailable N2 gas or oxidized to nitrate and retained in a more bioavailable form. We present an analysis of rates of microbial nitrogen transformations in the oxygen deficient zone (ODZ) within the eastern tropical North Pacific Ocean (ETNP). We determined rates using a novel one-dimensional model using the distribution of nitrite and nitrate concentrations, along with their natural abundance nitrogen (N) and oxygen (O) isotope profiles. We predict rate profiles for nitrate reduction, nitrite reduction, and nitrite oxidation throughout the ODZ, as well as the contributions of anammox to nitrite reduction and nitrite oxidation. Nitrate reduction occurs at a maximum rate of 25 nM d−1 at the top of the ODZ, at the same depth as the maximum rate of nitrite reduction, 15 nM d−1. Nitrite oxidation occurs at maximum rates of 10 nM d−1 above the secondary nitrite maximum, but also in the secondary nitrite maximum, within the ODZ. Anammox contributes to nitrite oxidation within the ODZ but cannot account for all of it. Nitrite oxidation within the ODZ that is not through anammox is also supported by microbial gene abundance profiles. Our results suggest the presence of nitrite oxidation within the ETNP ODZ, with implications for the distribution and physiology of marine nitrite-oxidizing bacteria, and for total nitrogen loss in the largest marine ODZ.
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ArticleNitrite oxidation exceeds reduction and fixed nitrogen loss in anoxic Pacific waters(Elsevier, 2020-08-20) Babbin, Andrew ; Buchwald, Carolyn ; Morel, Francois M. M. ; Wankel, Scott D. ; Ward, Bess B.The diversity of nitrogen-based dissimilatory metabolisms in anoxic waters continues to increase with additional studies to the marine oxygen deficient zones (ODZs). Although the microbial oxidation of nitrite (NO2–) has been known for over a century, studies of the pathways and microbes involved have generally proceeded under the assumption that nitrite oxidation to nitrate requires dioxygen (O2). Anaerobic NO2– oxidation until now has been conclusively shown only for anammox bacteria, albeit only as a limited sink for NO2– in their metabolism compared to the NO2– reduced to N2. Here, using direct experimental techniques optimized for replicating in situ anoxic conditions, we show that NO2– oxidation is substantial, widespread, and consistent across the ODZs of the eastern tropical Pacific Ocean. Regardless of the specific oxidant, NO2– oxidation rates are up to an order of magnitude larger than simultaneous N2 production rates for which these zones are known, and cannot be explained by anammox rates alone. Higher rates of NO2– oxidation over reduction in anoxic waters are paradoxical but help to explain how anammox rates can be enhanced over denitrification in shallow anoxic waters (σθ < 26.4) at the edge of the ODZs but not within the ODZ core. Furthermore, nitrite oxidation may be the key to reconciliation of the perceived imbalance of the global fixed nitrogen loss budget.
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ArticleNitrogen cycling in the deep sedimentary biosphere : nitrate isotopes in porewaters underlying the oligotrophic North Atlantic(Copernicus Publications on behalf of the European Geosciences Union, 2015-12-21) Wankel, Scott D. ; Buchwald, Carolyn ; Ziebis, Wiebke ; Wenk, Christine B. ; Lehmann, Moritz F.Nitrogen (N) is a key component of fundamental biomolecules. Hence, its cycling and availability are central factors governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite limited organic matter delivery from overlying water, the extent and modes of nitrogen transformations have not been widely investigated. Here we use the N and oxygen (O) isotopic composition of porewater nitrate (NO3−) from a site in the oligotrophic North Atlantic (Integrated Ocean Drilling Program – IODP) to determine the extent and magnitude of microbial nitrate production (via nitrification) and consumption (via denitrification). We find that NO3- accumulates far above bottom seawater concentrations (~ 21 μM) throughout the sediment column (up to ~ 50 μM) down to the oceanic basement as deep as 90 m b.s.f. (below sea floor), reflecting the predominance of aerobic nitrification/remineralization within the deep marine sediments. Large changes in the δ15N and δ18O of nitrate, however, reveal variable influence of nitrate respiration across the three sites. We use an inverse porewater diffusion–reaction model, constrained by the N and O isotope systematics of nitrification and denitrification and the porewater NO3- isotopic composition, to estimate rates of nitrification and denitrification throughout the sediment column. Results indicate variability of reaction rates across and within the three boreholes that are generally consistent with the differential distribution of dissolved oxygen at this site, though not necessarily with the canonical view of how redox thresholds separate nitrate regeneration from dissimilative consumption spatially. That is, we provide stable isotopic evidence for expanded zones of co-occurring nitrification and denitrification. The isotope biogeochemical modeling also yielded estimates for the δ15N and δ18O of newly produced nitrate (δ15NNTR (NTR, referring to nitrification) and δ18ONTR), as well as the isotope effect for denitrification (15ϵDNF) (DNF, referring to denitrification), parameters with high relevance to global ocean models of N cycling. Estimated values of δ15NNTR were generally lower than previously reported δ15N values for sinking particulate organic nitrogen in this region. We suggest that these values may be, in part, related to sedimentary N2 fixation and remineralization of the newly fixed organic N. Values of δ18ONTR generally ranged between −2.8 and 0.0 ‰, consistent with recent estimates based on lab cultures of nitrifying bacteria. Notably, some δ18ONTR values were elevated, suggesting incorporation of 18O-enriched dissolved oxygen during nitrification, and possibly indicating a tight coupling of NH4+ and NO2− oxidation in this metabolically sluggish environment. Our findings indicate that the production of organic matter by in situ autotrophy (e.g., nitrification, nitrogen fixation) supplies a large fraction of the biomass and organic substrate for heterotrophy in these sediments, supplementing the small organic-matter pool derived from the overlying euphotic zone. This work sheds new light on an active nitrogen cycle operating, despite exceedingly low carbon inputs, in the deep sedimentary biosphere.
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DatasetPorewater measurements of nitrate and nitrite concentration and N and O isotopic ratios (d15N and d18O) collected from sites 3 and 10 on the North Atlantic Long Core Cruise R/V Knorr KN223 from October to December 2014(Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2019-03-15) Buchwald, Carolyn ; Spivack, Arthur J. ; Wankel, ScottPorewater measurements of nitrate and nitrite concentration and N and O isotopic ratios (d15N and d18O) collected from sites 3 and 10 on the North Atlantic Long Core Cruise R/V Knorr KN223 from October to December 2014. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/748792
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ArticleIsotopic constraints on nitrogen transformation rates in the deep sedimentary marine biosphere(American Geophysical Union, 2018-10-18) Buchwald, Carolyn ; Homola, Kira ; Spivack, Arthur J. ; Estes, Emily R. ; Wankel, Scott D.Little is known about the nature of microbial community activity contributing to the cycling of nitrogen in organic-poor sediments underlying the expansive oligotrophic ocean gyres. Here we use pore water concentrations and stable N and O isotope measurements of nitrate and nitrite to constrain rates of nitrogen cycling processes over a 34-m profile from the deep North Atlantic spanning fully oxic to anoxic conditions. Using a 1-D reaction-diffusion model to predict the distribution of nitrogen cycling rates, results converge on two distinct scenarios: (1) an exceptionally high degree of coupling between nitrite oxidation and nitrate reduction near the top of the anoxic zone or (2) an unusually large N isotope effect (~60‰) for nitrate reduction that is decoupled from the corresponding O isotope effect, which is possibly explained by enzyme-level interconversion between nitrite and nitrate.
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ArticleOxygen isotopic composition of nitrate and nitrite produced by nitrifying cocultures and natural marine assemblages(Association for the Sciences of Limnology and Oceanography, 2012-09) Buchwald, Carolyn ; Santoro, Alyson E. ; McIlvin, Matthew R. ; Casciotti, Karen L.The δ18O value of nitrate produced during nitrification (δ18ONO3,nit) was measured in experiments designed to mimic oceanic conditions, involving cocultures of ammonia-oxidizing bacteria or ammonia-oxidizing archaea and nitrite-oxidizing bacteria, as well as natural marine assemblages. The estimates of ranged from −1.5‰ ± 0.1‰ to +1.3‰ ± 1.4‰ at δ18O values of water (H2O) and dissolved oxygen (O2) of 0‰ and 24.2‰ vs. Vienna Standard Mean Ocean Water, respectively. Additions of 18O-enriched H2O allowed us to evaluate the effects of oxygen (O) isotope fractionation and exchange on . Kinetic isotope effects for the incorporation of O atoms were the most important factors for setting overall values relative to the substrates (O2 and H2O). These isotope effects ranged from +10‰ to +22‰ for ammonia oxidation (O2 plus H2O incorporation) and from +1‰ to +27‰ for incorporation of H2O during nitrite oxidation. values were also affected by the amount and duration of nitrite accumulation, which permitted abiotic O atom exchange between nitrite and H2O. Coculture incubations where ammonia oxidation and nitrite oxidation were tightly coupled showed low levels of nitrite accumulation and exchange (3% ± 4%). These experiments had values of −1.5‰ to +0.7‰. Field experiments had greater accumulation of nitrite and a higher amount of exchange (22% to 100%), yielding an average value of +1.9‰ ± 3.0‰. Low levels of biologically catalyzed exchange in coculture experiments may be representative of nitrification in much of the ocean where nitrite accumulation is low. Abiotic oxygen isotope exchange may be important where nitrite does accumulate, such as oceanic primary and secondary nitrite maxima.
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ArticleEvidence for fungal and chemodenitrification based N2O flux from nitrogen impacted coastal sediments(Nature Publishing Group, 2017-06-05) Wankel, Scott D. ; Ziebis, Wiebke ; Buchwald, Carolyn ; Charoenpong, Chawalit N. ; de Beer, Dirk ; Dentinger, Jane ; Xu, Zhenjiang ; Zengler, KarstenAlthough increasing atmospheric nitrous oxide (N2O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N2O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N2O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N2O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N2O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N2O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N2O fluxes, especially in redox-dynamic sediments of coastal ecosystems.
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ArticleDiversity and spatial distribution of hydrazine oxidoreductase (hzo) gene in the oxygen minimum zone off Costa Rica(Public Library of Science, 2013-10-31) Kong, Liangliang ; Jing, Hongmei ; Kataoka, Takafumi ; Buchwald, Carolyn ; Liu, HongbinAnaerobic ammonia oxidation (anammox) as an important nitrogen loss pathway has been reported in marine oxygen minimum zones (OMZs), but the community composition and spatial distribution of anammox bacteria in the eastern tropical North Pacific (ETNP) OMZ are poorly determined. In this study, anammox bacterial communities in the OMZ off Costa Rica (CRD-OMZ) were analyzed based on both hydrazine oxidoreductase (hzo) genes and their transcripts assigned to cluster 1 and 2. The anammox communities revealed by hzo genes and proteins in CRD-OMZ showed a low diversity. Gene quantification results showed that hzo gene abundances peaked in the upper OMZs, associated with the peaks of nitrite concentration. Nitrite and oxygen concentrations may therefore colimit the distribution of anammox bacteria in this area. Furthermore, transcriptional activity of anammox bacteria was confirmed by obtaining abundant hzo mRNA transcripts through qRT-PCR. A novel hzo cluster 2x clade was identified by the phylogenetic analysis and these novel sequences were abundant and widely distributed in this environment. Our study demonstrated that both cluster 1 and 2 anammox bacteria play an active role in the CRD-OMZ, and the cluster 1 abundance and transcriptional activity were higher than cluster 2 in both free-living and particle-attached fractions at both gene and transcriptional levels.
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ArticleDynamics of extracellular superoxide production by Trichodesmium colonies from the Sargasso Sea(John Wiley & Sons, 2016-05-12) Hansel, Colleen M. ; Buchwald, Carolyn ; Diaz, Julia M. ; Ossolinski, Justin E. ; Dyhrman, Sonya T. ; Van Mooy, Benjamin A. S. ; Polyviou, DespoReactive oxygen species (ROS) are key players in the health and biogeochemistry of the ocean and its inhabitants. The vital contribution of microorganisms to marine ROS levels, particularly superoxide, has only recently come to light, and thus the specific biological sources and pathways involved in ROS production are largely unknown. To better understand the biogenic controls on ROS levels in tropical oligotrophic systems, we determined rates of superoxide production under various conditions by natural populations of the nitrogen-fixing diazotroph Trichodesmium obtained from various surface waters in the Sargasso Sea. Trichodesmium colonies collected from eight different stations all produced extracellular superoxide at high rates in both the dark and light. Colony density and light had a variable impact on extracellular superoxide production depending on the morphology of the Trichodesmium colonies. Raft morphotypes showed a rapid increase in superoxide production in response to even low levels of light, which was not observed for puff colonies. In contrast, superoxide production rates per colony decreased with increasing colony density for puff morphotypes but not for rafts. These findings point to Trichodesmium as a likely key source of ROS to the surface oligotrophic ocean. The physiological and/or ecological factors underpinning morphology-dependent controls on superoxide production need to be unveiled to better understand and predict superoxide production by Trichodesmium and ROS dynamics within marine systems.
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ThesisNitrogen cycling in oxygen deficient zones : insights from δ15N and δ18O of nitrite and nitrate(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2013-02) Buchwald, CarolynThe stable isotopes, δ15N and δ18O, of nitrite and nitrate can be powerful tools used to interpret nitrogen cycling in the ocean. In order to interpret isotope profiles, the isotope systematics of each process involved must be known. This thesis describes numerous experiments using both cultures of nitrifying organisms as well as natural seawater samples to determine the oxygen isotope systematics of nitrification. These experiments show that the accumulation of nitrite has a large effect on the resulting δ18ONO3. Also, the δ18ONO2 was developed as a unique tracer because it undergoes abiotic equilibration with water δ18O at a predictable rate based on pH, temperature and salinity. This rate, its dependencies, and how the δ18ONO2 values can be used as not only biological source indicators but also indicators of age are described. Finally, using the isotope systematics of nitrification as well as the properties of nitrite oxygen isotope exchange described in this thesis, the final chapter interprets multi-isotope nitrate and nitrite profiles in the Costa Rica Upwelling Dome using a simple 1D model. Overall, this thesis describes new nitrogen and oxygen isotopic tracers and uses them to elucidate the complicated nitrogen biogeochemistry in oxygen deficient zones.