Aquatic ecosystems, ranging from the open ocean to coastal environments, are driven by intricate nitrogen (N) cycling processes that are essential for maintaining the health of these environments and the food webs they sustain. At the base of these food webs, primary producers such as phytoplankton play a pivotal role by assimilating N from various sources through both photoautotrophic and mixotrophic pathways. Autotrophy converts inorganic N to organic N, predominantly in the form of proteins and amino acids (AAs), which serve as the primary carriers of N as it moves through the food webs.
Identifying the sources of N in aquatic ecosystems and their associated food webs is crucial for understanding and modeling these cycling processes. This can be achieved through contemporary measurements of dissolved inorganic N (e.g., nitrate and ammonia) concentrations and stable isotopes, as well as by reconstructing historical N sources and usage patterns using organic N and their isotopes in biogenic particles and sediments. These investigations provide valuable insights into both the past and present dynamics of N cycling in open ocean and coastal environments.
Moreover, N cycling processes have significant implications for climate, as processes such as denitrification and nitrification can lead to the production of nitrous oxide (N₂O), a potent greenhouse gas with long-term impacts on global warming. Understanding the factors that regulate N₂O production and release is therefore essential for predicting the broader environmental consequences of N cycling.
This session aims to bring together researchers utilizing a diverse array of methodologies, including isotope geochemistry, biomolecular tools, and numerical modeling, to explore N cycling in aquatic ecosystems and their associated food webs across both open ocean and coastal areas. By sharing insights and findings, this session seeks to deepen our understanding of N cycling processes across different aquatic environments, ultimately contributing to a more comprehensive understanding of how N cycling influences ecosystem structure and function across various spatial and temporal scales.
Lead Organizer: Lin Zhang, Texas A and M University Corpus Christi (lin.zhang@tamucc.edu)
Co-organizers:
Mark Altabet, University of Massachusetts Dartmouth (maltabet@umassd.edu)
Annie Bourbonnais, University of South Carolina (abourbonnais@seoe.sc.edu)
Pat Glibert, University of Maryland Center for Environmental Science (glibert@umces.edu)
Wingman (Charlotte) Lee, Texas A&M University-Corpus Christi (wlee4@islander.tamucc.edu)
Presentations
02:30 PM
SPATIOTEMPORAL DYNAMICS OF NITRIFYING MICROBIAL COMMUNITIES IN THE CAPE FEAR RIVER ESTUARY (9139)
Primary Presenter: Parker Lawrence, University of North Carolina at Wilmington (pbl5777@uncw.edu)
Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) play important roles in nitrification, a key process in the nitrogen cycle that contributes to the transformation of ammonium (NH₄⁺) into nitrite (NO₂⁻) and ultimately nitrate (NO₃⁻). Nitrification is particularly critical in estuarine environments as it can alter the composition of inorganic nitrogen exported to coastal waters. In the Cape Fear River Estuary (CFRE) near Wilmington, NC (southeastern USA), we conducted time-series sampling to assess spatiotemporal changes in abundance, diversity, and activity of nitrifying microbial communities, along with other environmental parameters such as temperature, salinity, dissolved oxygen, and nutrient concentrations. Using quantitative PCR (qPCR), we found that AOA exhibit an annual, recurrent ‘bloom’ each summer, with populations increasing 3-4 orders of magnitude above their typical abundance, aligned with previous work in coastal Georgia, USA. This AOA bloom coincided with a pronounced NO₂⁻peak, suggesting a decoupling of ammonia oxidation from subsequent nitrite oxidation processes. A strong and significant correlation between AOA gene abundance and NO₂⁻concentration was identified in the CFRE, whereas no correlation was found with AOB and NO₂⁻. These results suggest that AOA-mediated ammonia oxidation is a primary factor in the seasonally recurring NO₂⁻accumulation within the CFRE. Understanding these seasonal dynamics is critical for identifying the specific environmental triggers that drive AOA blooms, and any impacts they may have on coastal ecosystems.
02:45 PM
Nitrogen Isotopes in Amino Acids and Nitrate Reveal Particle Production and Transformation in a Marine Oxygen-Deficient Zone (8788)
Primary Presenter: Lin Zhang, Texas A and M University Corpus Christi (lin.zhang@tamucc.edu)
The eastern tropical North Pacific oxygen deficient zone (ETNP-ODZ) hosts unique assemblages of phytoplankton, microbes, and zooplankton, suggesting distinct particle production and transformation processes. This study compares δ¹⁵N values of nitrate in seawater and amino acids (AAs), such as phenylalanine (Phe) and glutamic acid (Glu), in size-fractionated particles collected across a productivity gradient within the ETNP-ODZ. Results show that at the primary chlorophyll maximum (PCM), phytoplankton relied more on NO₃-, while at sites with a pronounced secondary chlorophyll maximum (SCM), lower δ¹⁵N-Phe and δ¹⁵N-Glu values indicate a contribution from primary producers utilizing recycled nitrogen. Reduced ¹⁵N enrichment in Phe of small particles, along with smaller differences in δ¹⁵N-Phe between large and small particles compared to oxic waters, suggests slower microbial degradation of small particles in the ODZ. At the lower oxycline, distinctive δ¹⁵N-Phe and δ¹⁵N-Glu signatures were observed, possibly related to chemoautotrophic production and zooplankton interactions. These findings underscore the need for further research on nitrogen cycling at the SCM, particle alteration by zooplankton, and chemoautotroph contributions to deep-sea particles in expanding ODZs under climate change.
03:00 PM
The role of nitrogen fixation in shaping the stoichiometry of marine particulate organic matter (9253)
Primary Presenter: Na Li, GEOMAR Helmholtz Centre for Ocean Research Kiel (nli@geomar.de)
The main source of bioavailable N in the ocean is N2-fixation by diazotrophs. This study investigates the impact of nitrogen fixation on the elemental (carbon:nitrogen:phosphorus = C:N:P) composition of marine particulate organic matter (POM) in marine environments. Using a global ocean optimality-based eco-physiological model, we demonstrate the influence of nitrogen fixation through two scenarios: The Optimal Nitrogen Fixation Scenario (ONFS) represents optimal (realistic) nitrogen fixation, and the Reduced Nitrogen Fixation Scenario (RNFS) has half the nitrogen fixation of ONFS. The physiological model predicts that N2-fixing diazotrophs have lower C:N (5.4:1) and higher N:P (19.3:1) ratios than non-N2-fixing phytoplankton (C:N=7.1, N:P=16.3) and the Redfield ratio (C:N=6.6, N:P=16). With optimal nitrogen fixation, our model better reproduces the relatively low C:N ratios and high N:P ratio of observations, compared to our simulation with reduced nitrogen fixation, particularly in the subtropical zones, although our model still underestimates the observed deviations from the Redfield ratio. Despite representing less than 2% of the total POM, diazotrophs significantly influence the latitudinal patterns of particulate C:N:P ratios by altering their own stoichiometry as well as affecting the stoichiometry of non-N2-fixing phytoplankton and detritus through ecological interactions. A sensitivity analysis reveals that phytoplankton subsistence quotas and grazing pressure are key traits determining global stoichiometry. These results highlight the importance of accurately representing the nitrogen cycle, specifically nitrogen fixation, in reproducing the stoichiometry of marine particulate organic matter in global ocean models.
03:15 PM
A NEW PARADIGM FOR THE MARINE DISSOLVED ORGANIC NITROGEN RESERVOIR: REFRACTORY HETEROCYCLIC NITROGEN OF BACTERIAL ORIGIN? (9098)
Primary Presenter: Matthew McCarthy, UC Santa Cruz (mdmccar@ucsc.edu)
Marine dissolved organic nitrogen (DON) is one of the largest standing reservoirs of fixed N. However, most ocean DON appears resistant to biological utilization, representing a central control on N biogeochemical cycling in the modern ocean. However, the molecular composition of this refractory DON (RDON), and the reasons for its long-term persistence, have never been understood. Past characterization focused almost entirely on the high molecular weight fraction, which is now recognized as younger and more labile, consistent with its amide functionality. In contrast, the vastly larger low molecular weight (LMW), RDON pool has received relatively little attention. Here we synthesize a suite of new results focused on the quantitatively dominant, refractory LMW DON pool, pointing toward a fundamentally new view of the ocean’s dominant DON composition. We show that solid-state 15N NMR using multi-cross polarization pulse sequences yields good quantitation, even for non-protonated N functionalities. Data acquired with these sequences indicates that essentially the entire LMW DON pool, representing most DON in the world ocean, is composed of previously unknown N heterocyclic material. Amino acid biomarkers from the same samples indicate bacterial origin, and suggest that most RDON may be “preformed” as refractory N compounds in the surface ocean. Finally, these data suggest that most amino acids in RDON may not be proteinaceous, as has been long assumed, but could derive from unknown bacterial natural products.
03:30 PM
Vegetation enhances nitrogen removal in stormwater control measures in coastal South Carolina (9340)
Primary Presenter: Darcy Perin, University of South Carolina (dperin@email.sc.edu)
Stormwater Control Measures (SCM) are used to mitigate stormwater impacts on downstream receiving waters. SCMs tend to be highly effective at removing particle-associated nutrients, such as phosphorus, but they are often much less effective at removing dissolved nitrogen. In excess amounts, inorganic nitrogen can lead to coastal eutrophication, harmful algal blooms, and coastal hypoxia (i.e. “dead zones”). In this research, we aimed to provide an estimate of nitrogen removal rates (i.e., by denitrification) and net N2 fluxes across the sediment-water interface for a series of stormwater detention ponds with varying amounts of vegetation. Rates of removal were significantly higher (p<0.001) in the vegetated stormwater detention ponds (2.73 – 42.98 µmol N m-2 h-1) compared to those without vegetation (0.6 – 3.5 µmol N m-2 h-1). Additionally, average net N2 fluxes were positive for vegetated ponds (2.03 – 22.23 µmol N m-2 h-1) but generally negative for unvegetated ponds (-20.96 – 1.68 µmol N m-2 h-1). These results indicate that sediments of conventional (unvegetated) stormwater ponds are significant sources of nitrogen. However, when ponds are intentionally or passively allowed to grow emergent vegetation (especially vegetated littoral shelves), they switch roles and become net sinks due to a substantial increase in rates of denitrification, demonstrating stormwater ponds have the potential to alleviate some of the negative effects of urbanization and improve overall water quality with encouraged use of vegetation.
03:45 PM
Ocean warming enhances iron use efficiencies of marine ammonia-oxidizing archaea (9097)
Primary Presenter: WEI QIN, University of Oklahoma (qinwei2010@gmail.com)
Ammonia-oxidizing archaea (AOA) are among the most abundant microorganisms in the ocean, playing a fundamental role in the marine nitrogen cycle. Although temperature and trace metal availability each individually influence the growth and activity of marine AOA, there is still only a very limited understanding of the interactive effects of these two major factors on AOA in the rapidly changing ocean. Here, we show that the iron requirements of the model marine AOA species Nitrosopumilus maritimus SCM1 are highly sensitive to temperature changes. A 5ºC increase in growth temperature reduced SCM1 iron requirements by > 80%, and was associated with a substantial increase in iron use efficiencies (IUE, mol C fixed/hr/mol cellular Fe) under iron-limited and warming conditions. Thermally enhanced IUE thus enables SCM1 to more efficiently utilize scarce available iron supplies to support its growth. Based on comprehensive analysis of whole-cell proteomic responses to iron limitation at various temperatures, our study revealed that the proteomic profiles under iron-limited conditions were noticeably distinct from those observed under iron-replete conditions, and the profiles at 32˚C differed from those at lower temperatures (23 ˚C and 27 ˚C). In particular, the cystathionine β-synthase (CBS) domain and universal stress protein A (UspA) may have a specific function as a high temperature response protein in N. maritimus. The translation of a ferredoxin gene was depressed at growth limiting iron concentrations. In contrast, the expression of a copper-dependent plastocyanin gene was significantly increased in response to iron limitation. The downregulation of ferredoxin and upregulation of plastocyanin were particularly pronounced as growth temperatures increased, suggesting a temperature-dependent regulation mechanism Together, these findings suggest that marine AOA may be able to leverage current and future elevated temperatures to better adapt to living in widespread iron-depleted regions of the ocean.
SS18B - Nitrogen Cycling Processes in Aquatic Ecosystems and Associated Food Webs
Description
Time: 2:30 PM
Date: 29/3/2025
Room: W207CD