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
09:00 AM
Closing the gaps in freshwater diazotrophy across the global ecospace (9682)
Primary Presenter: Edo Bar-Zeev, Ben Gurion University of the Negev (edobarzeev@gmail.com)
New evidence and previous estimations indicate that dinitrogen fixation by diazotrophs may play a central role in the freshwater biosphere along the ecological scales. And yet little is currently known on freshwater DIAZOTROPHY, namely the diversity, abundance and N2 fixation rates across the global freshwater ecospace, while even less is known on the environmental factors that control them. This research closes the knowledge gaps related to freshwater diazotrophy: from the abiotic factors that control diazotrophs abundance, diversity, and activity at the global scale. Achieving this global objective was done by sending a custom kit to 91 research groups from 68 countries that sampled 230 freshwater locations across the world. These samples were analysed for diazotrophs abundance, diversity and N2 fixation rates as well as organic carbon, nitrogen and various nutrients. Sampling across the world was carried during April-May 2023, thus springtime at the northern hemisphere and autumn at the southern hemisphere. A few unicellular diazotrophs hotspots (>15 x107 cells L-1) were apparent across the freshwater ecospace. Corresponding N2 fixation rates were relatively low (<4 nmol N L-1 d-1) at the northern hemisphere, yet few regions exhibited high values (>50 nmol N L-1 d-1) were apparent at Northern US and Spain. Alpha diversity was variable (<7.5), with high % of NCDs. Overall, this global research generates new insights on the environmental factors that control N fixation and its significance of freshwater diazotrophy to new N production across the freshwater ecospace.
09:15 AM
THE AVAILABILTY OF UREA AND ITS MICROBIAL UPTAKE IN THE SOUTHEASTERN MEDITERRANEAN SEA (9675)
Primary Presenter: Ilana Berman-Frank, University of Haifa (iberman2@univ.haifa.ac.il)
Urea is an important bioavailable organic nitrogen (N) compound directly utilized by heterotrophic bacteria as well as some photoautotrophs. In this study, the distribution of urea was determined in the upper water column of the southeastern Mediterranean Sea during three consecutive periods: 1) a thermally stratified oligotrophic period depleted in dissolved inorganic N and phosphorus; 2) temporary mesotrophy after a winter storm that injected a major pulse of nutrients into the photic zone; 3) decline in dissolved inorganic nutrients during the subsequent seasonal thermal stratification. Urea and total dissolved organic nitrogen (DON) concentrations peaked during the winter period after Storm Carmel (Dec 2021) and decreased during the following summer. We show evidence of urea removal above the nitracline, a zone of high nitrification rates, with increased urea below that depth. Our study showed that bioavailable N was predominantly taken up by the microbial populations in the following order ammonia>urea>nitrate, excluding immediately after Storm Carmel when the preferred utilization was ammonia> nitrate>urea. The in-situ uptake ratio of inorganic C:bioavailable N ranged from 1.5 to 4 while the measured C:N ratio was 8-10. We hypothesize that this discrepancy results from microbial utilization of dissolved organic carbon, not labelled by the spiked sodium bicarbonate, and thus not accounted for. The highest rates of in-situ N uptake were for urea, which emphasises the importance of DON uptake as a source of N in the extreme inorganic N-depleted southeastern Mediterranean.
09:30 AM
DISSOLVED ORGANIC NITROGEN AND CARBON DISTRIBUTIONS AND DYNAMICS ACROSS THE ARCTIC OCEAN POLAR SURFACE LAYER (9408)
Primary Presenter: Claire Fandel, University of Miami (caf185@miami.edu)
The Arctic Ocean is notably deficient in inorganic nutrients, making dissolved organic nitrogen (DON) stand out as the principal reservoir of fixed nitrogen in this environment. However, due to the region’s inaccessibility, our understanding of the distributions and dynamics of DON in the Arctic Ocean remains limited. Here we combine hydrographic sections (GEOTRACES section GN01 and GN04; US GO-SHIP section ARC01; occupied in 2015) for the first trans-Arctic examination of DON in the polar surface layer (PSL). The Transpolar Drift harbors the highest concentrations of DON of all basins, alongside elevated dissolved organic carbon (DOC), meteoric water fractions (i.e., from rivers), and inorganic nutrients. This influence extends into the adjacent Amundsen Basin, where DON and DOC concentrations remain high. Conversely, the Nansen Basin and the Barents Sea, largely influenced by inflowing Atlantic water, exhibit the lowest DON and DOC of all basins. In the Amerasian Basin, we observe a non-conservative behavior between meteoric water and DON, likely due to the extended residence time in the PSL, deepening of the nutricline by the Beaufort Gyre, and influence of seasonal ice melt. This study also reports the first trans-Arctic measurements of δ15N of DON, with values ranging from 0.14‰ to 9.21‰. Overall, these findings underscore the significant role that regional hydrology and biogeochemical processes play in shaping the distributions of DON in the Arctic Ocean and highlight the complexity of Arctic nitrogen dynamics.
09:45 AM
SINGLE CELL NITROGEN AND CARBON UPTAKE RATES OF CENTRAL ARCTIC OCEAN PHYTOPLANKTON (9720)
Primary Presenter: Hanna Farnelid, Linnaeus University (hanna.farnelid@lnu.se)
The central Arctic Ocean is rapidly warming which has led to a sharp reduction in sea ice with increased light penetration, phytoplankton abundance and primary production. While pelagic diatoms (20-200 µm) are known to be important primary producers, increased stratification due to warming is likely to increase the contribution of nanoplankton (2-20 µm) and picophytoplankton (<2 µm) which have an advantage in nutrient acquisition. Such shifts have the potential to reduce overall primary production and carbon (C) export. In this study, we investigated nitrogen (N) uptake preferences in the central Arctic Ocean during summer. Short-term incubation experiments using dual stable isotope labeling (15N and 13C) were performed to measure N assimilation (nitrate, ammonium, urea and amino acids) and C fixation at six locations representing different environmental conditions and phytoplankton community composition. Total community uptake rates were measured with isotope ratio mass spectrometer (IRMS) and single cell rates using secondary-ion mass spectrometry (SIMS) and NanoSIMS to assess the contribution of different size classes of phytoplankton to total uptake. The results show that picophytoplankton have different nutrient acquisition strategies compared to diatoms and dinoflagellates. For inorganic N sources, N-specific uptake rates were generally well correlated with C fixation. While N-specific uptake of amino acids was low, the rates were highest for ammonium illustrating the importance of regenerated nutrients to the success of picophytoplankton in the future Arctic Ocean.
10:00 AM
RECYCLING VS. LOSS OF FIXED NITROGEN IN THE BAY OF BENGAL (9112)
Primary Presenter: Victor Fernandez Juarez, University of Southern Denmark (SDU) (victor.fj@biology.sdu.dk)
The Bay of Bengal (BoB) harbors one of four major oceanic oxygen minimum zones (OMZs). While other OMZs act as sinks for marine fixed nitrogen (N) via N2 production, N loss in BoB is minimal. A previous study in the northern BoB found that N loss was limited by nitrite (NO₂⁻) availability due to efficient NO₂⁻ oxidation at nanomolar O2 levels. Here, we expand this analysis to the southern half of BoB. In April/May 2024, aboard the R/V Sonne (SO305 BIOCAT), we sampled 8 stations, spanning 7°N to 15°N and 84°E to 89°E, revealing a south-to-north decline in O2 levels that reached <1 µM north of 13°N. Microbial nitrogen turnover was evaluated using ¹⁵N-labeled substrates to assess rates of anammox, denitrification, nitrate (NO3⁻) reduction, and NO₂⁻ oxidation. Our findings indicate that BoB contains "hotspots" with significant N loss, primarily driven by anammox. Peak anammox rates reached 8 nM N d⁻¹ in incubations with ¹⁵NO₂⁻, coinciding with the detection of a secondary NO₂⁻ maximum (0.35 µM). At most stations, however, nitrogen recycling processes outpaced N loss. While NO3⁻ reduction was detected throughout the transect, the resulting NO₂⁻ was rapidly consumed by nitrite oxidation, which hence outcompeted N2-producing anammox bacteria. Ongoing meta-genomics analyses will further complement these results. In conclusion, this study offers one of the most comprehensive assessments of nitrogen cycling in the BoB, confirming the potential for N loss at nanomolar O2 yet highlighting NO2- accumulation as critical for changing the balance between recycling and loss of fixed N.
10:15 AM
Physiological response of small eukaryotic phytoplankton to differing nitrogen regimes in the North Pacific Subtropical Gyre (9050)
Primary Presenter: Raquel Flynn, University of North Carolina at Chapel Hill (raqflynn@unc.edu)
In the North Pacific Subtropical Gyre, the physical supply of nutrients to the mixed layer (ML) is relatively small, however, small injects of NO3- into the euphotic zone have been shown to stimulate eukaryotic phytoplankton blooms and enhance carbon export. To investigate the response of eukaryotic phytoplankton to inputs of new (e.g., NO3-) and recycled (e.g., NH4+) nitrogen (N) sources, we conducted mesocosm experiments using seawater collected from within the ML and at the deep chlorophyll maximum (DCM) during late-summer. Various N regimes were simulated using assorted N amendments (as either NH4+, NO3- or deep seawater), and metatranscriptomic responses of important phytoplankton groups were assayed. The physiological response of the ML and DCM communities differed, with the ML community expressing genes related to N acquisition regardless of N availability while the DCM community upregulated specific genes in response to N inputs. In addition, smaller cells (<5 μm) dominated biomass, primary productivity and generally utilized recycled N (f-ratio of 0.2±0.1) in the ML while larger cells (>5 μm) dominated at the DCM and relied more heavily on new N (f-ratio of 0.4±0.3). The study provides evidence for niche partitioning between the two layers: low-N supply, small cell dominance and constitutive gene expression in the ML contrasts pulsed N supply, large cell dominance and responsive gene expression at the DCM. These results highlight the physiological differences observed between the ML and DCM communities, and how they may respond to changing N supply mechanisms in current and future oceans.
SS18A - Nitrogen Cycling Processes in Aquatic Ecosystems and Associated Food Webs
Description
Time: 9:00 AM
Date: 29/3/2025
Room: W207CD