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1. A global dataset of nitrogen fixation rates across inland and coastal waters

   https://doi.org/10.1002/lol2.10459  

   Fulweiler, Robinson W; Berberich, Megan E; Rinehart, Shelby A; Taylor, Jason M; Kelly, Michelle C; Ray, Nicholas E; Oczkowski, Autumn; Balint, Sawyer J; Geisser, Alexandra H; Mahoney, Catherine R; et al (January 2025, Limnology and Oceanography Letters)

   Biological nitrogen fixation is the conversion of dinitrogen (N2) gas into bioavailable nitrogen by microorganisms with consequences for primary production, ecosystem function, and global climate. Here we present a compiled dataset of 4793 nitrogen fixation (N2-fixation) rates measured in the water column and benthos of inland and coastal systems via the acetylene reduction assay, 15N2 labeling, or N2/Ar technique. While the data are distributed across seven continents, most observations (88%) are from the northern hemisphere. 15N2 labeling accounted for 67% of water column measurements, while the acetylene reduction assay accounted for 81% of benthic N2-fixation observations. Dataset median area-, volume-, and mass-normalized N2-fixation rates are 7.1 μmol N2-N m−2 h−1, 2.3 × 10−4 μmol N2-N L−1 h−1, and 4.8 × 10−4 μmol N2-N g−1 h−1, respectively. This dataset will facilitate future efforts to study and scale N2-fixation contributions across inland and coastal aquatic environments. 

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2. Global dataset of nitrogen fixation rates across inland and coastal waters based on a coordinated synthesis effort

   https://doi.org/10.6073/pasta/dd33573628ac1f61204a9a77a1feef9e  

   Fulweiler, Robinson W; Berberich, Megan E; Rinehart, Shelby A; Taylor, Jason M; Kelly, Michelle Catherine; Ray, Nicholas E; Oczkowski, Autumn; Benavides, Mar; Church, Matthew J; Loeks, Brianna; et al (January 2024, Environmental Data Initiative)

   Biological nitrogen fixation converts inert di-nitrogen gas into bioavailable nitrogen and can be an important source of bioavailable nitrogen to organisms. This dataset synthesizes the aquatic nitrogen fixation rate measurements across inland and coastal waters. Data were derived from papers and datasets published by April 2022 and include rates measured using the acetylene reduction assay (ARA), 15N2 labeling, or the N2/Ar technique. The dataset is comprised of 4793 nitrogen fixation rates measurements from 267 studies, and is structured into four tables: 1) a reference table with sources from which data were extracted, 2) a rates table with nitrogen fixation rates that includes habitat, substrate, geographic coordinates, and method of measuring N2 fixation rates, 3) a table with supporting environmental and chemical data for a subset of the rate measurements when data were available, and 4) a data dictionary with definitions for each variable in each data table. This dataset was compiled and curated by the NSF-funded Aquatic Nitrogen Fixation Research Coordination Network (award number 2015825). 

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3. Open ocean and coastal strains of the N2-fixing cyanobacterium UCYN-A have distinct transcriptomes

   https://doi.org/10.1371/journal.pone.0272674  

   Muñoz-Marín, María_del Carmen; Magasin, Jonathan D; Zehr, Jonathan P (May 2023, PLOS ONE)

   Kothe, Erika (Ed.)

   Decades of research on marine N₂fixation focused onTrichodesmium, which are generally free-living cyanobacteria, but in recent years the endosymbiotic cyanobacteriumCandidatusAtelocyanobacterium thalassa (UCYN-A) has received increasing attention. However, few studies have shed light on the influence of the host versus the habitat on UCYN-A N₂fixation and overall metabolism. Here we compared transcriptomes from natural populations of UCYN-A from oligotrophic open-ocean versus nutrient-rich coastal waters, using a microarray that targets the full genomes of UCYN-A1 and UCYN-A2 and known genes for UCYN-A3. We found that UCYN-A2, usually regarded as adapted to coastal environments, was transcriptionally very active in the open ocean and appeared to be less impacted by habitat change than UCYN-A1. Moreover, for genes with 24 h periodic expression we observed strong but inverse correlations among UCYN-A1, A2, and A3 to oxygen and chlorophyll, which suggests distinct host-symbiont relationships. Across habitats and sublineages, genes for N₂fixation and energy production had high transcript levels, and, intriguingly, were among the minority of genes that kept the same schedule of diel expression. This might indicate different regulatory mechanisms for genes that are critical to the symbiosis for the exchange of nitrogen for carbon from the host. Our results underscore the importance of N₂fixation in UCYN-A symbioses across habitats, with consequences for community interactions and global biogeochemical cycles. 

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4. The Angola Gyre is a hotspot of dinitrogen fixation in the South Atlantic Ocean

   https://doi.org/10.1038/s43247-022-00474-x  

   Marshall, Tanya; Granger, Julie; Casciotti, Karen L.; Dähnke, Kirstin; Emeis, Kay-Christian; Marconi, Dario; McIlvin, Matthew R.; Noble, Abigail E.; Saito, Mak A.; Sigman, Daniel M.; et al (June 2022, Communications Earth & Environment)

   Abstract Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate estimates vary five-fold, with models attributing most dinitrogen fixation to the western basin. From hydrographic properties and nitrate isotope ratios, we show that the Angola Gyre in the eastern tropical South Atlantic supports the fixation of 1.4–5.4 Tg N.a⁻¹, 28-108% of the existing (highly uncertain) estimates for the basin. Our observations contradict model diagnoses, revealing a substantial input of newly-fixed nitrogen to the tropical eastern basin and no dinitrogen fixation west of 7.5˚W. We propose that dinitrogen fixation in the South Atlantic occurs in hotspots controlled by the overlapping biogeography of excess phosphorus relative to nitrogen and bioavailable iron from margin sediments. Similar conditions may promote dinitrogen fixation in analogous ocean regions. Our analysis suggests that local iron availability causes the phosphorus-driven coupling of oceanic dinitrogen fixation to nitrogen loss to vary on a regional basis. 

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5. Nutrient-Colimited Trichodesmium as a Nitrogen Source or Sink in a Future Ocean

   https://doi.org/10.1128/AEM.02137-17  

   Walworth, Nathan G.; Fu, Fei-Xue; Lee, Michael D.; Cai, Xiaoni; Saito, Mak A.; Webb, Eric A.; Hutchins, David A.; Liu, Shuang-Jiang (February 2018, Applied and Environmental Microbiology)

   ABSTRACT Nitrogen-fixing (N 2 ) cyanobacteria provide bioavailable nitrogen to vast ocean regions but are in turn limited by iron (Fe) and/or phosphorus (P), which may force them to employ alternative nitrogen acquisition strategies. The adaptive responses of nitrogen fixers to global-change drivers under nutrient-limited conditions could profoundly alter the current ocean nitrogen and carbon cycles. Here, we show that the globally important N 2 fixer Trichodesmium fundamentally shifts nitrogen metabolism toward organic-nitrogen scavenging following long-term high-CO 2 adaptation under iron and/or phosphorus (co)limitation. Global shifts in transcripts and proteins under high-CO 2 /Fe-limited and/or P-limited conditions include decreases in the N 2 -fixing nitrogenase enzyme, coupled with major increases in enzymes that oxidize trimethylamine (TMA). TMA is an abundant, biogeochemically important organic nitrogen compound that supports rapid Trichodesmium growth while inhibiting N 2 fixation. In a future high-CO 2 ocean, this whole-cell energetic reallocation toward organic nitrogen scavenging and away from N 2 fixation may reduce new-nitrogen inputs by Trichodesmium while simultaneously depleting the scarce fixed-nitrogen supplies of nitrogen-limited open-ocean ecosystems. IMPORTANCE Trichodesmium is among the most biogeochemically significant microorganisms in the ocean, since it supplies up to 50% of the new nitrogen supporting open-ocean food webs. We used Trichodesmium cultures adapted to high-CO 2 conditions for 7 years, followed by additional exposure to iron and/or phosphorus (co)limitation. We show that “future ocean” conditions of high CO 2 and concurrent nutrient limitation(s) fundamentally shift nitrogen metabolism away from nitrogen fixation and instead toward upregulation of organic nitrogen-scavenging pathways. We show that the responses of Trichodesmium to projected future ocean conditions include decreases in the nitrogen-fixing nitrogenase enzymes coupled with major increases in enzymes that oxidize the abundant organic nitrogen source trimethylamine (TMA). Such a shift toward organic nitrogen uptake and away from nitrogen fixation may substantially reduce new-nitrogen inputs by Trichodesmium to the rest of the microbial community in the future high-CO 2 ocean, with potential global implications for ocean carbon and nitrogen cycling. 

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