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1. Low rates of dinitrogen fixation in the eastern tropical South Pacific

   https://doi.org/10.1002/lno.11159  

   Chang, Bonnie X. ; Jayakumar, Amal ; Widner, Brittany ; Bernhardt, Peter ; Mordy, Calvin W. ; Mulholland, Margaret R. ; Ward, Bess B. ( March 2019 , Limnology and Oceanography)

   Recent work has suggested that the oxygen deficient zone (ODZ) and overlying surface waters of the eastern tropical South Pacific (ETSP) is a potential niche for dinitrogen (N₂) fixation. Rates of dinitrogen fixation were measured in the ETSP above and within the ODZ in July 2013 using a modified¹⁵N₂bubble addition method, wherein a bubble was added, mixed, and then removed, and the isotopic enrichment of the dissolved N₂was measured directly for each incubation. N₂fixation rates in the euphotic zone ranged from below detection to 3.9 nmol L⁻¹d⁻¹and were below detection at all depths surveyed within the ODZ. Depth‐integrated rates ranged from below detection to 289.7μmol m⁻²d⁻¹. DNA and RNA of diversenifHgenes were detected at both surface waters and in the ODZ. However, the results of this study suggest that N₂fixation rates were low and contribute little to N cycling in the ETSP.

    

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2. Quantifying N2 fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets

   https://doi.org/10.3389/fmars.2023.1249115  

   Forrer, Heather J. ; Bonnet, Sophie ; Thomas, Rachel K. ; Grosso, Olivier ; Guieu, Cecile ; Knapp, Angela N. ( October 2023 , Frontiers in Marine Science)

   The spatial distribution of marine di-nitrogen (N₂) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived δ¹⁵N budgets, we quantified rates of N₂fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (<300 m) hydrothermal vents located along the arc provide significant dissolved iron to the euphotic zone, stimulating N₂fixation. Here we compare measurements of water column δ¹⁵N_NO3+NO2with sinking particulate δ¹⁵N collected by short-term sediment traps deployed at 170 m and 270 m at stations in close proximity to subsurface hydrothermal activity, and the δ¹⁵N of N₂fixation. Results from the δ¹⁵N budgets yield high geochemically-based N₂fixation rates (282 to 638 µmol N m^-2d^-1) at stations impacted by hydrothermal activity, supporting 64 to 92% of export production in late spring. These results are consistent with contemporaneous¹⁵N₂uptake rate estimates and molecular work describing highTrichodesmiumspp. and other diazotroph abundances associated with elevated N₂fixation rates. Further, the δ¹⁵N of sinking particulate N collected at 1000 m over an annual cycle revealed sinking fluxes peaked in the summer and coincided with the lowest δ¹⁵N, while lower winter sinking fluxes had the highest δ¹⁵N, indicating isotopically distinct N sources supporting export seasonally, and aligning with observations from most other δ¹⁵N budgets in oligotrophic regions. Consequently, the significant regional N₂fixation input to the late spring/summer Western Tropical South Pacific results in the accumulation of low-δ¹⁵N_NO3+NO2in the upper thermocline that works to lower the elevated δ¹⁵N_NO3+NO2generated in the oxygen deficient zones in the Eastern Tropical South Pacific.

    

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3. A coastal N2 fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning

   https://doi.org/10.1002/lno.11727  

   Selden, Corday R. ; Chappell, P. Dreux ; Clayton, Sophie ; Macías‐Tapia, Alfonso ; Bernhardt, Peter W. ; Mulholland, Margaret R. ( April 2021 , Limnology and Oceanography)

   In the North Atlantic Ocean, dinitrogen (N₂) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N₂fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N₂fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the¹⁵N₂bubble release method and used to build empirical models of regional N₂fixation via a random forest machine learning approach. N₂fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N₂fixation rates (> 100 nmol N L⁻¹d⁻¹), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N₂fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

    

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4. Alphaproteobacteria facilitate Trichodesmium community trimethylamine utilization

   https://doi.org/10.1111/1462-2920.15773  

   Conover, Asa E. ; Morando, Michael ; Zhao, Yiming ; Semones, Jacob ; Hutchins, David A. ; Webb, Eric A. ( October 2021 , Environmental Microbiology)

   In the surface waters of the warm oligotrophic ocean, filaments and aggregated colonies of the nitrogen (N)‐fixing cyanobacteriumTrichodesmiumcreate microscale nutrient‐rich oases. These hotspots fuel primary productivity and harbour a diverse consortium of heterotrophs. Interactions with associated microbiota can affect the physiology ofTrichodesmium, often in ways that have been predicted to support its growth. Recently, it was found that trimethylamine (TMA), a globally abundant organic N compound, inhibits N₂fixation in cultures ofTrichodesmiumwithout impairing growth rate, suggesting thatTrichodesmiumcan use TMA as an alternate N source. In this study,¹⁵N‐TMA DNA stable isotope probing (SIP) of aTrichodesmiumenrichment was employed to further investigate TMA metabolism and determine whether TMA‐N is incorporated directly or secondarily via cross‐feeding facilitated by microbial associates. Herein, we identify two members of the marineRoseobacterclade (MRC) of Alphaproteobacteria as the likely metabolizers of TMA and provide genomic evidence that they converted TMA into a more readily available form of N, e.g., ammonium (NH₄⁺), which was subsequently used byTrichodesmiumand the rest of the community. The results implicate microbiome‐mediated carbon (C) and N transformations in modulating N₂fixation and thus highlight the involvement of host‐associated heterotrophs in global biogeochemical cycling.

    

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5. Sinking Trichodesmium fixes nitrogen in the dark ocean

   https://doi.org/10.1038/s41396-022-01289-6  

   Benavides, Mar ; Bonnet, Sophie ; Le Moigne, Frédéric A. C. ; Armin, Gabrielle ; Inomura, Keisuke ; Hallstrøm, Søren ; Riemann, Lasse ; Berman-Frank, Ilana ; Poletti, Emilie ; Garel, Marc ; et al ( July 2022 , The ISME Journal)

   The photosynthetic cyanobacterium Trichodesmium is widely distributed in the surface low latitude ocean where it contributes significantly to N2 fixation and primary productivity. Previous studies found nifH genes and intact Trichodesmium colonies in the sunlight-deprived meso- and bathypelagic layers of the ocean (200–4000 m depth). Yet, the ability of Trichodesmium to fix N2 in the dark ocean has not been explored. We performed 15N2 incubations in sediment traps at 170, 270 and 1000 m at two locations in the South Pacific. Sinking Trichodesmium colonies fixed N2 at similar rates than previously observed in the surface ocean (36–214 fmol N cell−1 d−1). This activity accounted for 40 ± 28% of the bulk N2 fixation rates measured in the traps, indicating that other diazotrophs were also active in the mesopelagic zone. Accordingly, cDNA nifH amplicon sequencing revealed that while Trichodesmium accounted for most of the expressed nifH genes in the traps, other diazotrophs such as Chlorobium and Deltaproteobacteria were also active. Laboratory experiments simulating mesopelagic conditions confirmed that increasing hydrostatic pressure and decreasing temperature reduced but did not completely inhibit N2 fixation in Trichodesmium. Finally, using a cell metabolism model we predict that Trichodesmium uses photosynthesis-derived stored carbon to sustain N2 fixation while sinking into the mesopelagic. We conclude that sinking Trichodesmium provides ammonium, dissolved organic matter and biomass to mesopelagic prokaryotes.

    

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