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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. 

The oceanic dissolved organic phosphorus (DOP) pool is mainly composed of P-esters and, to a lesser extent, equally abundant phosphonate and P-anhydride molecules. In phosphate-limited ocean regions, diazotrophs are thought to rely on DOP compounds as an alternative source of phosphorus (P). While both P-esters and phosphonates effectively promote dinitrogen (N 2 ) fixation, the role of P-anhydrides for diazotrophs is unknown. Here we explore the effect of P-anhydrides on N 2 fixation at two stations with contrasting biogeochemical conditions: one located in the Tonga trench volcanic arc region (“volcano,” with low phosphate and high iron concentrations), and the other in the South Pacific Gyre (“gyre,” with moderate phosphate and low iron). We incubated surface seawater with AMP (P-ester), ATP (P-ester and P-anhydride), or 3polyP (P-anhydride) and determined cell-specific N 2 fixation rates, nifH gene abundance, and transcription in Crocosphaera and Trichodesmium . Trichodesmium did not respond to any DOP compounds added, suggesting that they were not P-limited at the volcano station and were outcompeted by the low iron conditions at the gyre station. Conversely, Crocosphaera were numerous at both stations and their specific N 2 fixation rates were stimulated by AMP at the volcano station and slightly by 3polyP at both stations. Heterotrophic bacteria responded to ATP and 3polyP additions similarly at both stations, despite the contrasting phosphate and iron availability. The use of 3polyP by Crocosphaera and heterotrophic bacteria at both low and moderate phosphate concentrations suggests that this compound, in addition to being a source of P, can be used to acquire energy for which both groups compete. P-anhydrides may thus leverage energy restrictions to diazotrophs in the future stratified and nutrient-impoverished ocean. 

Abstract The unicellular diazotrophic cyanobacterium Crocosphaera contributes significantly to fixed nitrogen inputs in the oligotrophic ocean. In the western tropical South Pacific Ocean (WTSP), these diazotrophs abound thanks to the phosphorus-rich waters provided by the South Equatorial Current, and iron provided aeolian and subsurface volcanic activity. East of the WTSP, the South Pacific Gyre (SPG) harbors the most oligotrophic and transparent waters of the world's oceans, where only heterotrophic diazotrophs have been reported before. Here, in the SPG, we detected unexpected accumulation of Crocosphaera at 50 m with peak abundances of 5.26 × 105 nifH gene copies l–1. The abundance of Crocosphaera at 50 m was in the same order of magnitude as those detected westwards in the WTSP and represented 100% of volumetric N2 fixation rates. This accumulation at 50 m was likely due to a deeper penetration of UV light in the clear waters of the SPG being detrimental for Crocosphaera growth and N2 fixation activity. Nutrient and trace metal addition experiments did not induce any significant changes in N2 fixation or Crocosphaera abundance, indicating that this population was not limited by the resources tested and could develop in high numbers despite the oligotrophic conditions. Our findings indicate that the distribution of Crocosphaera can extend into subtropical gyres and further understanding of their controlling factors is needed. 

Shallow submarine hydrothermal vents can supply the iron needed to fuel phytoplankton blooms. 

Abstract 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. 

Abstract. In marine ecosystems, most physiological, ecological, or physical processes are size dependent. These include metabolic rates, the uptake of carbon and other nutrients, swimming and sinking velocities, and trophic interactions, which eventually determine the stocks of commercial species, as well as biogeochemical cycles and carbon sequestration. As such, broad-scale observations of plankton size distribution are important indicators of the general functioning and state of pelagic ecosystems under anthropogenic pressures. Here, we present the first global datasets of the Pelagic Size Structure database (PSSdb), generated from plankton imaging devices. This release includes the bulk particle normalized biovolume size spectrum (NBSS) and the bulk particle size distribution (PSD), along with their related parameters (slope, intercept, and R2) measured within the epipelagic layer (0–200 m) by three imaging sensors: the Imaging FlowCytobot (IFCB), the Underwater Vision Profiler (UVP), and benchtop scanners. Collectively, these instruments effectively image organisms and detrital material in the 7–10 000 µm size range. A total of 92 472 IFCB samples, 3068 UVP profiles, and 2411 scans passed our quality control and were standardized to produce consistent instrument-specific size spectra averaged to 1° × 1° latitude and longitude and by year and month. Our instrument-specific datasets span most major ocean basins, except for the IFCB datasets we have ingested, which were exclusively collected in northern latitudes, and cover decadal time periods (2013–2022 for IFCB, 2008–2021 for UVP, and 1996–2022 for scanners), allowing for a further assessment of the pelagic size spectrum in space and time. The datasets that constitute PSSdb's first release are available at https://doi.org/10.5281/zenodo.11050013 (Dugenne et al., 2024b). In addition, future updates to these data products can be accessed at https://doi.org/10.5281/zenodo.7998799. 

ABSTRACT Mixotrophy, the combination of heterotrophic and autotrophic nutrition modes, is emerging as the rule rather than the exception in marine photosynthetic plankton. Trichodesmium, a prominent diazotroph ubiquitous in the (sub)tropical oceans, is generally considered to obtain energy via autotrophy. While the ability of Trichodesmium to use dissolved organic phosphorus when deprived of inorganic phosphorus sources is well known, the extent to which this important cyanobacterium may benefit from other dissolved organic matter (DOM) resources is unknown. Here we provide evidence of carbon-, nitrogen- and phosphorus-rich DOM molecules enhancing N2 fixation rates and nifH gene expression in natural Trichodesmium colonies collected at two stations in the western tropical South Pacific. Sampling at a third station located in the oligotrophic South Pacific Gyre revealed no Trichodesmium but showed presence of UCYN-B, although no nifH expression was detected. Our results suggest that Trichodesmium behaves mixotrophically in response to certain environmental conditions, providing them with metabolic plasticity and adding up to the view that mixotrophy is widespread among marine microbes. 

Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N₂) fixation. Here, as in other marine environments across the oceans, N₂ fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N₂ fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N₂ fixation in the WTSP, we measured N₂ fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N₂ fixation in the aphotic ocean. N₂ fixation rates were low (average 0.63±0.07nmolNL⁻¹d⁻¹) but consistently detected across all depths and stations, representing ∼ 6–88% of photic N₂ fixation. N₂ fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N₂ fixation activity at all depths sampled during our study suggest that aphotic N₂ fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.