Abstract
AbstractThe Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (<20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.
Highlights
One of the major sources of reactive N (Nr) to the global ocean is dinitrogen (N2) fixation (Gruber & Galloway, 2008), the assimilation of N2 gas into biomass
Our findings suggest that diazotrophs inhabit the Eastern Tropical North Pacific (ETNP) region and fix N2 at elevated rates in euphotic waters near the coast but are largely inactive in O2 minimum zones (OMZs) and oxygen deficient zones (ODZs) waters, as well as in the water column below, likely due to energy or C limitation at the OMZ depth horizon
Our observation of high rates (>500 μmol N·m−2·day−1) at inshore stations supports the growing body of evidence that ocean margins contribute a greater amount of newly fixed N than previously thought (Mulholland et al, 2019; Tang et al, 2019)
Summary
One of the major sources of reactive N (Nr) to the global ocean is dinitrogen (N2) fixation (Gruber & Galloway, 2008), the assimilation of N2 gas into biomass. Despite the abundance of N2 in marine systems, only selected prokaryotic “N2 fixers” (diazotrophs) have the genetic capacity to mediate its intracellular reduction to ammonia, which can be assimilated via common metabolic pathways (Berges & Mulholland, 2008). Where present, these organisms can increase the Nr pool and stimulate primary production in N‐limited ocean regions, thereby enhancing atmospheric drawdown of carbon dioxide (CO2) and, potentially, export of this carbon (C) through the biological pump (e.g., Karl et al, 2012). The sensitivities and physiological ranges of these groups likely differ from those of long‐cultured and well‐studied cyanobacterial diazotrophs like Trichodesmium, complicating our understanding of the environmental factors that regulate the magnitude and distribution of N2 fixation rates (NFR) in the ocean
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