Abstract
Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles.
Highlights
Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate
Low temperature, and high bioavailable nitrogen have traditionally been assumed to limit N2 fixation, recent studies have reported significant diazotrophic activities in darker, colder, and more nitrogen-rich environments, thereby broadening the putative biogeography of marine N2 fixation[12,13,14,15,16]. The discovery of these new niches has been accompanied by increasing appreciation for the large diversity of species fixing N2, ranging from the well-known Trichodesmium and diatom-diazotroph associations (DDA) to more recentlyrecognized unicellular cyanobacteria[17] and non-cyanobacterial diazotrophs[18]
When integrated over 24-h N2 fixation diel cycles, continuous estimates of daily surface N2 fixation rates (≤0.19–97.6 nmol N L−1 d−1) were in line with discrete N2 fixation rates concurrently determined by 15N2 incubations (n = 7, r = 0.97, p < 0.01, Fig. 1b and Methods)
Summary
Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. The limited number of observations impede our ability to close regional marine nitrogen budgets, scale estimates globally, and identify factors controlling N2 fixation[10]. While our field observations identify phosphorus, P*, and [Chl] as predictors of spatial variations in N2 fixation in our study area, a meta-analysis of published results shows that none of the putative regulating factors of N2 fixation can satisfactorily explain variations in volumetric rates globally (Supplementary Fig. 6).
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