Revisiting the distribution of oceanic N2 fixation and estimating diazotrophic contribution to marine production

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.

Fig. 1

N₂ fixation rates (NF) measured and calculated in August 2015 and 2016 over the western North Atlantic Ocean. a In situ hourly surface volumetric N₂ fixation rates determined by the continuous underway incubation method, overlaid on chlorophyll-a concentrations measured by the MODIS satellite during the respective cruise periods (Aug 3–12 in both 2015 and 2016). Note that chlorophyll-a concentrations are shown on a logarithmic scale. b Calculated daily surface volumetric N₂ fixation rates overlaid on the August climatology of surface excess phosphate P* $\left({\mathrm{P}}^{*}=\left[{\mathrm{PO}}_4^{3-}\right]-\left[{\mathrm{NO}}_3^{-}\right]∕16\right)$. Nutrient data were obtained from the World Ocean Atlas 2013 version 2. Red vertical bars represent N₂ fixation rates determined by the discrete dissolved ¹⁵N₂ incubation method during the 2015 cruise. c Estimated depth-integrated N₂ fixation rates overlaid on the August climatology of subsurface excess nitrogen N* $\left({\mathrm{N}}^{*}=\left[{\mathrm{NO}}_3^{-}\right]-\left[{\mathrm{PO}}_4^{3-}\right]\times 16\right)$

Fig. 2

Absolute rRNA gene abundances of various diazotrophs and their potential hosts in surface seawater collected during the 2015 cruise. The inset map shows the locations where molecular samples were collected. The distributions of diazotrophs (a) and hosts (b) are compared with surface daily N₂ fixation rates (solid red line). Diazotrophs and hosts below detection limits at different sampling locations are not shown in the figure. “Others” in panel a includes diazotrophic proteobacteria belonging to Bradyrhizobium, Mesorhizobium, Novosphingbium, and Paenibacillus

Fig. 3

Distribution of Net Community Production (NCP) and contribution of N₂ fixation to NCP. a Color-coded NCP with negative values shown in grey color scale. b Distribution of the ratio of N₂ fixation-supported carbon production to NCP. Circle size in b represents the magnitude of NCP, contextualizing the large uncertainty in calculated ratios when the denominator (i.e., NCP) is small

Fig. 4

Histograms comparing N₂ fixation rates in open and coastal oceans. a The magnitude of depth-integrated coastal N₂ fixation rates (red) is slightly larger than N₂ fixation rates in the open oceans (blue). Volumetric N₂ fixation rates at all depths (b) and at the surface (c) are generally higher in coastal regions than in the open ocean. d Integration depths used to calculate the depth-integrated N₂ fixation rates from the volumetric N₂ fixation rates. Geometric means and geometric standard deviations are shown in the legends