Abstract
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 ± 0.07 nmol N L−1 d−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 650 dbar, 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.
Highlights
Pelagic N2 fixation is considered the greatest input of fixed nitrogen to the oceans, adding up to ∼ 100–107 Tg N per year (Galloway et al, 2004; Codispoti, 2007; Gruber and Galloway, 2008; Jickells et al, 2017)
All variables show a clear divide between the Melanesian Archipelago waters (MA; stations SD1 to SD12) and the South Pacific Gyre (GY; eastwards of SD12; Moutin et al, 2017)
The aphotic N2 fixation activity measured in the western tropical South Pacific (WTSP) was low but consistently detected across all depths and stations (Fig. 1), representing on average 13 and 51 % of photic N2 fixation in the MA and GY waters, respectively (Bonnet et al, 2018), or 6–88 % of overall photic N2 fixation across the whole OUTPACE transect
Summary
Pelagic N2 fixation is considered the greatest input of fixed nitrogen to the oceans, adding up to ∼ 100–107 Tg N per year (Galloway et al, 2004; Codispoti, 2007; Gruber and Galloway, 2008; Jickells et al, 2017). In the sunlit layer of the warm oligotrophic tropical and subtropical oceans, cyanobacterial diazotrophs such as Trichodesmium, UCYNB and diatom–diazotroph associations (DDAs) dominate fixed nitrogen inputs via N2 fixation (Zehr, 2011). In colder and less oligotrophic waters at higher latitudes, other diazotrophs including UCYN-A and non-cyanobacterial groups may be more competitive (Moisander et al, 2010, 2014; Bonnet et al, 2015; Langlois et al, 2015), considerably expanding the latitudinal range over which N2 fixation is considered significant in predictive biogeochemical models. Due to the immense volume of the dark ocean, aphotic N2 fixation could influence the global nitrogen budget substantially. The number of published aphotic N2 fixation rates is scant and our understanding of the metabolism and ecology of aphotic diazotrophs is still limited, hindering our ability to evaluate their impact on global fixed nitrogen inputs (Moisander et al, 2017)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
