Nitrogen fixation in the South Atlantic Gyre and the Benguela Upwelling System

Abstract

Dinitrogen (N2) fixation is recognized as an important input of new nitrogen (N) to the open ocean gyres, contributing to the export of organic matter from surface waters. However, very little N2‐fixation research has focused on the South Atlantic Gyre, where dust deposition of iron (Fe), an important micronutrient for diazotrophs, is seasonally low. Recent modeling efforts suggest that N2‐fixation may in fact be closely coupled to, and greatest in, areas of denitrification, as opposed to the oceanic gyres. One of these areas, the Benguela Upwelling System, lies to the east of the South Atlantic Gyre. In this study we show that N2‐fixation in surface waters across the South Atlantic Gyre was low overall (<1.5 nmol N l−1 d−1) with highest rates seen in or near the Benguela Upwelling System (up to ∼8 nmol N l−1 d−1). Surface water dissolved Fe (dFe) concentrations were very low in the gyre (∼0.3 nM or lower), while soluble reactive phosphorus (SRP) concentrations were relatively high (∼0.15 μM). N2‐fixation rates across the entire sampling area were significantly positively correlated to dFe, but also to SRP and NO3−. Thus, high NO3− concentrations did not exclude N2‐fixation in the upwelling region, which provides evidence that N2‐fixation may be occurring in previously unrecognized waters, specifically near denitrification zones. However the gene encoding for a nitrogenase component (nifH) was not detected from known diazotrophs at some stations in or near the upwelling where N2‐fixation was greatest, suggesting the presence of unknown diazotrophs in these waters.

Global distribution and vertical patterns of a prymnesiophyte–cyanobacteria obligate symbiosis

“…Our CARD-FISH observations expanded the geographic coverage of UCYN-A1 in symbiosis to the South Atlantic gyre and the southern Indian Ocean. In these regions, no records of the prymnesiophyte phylotypes or UCYN-A were documented previously, except occasional detections of UCYN-A nifH sequences (no clade distinction) in the Benguela upwelling system, in the Arabian Sea and near Madagascar (Mazard et al, 2004;Moisander et al, 2010;Sohm et al, 2011a). Moreover, our metabarcoding analysis represented the first detection of both hosts in the Red Sea and widened the presence of the UCYN-A2 host in the South Pacific, the Arabian Sea and the Mediterranean Sea, where only few nifH clones of UCYN-A2 were reported (Man-Aharonovich et al, 2007).…”

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

“…Recent studies demonstrate the presence and activity of diazotrophs in DIN-enriched environments such as the eastern tropical North Atlantic (Voss et al, 2004), the western English Channel (Rees et al, 2009), the Mekong River plume in the South China Sea (Grosse et al, 2010), the temperate NE American coast (Mulholland et al, 2012), and mesohaline temperate estuaries between the Baltic Sea and the North Sea (Bentzon-Tilia et al, 2015). Evidences of BNF have also been reported in upwelling systems such as the eastern equatorial Atlantic (Voss et al, 2004;Foster et al, 2009;Subramaniam et al, 2013), the Benguela upwelling system (Sohm et al, 2011;Benavides et al, 2014), and the region off central Chile (Raimbault and Garcia, 2008;.…”

Trichodesmium genome maintains abundant, widespread noncoding DNA in situ, despite oligotrophic lifestyle

“…marine microbiology | oligotrophic | evolution genomics | nitrogen fixation T he low availability of N (and fixed carbon) in the midlatitude upper oceans provides an important niche for autotrophic organisms that can fix atmospheric nitrogen, which can exert control over global primary production (1)(2)(3). Nitrogen fixation is a prokaryotic process with a high-energy demand, and oceanic cyanobacteria are known to be significant sources of this "new" nitrogen (nitrogen that is fixed from the atmosphere or NO 3 advected from depth) (4,5). Molecular field data have shown that a handful of cyanobacterial diazotrophs responsible for oligotrophic nitrogen fixation can reach relatively high cell numbers (6)(7)(8)(9)(10)(11)(12) and be of significant biogeochemical importance (13)(14)(15)(16).…”