Low rates of nitrogen fixation in eastern tropical South Pacific surface waters

Abstract

An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N 2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N 2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N 2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 μmol N m −2 d −1 based on sediment trap fluxes. Whereas the fractional importance of N 2 fixation did not change, the N 2 -fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N 2 -fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N 2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N 2 -fixation rates were higher, up to 100 μmol N m, and detected N 2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N 2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N 2 fixation in the ETSP.nitrogen fixation | eastern tropical south Pacific | nitrogen budgets | nitrate | nitrogen isotopes

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

“…The BNF activity detected in this study is a further evidence that high nitrate concentrations (and high N:P ratios) do not inhibit BNF activity, such as it was also observed in other upwelling regions (e.g., Voss et al, 2004;Sohm et al, 2011;Subramaniam et al, 2013;Benavides et al, 2014). Indeed, recent studies demonstrated that BNF can occur upon exposure to elevated DIN (nitrate and/or ammonium), as long as phosphate is available (Knapp, 2012), and point toward iron as the primary control factor of BNF (Knapp et al, 2016;Bonnet et al, 2017). Thus, the sensitivity of marine BNF to high DIN is not clear.…”

Diazotrophs: Overlooked Key Players within the Coral Symbiosis and Tropical Reef Ecosystems?

“…Rates of N 2 fixation can either be expressed indirectly as ethylene (C 2 H 4 ) produced from acetylene (C 2 H 2 ) using the acetylene reduction assay (ARA), or directly as the amount of nitrogen assimilated using 15 N 2 tracer experiments (see Box 1). The lowest rates have been recorded in the water column (Capone, 2001;Foster et al, 2009;Knapp et al, 2016), compared to benthic environments such as reef sediments and hard substrates which often present high N 2 fixation activities (Davey et al, 2008;O'Neil and Capone, 2008;Bednarz et al, 2015b;Rix et al, 2015). Epilithic communities recovering coral rubbles or dead coral skeletons can indeed fix up to 12 and 27 nmol N cm −2 h −1 , respectively (Davey et al, 2008).…”

Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets

“…More recently, the Eastern Tropical South Pacific (ETSP) has seen increased sampling due to nutrient distribution-based modelling predictions that the highest global N 2 fixation rates would be found in surface waters above 25 and adjacent to oxygen deficient zones (ODZs), where significant phosphorus (P) would be available to support N 2 fixation (Deutsch et al, 2007). However, field campaigns have found exceedingly low rates of N 2 fixation in the ETSP gyre (Knapp et al, 2016a;Raimbault and Garcia, 2008 2008), which have been attributed to limited iron availability (Dekaezemacker et al, 2013).…”

“…Multiplying the fraction of export production supported by N 2 fixation (x) by the PN sink mass flux provides a time-integrated N 2 fixation rate that can be compared with 15 N 2 incubation-based N 2 fixation rate measurements (Knapp et al, 2016a). Here it is hypothesized that both rates of N 2 fixation and its 10 importance for fuelling export production will be higher at stations in the western vs. central and eastern regions of the WTSP because of their closer proximity to iron sources (Guieu et al, under review).…”