Insights into nitrogen fixation below the euphotic zone: trials in an oligotrophic marginal sea and global compilation (original) (raw)
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N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)
Biogeosciences, 2018
We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (February-March 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N 2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N 2 fixation rates were extremely high at both LD A (593 ± 51 µmol N m −2 d −1) and LD B (706 ± 302 µmol N m −2 d −1), and the diazotroph community was dominated by Trichodesmium. N 2 fixation rates were lower (59 ± 16 µmol N m −2 d −1) at LD C, and the diazotroph community was dominated by unicellular N 2-fixing cyanobacteria (UCYN). At all stations, N 2 fixation was the major source of new N (> 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N 2 fixation contributed circa 13-18 % of primary production in the MA region and 3 % in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratio = particulate carbon export / primary production) was maximum at LD A (9.7 %) and was higher than the e ratio in most studied oligotrophic regions (< 5 %), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 % of the PC export at LD A, while their contribution was 5 and < 0.1 % at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper Published by Copernicus Publications on behalf of the European Geosciences Union. 2566 M. Caffin et al.: N 2 fixation as a dominant new N source layer appears as a characteristic of the WTSP during the summer season.
The western tropical South Pacific (WTSP) Ocean has been recognized as a global hotspot of dinitrogen 25 (N 2) fixation. Here, as in other marine environments across the oceans, N 2 fixation studies have focused in the sunlit layer. However, studies have confirmed the importance of aphotic N 2 fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N 2 fixation in the WTSP, here we measure N 2 fixation rates and identify diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French 30 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 Mass Spectrometry (FTICRMS). N 2 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 N 2 fixation. N 2 fixation rates were not significantly correlated to DOM compounds. The analysis of nifH gene 35
New estimation of N 2 fixation in the western and central Pacific Ocean and its marginal seas
Global Biogeochemical Cycles, 2010
1] The distribution of N 2 fixation was examined using a 15 N 2 tracer with accompanying measurements of abundance of Trichodesmium spp. and Richelia intracellularis, nitrate plus nitrite (N+N) and soluble reactive phosphorus at the nanomolar level, and primary production in the western and central Pacific Ocean. N 2 fixation occurred only in >∼20°C oligotrophic (i.e., N+N < 100 nM) waters except at a station in the equatorial upwelling zone where N+N was 1880 nM. High N 2 fixation rates were observed in the Kuroshio and East China Sea (KECS) and near Fiji and other isolated islands with concomitant high abundance of Trichodesmium spp. In contrast, N 2 fixation in the western and central oligotrophic North Pacific (WCONP) was significantly lower, and Trichodesmium spp. were rarely observed. These observations hint that KECS and waters around isolated islands are N 2 fixation "hot spots" because of the occurrence of Trichodesmium spp. The average N 2 fixation rate in the KECS of 232 ± 54.8 (±SE, n = 13) mmol N m −2 d −1 was almost 1 order of magnitude higher than that in the WCONP of 39.2 ± 7.51 (n = 26) mmol N m −2 d −1 . On the basis of these estimates and reported values obtained using 15 N 2 , depth-integrated N 2 fixation in the North Pacific was estimated to be 2.6 ± 0.3 × 109 (n = 63) mol N d −1 , which is less than half of previous estimates. This difference was ascribed primarily to the unavailability of N 2 fixation rates in the WCONP, which occupies a vast area of the subtropical North Pacific, and the use of data obtained in the hot spots which represent small areas that likely led to the previous overestimation.
Aphotic N2 Fixation in the Eastern Tropical South Pacific Ocean
PLoS ONE, 2013
We examined rates of N 2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niñ o (2010) and La Niñ a (2011). Replicated vertical profiles performed under oxygen-free conditions show that N 2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 mmol N m 22 d 21 in 2010 and 24614 to 118687 mmol N m 22 d 21 in 2011.
Global Marine N2 Fixation Estimates: From Observations to Models
Frontiers in Microbiology, 2018
Fixed nitrogen (N) limits productivity across much of the low-latitude ocean. The magnitude of its inventory results from the balance of N input and N loss, the latter largely occurring in regionally well-defined low-oxygen waters and sediments (denitrification and anammox). The rate and distribution of N input by biotic N 2 fixation, the dominant N source, is not well known. Here we compile N 2 fixation estimates from experimental measurements, tracer-based geochemical and modeling approaches, and discuss their limitations and uncertainties. The lack of adequate experimental data coverage and the insufficient understanding of the controls of marine N 2 fixation result in high uncertainties, which make the assessment of the current N-balance a challenge. We suggest that a more comprehensive understanding of the environmental and ecological interaction of marine N 2 fixers is required to advance the field toward robust N 2 fixation rates estimates and predictions.
The conundrum of marine N2 fixation
American Journal of Science, 2005
Over the last 30 years, immense progress has been made in understanding the global significance of marine nitrogen (N 2 ) fixation. Development of enzymological, isotopic and molecular techniques for identifying N 2 fixers and quantifying N 2 fixation rates, as well as more frequent and extensive field campaigns have fuelled such progress. Ship-based and laboratory experiments have revealed a large suite of previously unknown physiological characteristics of diazotrophs. More recently, geochemical estimates of N 2 fixation, based upon N and phosphorus (P) stoichiometry, stable N isotope studies and carbon (C) anomalies in nutrient starved regions of the ocean, have provided basin and global-scale estimates of N 2 fixation. While these achievements have revolutionized our understanding of the role N 2 fixers play in global marine N and C cycles, there remain fundamental challenges in the study of N 2 fixation. We here summarize the advances made and highlight the conundrums that remain regarding the basin and global scale quantification of N 2 fixation, as well as the climatological, physical and biological factors that enable or constrain the distribution and growth of diazotrophs.