Diversity and activity of nitrogen‐fixing communities across ocean basins

Gradoville, Mary R.; Bombar, Deniz; Crump, Byron C.; Letelier, Ricardo M.; Zehr, Jonathan P.; White, Angelicque E.

doi:10.1002/lno.10542

Cited by 100 publications

(113 citation statements)

References 67 publications

(134 reference statements)

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“…The samples were analyzed by continuous-flow isotope ratio mass spectrometry (IRMS) using an Integra2 Analyser (Integra CN), calibrated with International Atomic Energy Agency standards IAEA 310-A for nitrogen (47.2‰) and IAEA 303-B for carbon (466‰). The minimum quantifiable N 2 fixation rate was 0.0041 nmol N L −1 d −1 , as determined by error propagation analysis (Gradoville et al, 2017). We note that the use of the traditional 15 N 2 "bubble method" (Montoya et al, 1996) instead of the "dissolved method" (Mohr et al, 2010) may have resulted in an underestimation of the actual rates (Grokopf et al, 2012), although the long incubation time (24 h) likely reduces this possibility.…”

Section: N 2 and Carbon Fixation Ratesmentioning

confidence: 80%

Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

et al. 2018

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Specialized prokaryotes performing biological dinitrogen (N 2 ) fixation ("diazotrophs") provide an important source of fixed nitrogen in oligotrophic marine ecosystems such as tropical and subtropical oceans. In these waters, cyanobacterial photosynthetic diazotrophs are well known to be abundant and active, yet the role and contribution of non-cyanobacterial diazotrophs are currently unclear. The latter are not photosynthetic (here called "heterotrophic") and hence require external sources of organic matter to sustain N 2 fixation. Here we added the photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to estimate the N 2 fixation potential of heterotrophic diazotrophs as compared to autotrophic ones. Additionally, we explored the influence of dissolved organic matter (DOM) on these diazotrophs along a coast to open ocean gradient in the surface waters of a subtropical coral lagoon (New Caledonia). Total N 2 fixation (samples not amended with DCMU) ranged from 0.66 to 1.32 nmol N L −1 d −1 . The addition of DCMU reduced N 2 fixation by >90%, suggesting that the contribution of heterotrophic diazotrophs to overall N 2 fixation activity was minor in this environment. Higher contribution of heterotrophic diazotrophs occurred in stations closer to the shore and coincided with the decreasing lability of DOM, as shown by various colored DOM and fluorescent DOM (CDOM and FDOM) indices. We tested the response of diazotrophs (in terms of nifH gene expression and bulk N 2 fixation rates) upon the addition of a mix of carbohydrates ("DOC" treatment), amino acids ("DON" treatment), and phosphonates and phosphomonesters ("DOP" treatment). While nifH expression increased significantly in Trichodesmium exposed to the DOC treatment, bulk N 2 fixation rates increased significantly only in the DOP treatment. The lack of nifH expression by gammaproteobacteria, in any of the DOM addition treatments applied, questions the contribution of non-cyanobacterial diazotrophs to fixed nitrogen inputs in the New Caledonian lagoon. While the metabolism and ecology of heterotrophic diazotrophs is currently elusive, a deeper understanding of their ecology and relationship with DOM is needed in the light of increased DOM inputs in coastal zones due to anthropogenic pressure.

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Section: N 2 and Carbon Fixation Ratesmentioning

confidence: 80%

Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

et al. 2018

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“…In addition, the 15 N enrichment of the ambient (unlabelled) PN was measured in one replicate at each station at the DCM and the subsurface and was used as the "initial" 15 N enrichment, as termed in Montoya et al (1996). The minimum quantifiable rate calculated using standard propagation of errors via the observed variability between replicate samples measured according to Gradoville et al (2017) was 0.035 nmol N L −1 d −1 . Integrated N 2 fixation rates over the studied layer were calculated by using the same method as for the iN-PP.…”

Section: Nitrogen Fixation Ratesmentioning

confidence: 99%

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

et al. 2018

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Abstract. 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 (FebruaryMarch 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.

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“…Diazotrophic organisms in this ecosystem range from unicellular cyanobacterial symbionts (termed Group A, Zehr et al, 1998Zehr et al, , 2001) to small and generally free-living cyanobacteria such as Crocosphaera spp. and heterotrophic diazotrophs (Church et al, 2005;Gradoville et al, 2017b), to more conspicuous and large cell-sized diazotrophs such as Trichodesmium and symbioses of Richelia and Calothrix with various diatoms (termed DDAs, diatom-diazotroph assemblages). These latter, larger organisms have been recognized as important contributors to upper ocean productivity and the annual flux of organic matter to the mesopelagic of the NPSG (Scharek et al, 1999;Dore et al, 2002;Karl et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Temporal Variability of Trichodesmium spp. and Diatom-Diazotroph Assemblages in the North Pacific Subtropical Gyre

2018

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In oligotrophic ocean regions such as the North Pacific Subtropical Gyre (NPSG), N 2 fixation (i.e., diazotrophy) by a diverse consortium of microorganisms has been shown to contribute significantly to new production and particle export. In 2015 and 2016, we measured near-monthly abundances of the large cell-sized (> 10 µm) diazotrophic genera Trichodesmium and diatom-associated Richelia and Calothrix spp. in the NPSG via microscopy and quantitative PCR of nifH genes. Of these genera, we find Trichodesmium to be the more abundant over our study period, with cell concentrations in the upper water column (0-45 m) ranging from 1 to 5,988 cells L −1 , while the sum of Richelia and Calothrix spp. abundances ranged from 4 to 157 heterocysts L −1 . Significant discrepancies between absolute abundances were noted between cell and gene-based approaches to biomass determination (nifH copies L −1 were up to 10 2 -10 3 higher than cell concentrations). Potential explanations for these striking discrepancies are discussed. Using the maximum N fixation rates per cell found in the existing literature for these genera, we estimate potential N 2 fixation rates via these large diazotroph communities to be between 0.01 and 1.5 nmol N L −1 d −1 . When comparing these rates to available 15 N 2 tracer measurements, we conclude that large diazotrophs were generally minor (<10%) contributors to bulk N 2 fixation in the surface ocean during our study period. Conversely, high concentrations of Trichodesmium observed in fall-winter of 2015 and 2016 were estimated to drive >50% of measured N 2 fixation rates. While these large cell-sized and heterogeneously distributed organisms may still disproportionately contribute to export, cell-abundance based rate estimates suggests that other diazotrophs are largely responsible for N 2 fixation rates measured in bottle-based incubations.

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Diversity and activity of nitrogen‐fixing communities across ocean basins

Cited by 100 publications

References 67 publications

Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

Temporal Variability of Trichodesmium spp. and Diatom-Diazotroph Assemblages in the North Pacific Subtropical Gyre

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Diversity and activity of nitrogen‐fixing communities across ocean basins

Cited by 100 publications

References 67 publications

Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

N&lt;sub&gt;2&lt;/sub&gt; fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

Temporal Variability of Trichodesmium spp. and Diatom-Diazotroph Assemblages in the North Pacific Subtropical Gyre

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N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)