The conundrum of marine N2 fixation

Mahaffey, Claire

doi:10.2475/ajs.305.6-8.546

Cited by 240 publications

(258 citation statements)

References 274 publications

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“…One of these diazotrophs, Trichodesmium, is able to form massive blooms known as ''sea-sawdust'', covering large areas of the surface ocean in the tropical and subtropical regions Mahaffey et al 2005). The first mention of Trichodesmium was made in 1770 by Captain Cook in the Coral Sea near Australia.…”

Section: Introductionmentioning

confidence: 99%

Interactions between CCM and N2 fixation in Trichodesmium

2010

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In view of the current increase in atmospheric pCO 2 and concomitant changes in the marine environment, it is crucial to assess, understand, and predict future responses of ecologically relevant phytoplankton species. The diazotrophic cyanobacterium Trichodesmium erythraeum was found to respond strongly to elevated pCO 2 by increasing growth, production rates, and N 2 fixation. The magnitude of these CO 2 effects exceeds those previously seen in other phytoplankton, raising the question about the underlying mechanisms. Here, we review recent publications on metabolic pathways of Trichodesmium from a gene transcription level to the protein activities and energy fluxes. Diurnal patterns of nitrogenase activity change markedly with CO 2 availability, causing higher diel N 2 fixation rates under elevated pCO 2 . The observed responses to elevated pCO 2 could not be attributed to enhanced energy generation via gross photosynthesis, although there are indications for CO 2 -dependent changes in ATP/ NADPH ? H ? production. The CO 2 concentrating mechanism (CCM) in Trichodesmium is primarily based on HCO 3 -uptake. Although only little CO 2 uptake was detected, the NDH complex seems to play a crucial role in internal cycling of inorganic carbon, especially under elevated pCO 2 . Affinities for inorganic carbon change over the day, closely following the pattern in N 2 fixation, and generally decrease with increasing pCO 2 . This down-regulation of CCM activity and the simultaneously enhanced N 2 fixation point to a shift in energy allocation from carbon acquisition to N 2 fixation under elevated pCO 2 levels. A strong light modulation of CO 2 effects further corroborates the role of energy fluxes as a key to understand the responses of Trichodesmium.

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Section: Introductionmentioning

confidence: 99%

Interactions between CCM and N2 fixation in Trichodesmium

2010

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“…This bioavailable N is an important source of N that supports primary production in oligotrophic marine ecosystems (Karl et al, 1997), where N availability often limits phytoplankton growth (Graziano et al, 1996;Mills et al, 2004). Rate measurements and geochemical estimates suggest N inputs via N 2 fixation are much smaller than the N loss processes through denitrification (including anaerobic ammonium oxidation) (Mahaffey et al, 2005). A better understanding of the identity, distributions and activity of microorganisms contributing to N 2 fixation in the oceans is necessary in order to balance the oceanic N budget (Zehr and Kudela, 2011).…”

Section: Introductionmentioning

confidence: 99%

Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean

et al. 2014

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In addition to the cyanobacterial N2-fixers (diazotrophs), there is a high nifH gene diversity of non-cyanobacterial groups present in marine environments, yet quantitative information about these groups is scarce. N2 fixation potential (nifH gene expression), diversity and distributions of the uncultivated diazotroph phylotype γ-24774A11, a putative gammaproteobacterium, were investigated in the western South Pacific Ocean. γ-24774A11 gene copies correlated positively with diazotrophic cyanobacteria, temperature, dissolved organic carbon and ambient O2 saturation, and negatively with depth, chlorophyll a and nutrients, suggesting that carbon supply, access to light or inhibitory effects of DIN may control γ-24774A11 abundances. Maximum nifH gene-copy abundance was 2 × 104 l−1, two orders of magnitude less than that for diazotrophic cyanobacteria, while the median γ-24774A11 abundance, 8 × 102 l−1, was greater than that for the UCYN-A cyanobacteria, suggesting a more homogeneous distribution in surface waters. The abundance of nifH transcripts by γ-24774A11 was greater during the night than during the day, and the transcripts generally ranged from 0–7%, but were up to 26% of all nifH transcripts at each station. The ubiquitous presence and low variability of γ-24774A11 abundances across tropical and subtropical oceans, combined with the consistent nifH expression reported in this study, suggest that γ-24774A11 could be one of the most important heterotrophic (or photoheterotrophic) diazotrophs and may need to be considered in future N budget estimates and models.

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“…The N : P ratio of Synechococcus and Prochlorococcus, small and abundant phytoplankton cells in the open ocean, varies from 13.3 to 33.2 and 15.9 to 24.4, respectively, during exponential growth, while the ratio can be as high as 100 during PO 3− 4 -limited growth (Bertilsson et al, 2003;(Heldal et al, 2003). Another cyanobacteria, the N 2 fixer Trichodesmium has an N : P ratio that varies from 42 to 125 (Karl et al, 1992), while in general diatoms have a ratio of ∼ 11 : 1 (Quigg et al, 2003;Letelier and Karl, 1996;Mahaffey et al, 2005). Excess downward dissolved organic nitrogen (DON) fluxes relative to NO − 3 are associated with Trichodesmium abundance (Vidal et al, 1999).…”

Section: Introductionmentioning

confidence: 99%

C : N : P stoichiometry at the Bermuda Atlantic Time-series Study station in the North Atlantic Ocean

et al. 2015

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Abstract. Nitrogen (N) and phosphorus (P) availability, in addition to other macro-and micronutrients, determine the strength of the ocean's carbon (C) uptake, and variation in the N : P ratio of inorganic nutrient pools is key to phytoplankton growth. A similarity between C : N : P ratios in the plankton biomass and deep-water nutrients was observed by Alfred C. Redfield around 80 years ago and suggested that biological processes in the surface ocean controlled deepocean chemistry. Recent studies have emphasized the role of inorganic N : P ratios in governing biogeochemical processes, particularly the C : N : P ratio in suspended particulate organic matter (POM), with somewhat less attention given to exported POM and dissolved organic matter (DOM). Herein, we extend the discussion on ecosystem C : N : P stoichiometry but also examine temporal variation in stoichiometric relationships. We have analyzed elemental stoichiometry in the suspended POM and total (POM + DOM) organic-matter (TOM) pools in the upper 100 m and in the exported POM and subeuphotic zone (100-500 m) inorganic nutrient pools from the monthly data collected at the Bermuda Atlantic Time-series Study (BATS) site located in the western part of the North Atlantic Ocean. C : N and N : P ratios in TOM were at least twice those in the POM, while C : P ratios were up to 5 times higher in TOM compared to those in the POM. Observed C : N ratios in suspended POM were approximately equal to the canonical Redfield ratio (C : N : P = 106 : 16 : 1), while N : P and C : P ratios in the same pool were more than twice the Redfield ratio. Average N : P ratios in the subsurface inorganic nutrient pool were ∼ 26 : 1, squarely between the suspended POM ratio and the Redfield ratio. We have further linked variation in elemental stoichiometry to that of phytoplankton cell abundance observed at the BATS site. Findings from this study suggest that elemental ratios vary with depth in the euphotic zone, mainly due to different growth rates of cyanobacterial cells. We have also examined the role of the Arctic Oscillation on temporal patterns in C : N : P stoichiometry. This study strengthens our understanding of the variability in elemental stoichiometry in different organic-matter pools and should improve biogeochemical models by constraining the range of non-Redfield stoichiometry and the net relative flow of elements between pools.

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The conundrum of marine N2 fixation

Cited by 240 publications

References 274 publications

Interactions between CCM and N2 fixation in Trichodesmium

Interactions between CCM and N2 fixation in Trichodesmium

Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean

C : N : P stoichiometry at the Bermuda Atlantic Time-series Study station in the North Atlantic Ocean

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