Hugo Berthelot

Hot spot of N-2 fixation in the western tropical South Pacific pleads for a spatial decoupling between N-2 fixation and denitrification

Nitrogen (N) is the building block of life. Quantifying the sources and sinks of N to the ocean is essential for predicting its productivity and potential carbon sequestration. In his paper, Gruber (1) seeks for “elusive marine nitrogen fixation” following results from Knapp et al. (2), who measured unexpectedly low N input through N2 fixation in the eastern tropical South Pacific (ETSP), seriously bringing into question the proposed close spatial coupling between N input (through N2 fixation) and loss (through denitrification) (3). Here, we compile data from recently published and unpublished studies revealing a hot spot of N2 fixation in the western tropical South Pacific (WTSP) arguing for a spatial decoupling between N sources and sinks in the South Pacific. Based on four cruises performed between 2012 and 2015 during austral winter and summer conditions, with a total of more than 600 15N2 incubations-based measurements, and particularly a 4,000-km zonal transect at ∼20°S in … [↵][1]1To whom correspondence should be addressed. Email: sophie.bonnet{at}univ-amu.fr. [1]: #xref-corresp-1-1

Mesopelagic N2 Fixation Related to Organic Matter Composition in the Solomon and Bismarck Seas (Southwest Pacific)

Dinitrogen (N2) fixation was investigated together with organic matter composition in the mesopelagic zone of the Bismarck (Transect 1) and Solomon (Transect 2) Seas (Southwest Pacific). Transparent exopolymer particles (TEP) and the presence of compounds sharing molecular formulae with saturated fatty acids and sugars, as well as dissolved organic matter (DOM) compounds containing nitrogen (N) and phosphorus (P) were higher on Transect 1 than on Transect 2, while oxygen concentrations showed an opposite pattern. N2 fixation rates (up to ~1 nmol N L-1 d-1) were higher in Transect 1 than in Transect 2, and correlated positively with TEP, suggesting a dependence of diazotroph activity on organic matter. The scores of the multivariate ordination of DOM molecular formulae and their relative abundance correlated negatively with bacterial abundances and positively with N2 fixation rates, suggesting an active bacterial exploitation of DOM and its use to sustain diazotrophic activity. Sequences of the nifH gene clustered with Alpha-, Beta-, Gamma- and Deltaproteobacteria, and included representatives from Clusters I, III and IV. A third of the clone library included sequences close to the potentially anaerobic Cluster III, suggesting that N2 fixation was partially supported by presumably particle-attached diazotrophs. Quantitative polymerase chain reaction (qPCR) primer-probe sets were designed for three phylotypes and showed low abundances, with a phylotype within Cluster III at up to 103 nifH gene copies L-1. These results provide new insights into the ecology of non-cyanobacterial diazotrophs and suggest that organic matter sustains their activity in the mesopelagic ocean.

High‐nitrogen fixation rates in the particulate and dissolved pools in the Western Tropical Pacific (Solomon and Bismarck Seas)

Dinitrogen (N-2) fixation rates were investigated in the euphotic layer of the Bismarck and Solomon Seas using N-15(2) incubation assays taking into account both the particulate and the dissolved pools. Average depth-integrated particulate N-2 fixation rates were 203 (range 43-399) and 1396 (range 176-3132)mol N m(-2) d(-1) in the Bismarck and Solomon Seas, respectively. In both seas, N-2 fixation measured in the dissolved pool was similar to particulate N-2 fixation, highlighting the potentially substantial underestimation of N-2 fixation in oceanic budgets when only particulate N-2 fixation is considered. Among the diazotroph phylotypes targeted using quantitative polymerase chain reaction amplification of nifH genes, Trichodesmium was the most abundant. Regression analyses suggest that it accounted for the major proportion of N-2 fixation. However, unicellular cyanobacterial and non-cyanobacterial diazotrophs were also occasionally abundant. This study reports high pelagic N-2 fixation rates and confirms that the Western Tropical South Pacific is a hot spot for marine N-2 fixation.

Dissolved organic matter uptake by Trichodesmium in the Southwest Pacific

The globally distributed diazotroph Trichodesmium contributes importantly to nitrogen inputs in the oligotrophic oceans. Sites of dissolved organic matter (DOM) accumulation could promote the mixotrophic nutrition of Trichodesmium when inorganic nutrients are scarce. Nano-scale secondary ion mass spectrometry (nanoSIMS) analyses of individual trichomes sampled in the South Pacific Ocean, showed significant 13C-enrichments after incubation with either 13C-labeled carbohydrates or amino acids. These results suggest that DOM could be directly taken up by Trichodesmium or primarily consumed by heterotrophic epibiont bacteria that ultimately transfer reduced DOM compounds to their host trichomes. Although the addition of carbohydrates or amino acids did not significantly affect bulk N2 fixation rates, N2 fixation was enhanced by amino acids in individual colonies of Trichodesmium. We discuss the ecological advantages of DOM use by Trichodesmium as an alternative to autotrophic nutrition in oligotrophic open ocean waters.