Hugo Berthelot

European Molecular Biology Laboratory | EMBL · EMBL Heidelberg

Bacterial degradation of sinking diatom aggregates is key for the availability of organic matter in the deep-ocean. Yet, little is known about the impact of aggregate colonization by different bacterial taxa on organic carbon and nutrient cycling within aggregates. Here, we tracked the carbon (C) and nitrogen (N) transfer from the diatom Leptocylin...

Picoplankton populations dominate the planktonic community in the surface oligotrophic ocean. Yet, their strategies in the acquisition and the partitioning of organic and inorganic sources of nitrogen (N) and carbon (C) are poorly described. Here, we measured at the single‐cell level the uptake of dissolved inorganic C (C‐fixation), C‐leucine, N‐le...

Phytoplankton is composed of a broad-sized spectrum of phylogenetically diverse microorganisms. Assessing CO2-fixation intra- and inter-group variability is crucial in understanding how the carbon pump functions, as each group of phytoplankton may be characterized by diverse efficiencies in carbon fixation and export to the deep ocean. We measured...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N2) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N2 fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in...

Nitrogen (N) is a limiting nutrient in vast regions of the world’s oceans, yet the sources of N available to various phytoplankton groups remain poorly understood. In this study, we investigated inorganic carbon (C) fixation rates and nitrate (NO3−), ammonium (NH4+) and urea uptake rates at the single cell level in photosynthetic pico-eukaryotes (P...

Here we report N2 fixation rates from a ∼ 4000 km transect in the western and central tropical South Pacific, a particularly undersampled region in the world ocean. Water samples were collected in the euphotic layer along a west to east transect from 160° E to 160° W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archi...

Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is r...

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) st...

Here we report quantification of N2 fixation rates over a ~ 4000 km transect in the western and central tropical South Pacific. Water samples were collected along a west to east transect from 160° E to 160° W, covering contrasting trophic regimes, from oligotrophy in the Melanesian archipelagoes (MA) waters to ultra-oligotrophy in the South Pacific...

Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood due to technical limitations. The main goals of this study were to (i) quantify ho...

We performed N budgets at three stations in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (Feb. Mar. 2015) and quantified all major N fluxes both entering the system (N2 fixation, nitrate eddy diffusion, atmospheric deposition) and leaving the system (PN export). Thanks to a Lagrangian strategy, we sampled the sam...

Dinitrogen (N2) fixation rates were investigated in the euphotic layer of the Bismarck and Solomon Seas using 15N2 incubation assays taking into account both the particulate and the dissolved pools. Average depth-integrated particulate N2 fixation rates were 203 (range 43-399) and 1396 (range 176-3132) μmol N m-2 in the Bismarck and Solomon Seas, r...

The semi-enclosed Solomon Sea in the southwestern tropical Pacific is on the pathway of a major oceanic circuit connecting the subtropics to the equator via energetic western boundary currents. Waters transiting through this area replenish the Pacific Warm Pool and ultimately feed the equatorial current system, in particular the equatorial undercur...

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 trich...

The VAHINE mesocosm experiment was designed to trigger a diazotroph bloom and to follow the subsequent transfer of diazotroph-derived nitrogen (DDN) in the rest of the food web. Three mesocosms (50 m3) located inside the Nouméa lagoon (New Caledonia, southwestern Pacific) were enriched with dissolved inorganic phosphorus (DIP) in order to promote N...

The globally important marine diazotrophic cyanobacterium Trichodesmium is abundant in the New Caledonian lagoon (southwestern Pacific Ocean) during austral spring/summer. We investigated the cellular processes mediating Trichodesmium mortality from large surface accumulations (blooms) in the lagoon. Trichodesmium cells (and associated microbiota)...

Nitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine productivity. Yet the fate of diazotroph-derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto- and heterotrophi...

Nitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine productivity. Yet the fate of diazotroph-derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto- and heterotrophi...

Studies investigating the fate of diazotrophs through the microbial food web are lacking, although N2 fixation can fuel up to 50 % of new production in some oligotrophic oceans. In particular, the role played by heterotrophic prokaryotes in this transfer is largely unknown. In the frame of the VAHINE (VAriability of vertical and tropHIc transfer of...

In oligotrophic tropical and subtropical oceans, where strong stratification can limit the replenishment of surface nitrate, dinitrogen (N2) fixation by diazotrophs can represent a significant source of nitrogen (N) for primary production. The VAHINE (VAriability of vertical and tropHIc transfer of fixed N2 in the south-wEst Pacific) experiment was...

In marine ecosystems, biological N2 fixation provides the predominant external source of nitrogen (N; 140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly fixed N, or diazotroph-derived N (hereafter named DDN) in marine ecosystems is poorly understood. Moreover, whether...

Biological dinitrogen (N2) fixation is the major source of new nitrogen (N) for the open ocean, and thus promotes marine productivity, in particular in the vast N-depleted regions of the surface ocean. Yet, the fate of the diazotroph-derived N (DDN) in marine ecosystems is poorly understood, and its transfer to auto- and heterotrophic surrounding p...

N2 fixation rates were measured daily in large (∼ 50 m3) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N2 fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The m...

In marine ecosystems, N2 fixation provides the predominant external source of nitrogen (N) (140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly-fixed N, or diazotroph-derived N (hereafter named DDN) in marine ecosystems is poorly understood. Moreover, it remains unclea...

In oligotrophic tropical and subtropical oceans, where strong stratification can limit the replenishment of surface nitrate, dinitrogen (N2) fixation by diazotrophs can represent a significant source of nitrogen (N) for primary production. The VAHINE experiment was designed to examine the fate of diazotroph derived nitrogen (DDN) in such ecosystems...

The VAHINE mesocosm experiment was designed to trigger a diazotroph bloom and to follow the subsequent transfer of diazotroph derived nitrogen (DDN) in the rest of the foodweb. Three mesocosms (50 m3) located inside the Nouméa lagoon (New Caledonia, South West Pacific) were enriched with dissolved inorganic phosphate (DIP) in order to promote N2 fi...

The globally important marine diazotrophic cyanobacterium Trichodesmium blooms regularly in the New Caledonian lagoons (Sowthwestern Pacific). We exploited the development of a Trichodesmium bloom in the lagoon waters outside the enclosed VAHINE mesocosms to specifically investigate the cellular processes mediating its decline. Trichodesmium cells...

Biological dinitrogen (N2) fixation is the major source of new nitrogen (N) for the open ocean, and thus promotes marine productivity, in particular in the vast N-depleted regions of the surface ocean. Yet, the fate of the diazotroph-derived N (DDN) in marine ecosystems is poorly understood and its transfer to auto- and heterotrophic surrounding pl...

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...

N2 fixation fuels ~ 50 % of new primary production in the oligotrophic South Pacific Ocean. The VAHINE mesocosm experiment designed to track the fate of diazotroph derived nitrogen (DDN) in the New Caledonia lagoon. Here, we examined the temporal dynamics of heterotrophic bacterial production during this experiment. Three replicate large-volume (~...

N2 fixation rates were measured daily in large (~ 50 m3) mesocosms deployed in the tropical South West Pacific coastal ocean (New Caledonia) to investigate the spatial and temporal dynamics of diazotrophy and the fate of diazotroph-derived nitrogen (DDN) in a low nutrient, low chlorophyll ecosystem. The mesocosms were intentionally fertilized with...

The dinitrogen (N2) fixed by diazotrophs and released as dissolved nitrogen (DN) has been compared in batch cultures of four marine diazotrophic cyanobacteria: the colony forming Trichodesmium IMS101 and the unicellular strains Cyanothece ATCC51142, Crocosphaera watsonii WH8501 and WH0003. Two approaches were conducted for this purpose. The first a...

In the oligotrophic ocean characterized by nitrate (NO3−) depletion in surface waters, dinitrogen (N2) fixation and dissolved organic nitrogen (DON) can represent significant nitrogen (N) sources for the ecosystem. In this study, we deployed large in situ mesocosms in New Caledonia in order to investigate (1) the contribution of N2 fixation and DON...

In the oligotrophic ocean characterized by nitrate (NO3-) depletion in surface waters, dinitrogen (N2) fixation and dissolved organic nitrogen (DON) can represent significant nitrogen (N) sources for the ecosystem. Here we deployed in New Caledonia large in situ mesocosms in order to investigate (1) the contribution of N2 fixation and DON use to pr...