Shallow hydroThermal sOurces of trace elemeNts: potential impacts on biological productivity and the bioloGicAl carbon pump


A nitrogen-fixation hot spot in the southwest subtropical ocean

The western tropical South Pacific (WTSP) ocean is a hot spot of atmospheric dinitrogen (N2) fixation, a process that naturally fertilizes the surface ocean with nitrogenous fertilizers, boosting biological productivity. It is estimated that this region contributes ~30% of global nitrogen fixation activity. Nitrogen-fixing (or diazotroph) organisms have high iron requirements compared to non-diazotroph phytoplankton, and their success in the WTSP is thought to be due to the alleviation of iron limitation in this region. Shallow Fe sources of hydrothermal origin (<500 m) were indeed discovered at the submarine volcanoes of Tonga's volcanic arc in 2015 by our team, resulting in high dissolved iron concentrations (> 4 nM) in the productive layer (~ 0-100 m). These inputs are thought to stimulate biological activity in this zone, particularly that of diazotrophs, creating a vast satellite-visible bloom of around 400,000 km2, a real oasis of life in the middle of the South Pacific marine desert. However, the potential impact of these shallow hydrothermal inputs on regional iron budgets, biological productivity and carbon sequestration had never been studied.

In search of the submarine volcanoes of Tonga's volcanic arc

The aim of the TONGA project was to investigate the control of oceanic productivity and carbon sequestration by micronutrients of shallow hydrothermal origin by:


Accurately quantifying iron (and other biogeochemically relevant compounds) inputs from shallow submarine volcanoes and associated hydrothermal sources along the Tonga volcanic arc (20 and 25ºS) for the productive layer in comparison with atmospheric iron inputs,


Studying the fate of shallow hydrothermal plumes in the water column at local and regional scales,


Investigating the bioavailability and potential impact of these hydrothermal inputs on planktonic communities and carbon export in the WTSP.

Major results

The 37-day expedition was a success: we identified and explored 2 contrasted volcanoes and revealed that the cumulative impact of multiple hydrothermal sources along the Tonga arc fertilizes the entire Lau basin (Tilliette et al., 2022, Figure 1). These fluids emitted along the Tonga volcanic arc have a substantial impact on iron concentrations in the productive layer through vertical diffusion. This enrichment stimulates biological activity, resulting in a vast chlorophyll patch (~400,000 km2). Diazotroph activity is 2 to 8 times higher and carbon export fluxes are 2 to 3 times greater in iron-enriched waters than in adjacent unfertilized waters. These results reveal a previously undescribed mechanism of natural iron fertilization of the oceans that feeds regional atmospheric CO2 sinks (Bonnet & Guieu et al., 2023, Science).

Scientific output

The TONGA project resulted in a review article in the prestigious journal Science (Bonnet, Guieu et al., 2023), 17 articles in international journals, including 4 in a special issue dedicated to the project in Frontiers in Marine Science and Frontiers in Microbiology, in which a further 13 articles are expected by early 2024: The valorization of the TONGA results is beyond that Special Issue, as more work are still in progress (that were not anticipated and more funding have been necessary and acquired).

Factual information

The TONGA project is a fundamental research project coordinated by Sophie Bonnet & Cécile Guieu. It involves ~100 other scientists from 21 international laboratories. The project began in December 2018 and lasted 54 months. It received ANR funding of €417,650 for an overall cost (not consolidated with permanent staff salaries) of around 2.2 M€.

© IRD - Hubert Bataille
Installation du matériel scientifique, Nouvelle-Calédonie

TONGA Publications (ongoing)

Accepted publications

Under revision in Frontiers in Marine Science, 2024
Iron-binding by dissolved organic matter in the Western Tropical South Pacific Ocean (GEOTRACES cruise GPpr14)

Mahieu, L., Whitby, H., Dulaquais, G., Tilliette, C., Guigue, C., Tedetti, M., Lefevre, D., Fourrier, P., Bressac, M., Sarthou, G., Bonnet, S., Guieu, C. and Salaun, P.

Read the paper online
The ISME Journal, Volume 17, 2023 - The ISME Journal Best Paper Award 2023
Diazotrophs are overlooked contributors to carbon and nitrogen export to the deep ocean.

Bonnet, S., Benavides, M., Le Moigne, F.A.C, Camps, M., Torremocha, A., Grosso, O., Dimier, C., Spungin, D., Berman-Frank, Garczarek, L. Cornejo-Castillo, F.

Read the paper online
Frontiers in Marine Science, Volume 10, 2023
Influence of shallow hydrothermal fluids release on the functioning of phytoplankton communities

Tilliette, C., Gazeau, F. Portlock, G., Bonnet, S., Guigue, C., Leblond, N., Lory, C., Marie, D., Montanes, M., Pulido-Villena, E., Sarthou, G., Tedetti, M., Vorrath, M.E., Whitby, H. and Guieu, C.

Read the paper online
Science, Volume 380, Issue 6647, pp. 812-817, 2023
Natural iron fertilization by shallow hydrothermal sources fuels diazotroph blooms in the Ocean

Bonnet, S., Guieu, C., Taillandier, V., Boulart, C., Bouruet-Aubertot, P., Gazeau, F., Bressac, M., Knapp, A. N., Cuypers, Y., González-Santana, D., Forrer, H. J., Grisoni, J.-M., Grosso, O., Habasque, J., Jardin-Camps, M., Leblond, N., Moigne, F. Le, Dhaussy, A. L., Lory, C., … Tilliette, C.

Read the paper online
Frontiers in Marine Science, 10, 1232923, 2023
Plankton community structure in response to hydrothermal iron inputs along the Tonga-Kermadec arc

Mériguet, Z., Vilain, M., Baudena, A., Tilliette, C., Habasque, J., Lebourges-Dhaussy, A., Bhairy, N., Guieu, C., Bonnet, S., and Lombard, F.

Read the paper online

Funding and partners

TONGA is funded by ANR, INSU LEFE-CYBER et LEFE-GMMC, the A-MIDEX fundation, the TGIR ‘Flotte Océanographique Française’ and IRD. TONGA has been endorsed as a GEOTRACES process study and received a letter of support from the IMBER international program. ​The TONGA consortium involves 90 scientists from 19 international institutions among which hydrothermal geochemists, physical oceanographers, trace element chemists (ocean and atmosphere), biogeochemists, biologists and modelers.

  • M.I.O Mediterranean Institute of Oceanography (Marseille, France)
  • LOV Laboratoire d’Océanographie de Villefranche, (Villefranche/mer, France)
  • GET Laboratoire de Géosciences-Environnement (Toulouse, France)
  • LEMAR Laboratoire des Sciences de l’Environnement Marin (Brest, France)
  • AD2M UMR Adaptation et Diversité en Milieu Marin (Roscoff, France)
  • LISA Laboratoire Inter-universitaire des Systèmes Atmosphériques (Paris, France)
  • LOPS Laboratoire d’Océanographie Physique et Spatiale (Brest, France)
  • LOCEAN Laboratoire d’Océanographie et du Climat (Paris, France)
  • LEGOS Laboratoire d’Etudes en Géophysique et Océano. spatiales (Toulouse, France)
  • AEL Laboratoire d'analyses environnementales (New Caledonia)
  • LaMP Laboratoire de Météorologie Physique (Clermont-Ferrand, France)
  • GEOAZUR (Nice, France)
  • DIMENC (Direction des Mines et de l’Environnement, New Caledonia)
  • IFREMER (Brest)
  • University of Liverpool (UK)
  • University of Tasmania (Australia)
  • New-York University Abu-Dhabi (Emirates)
  • Leibniz Institute for Baltic Sea Research (Germany)
  • Florida State University (USA)
  • Haifa University (Israel)