Objectives
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), https://www.science.org/doi/10.1126/science.abq4654. 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: https://www.frontiersin.org/research-topics/49099/hydrothermal-and-submarine-volcanic-activity-impacts-on-ocean-chemistry-and-plankton-dynamics. 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€.
