Periodic Reporting for period 1 - HOPE (How do diazotrophs shape the ocean biological carbon pump? A global approach, from the single cell to the ecosystem)
Reporting period: 2023-01-01 to 2025-06-30
Diazotrophs regulate marine productivity in 60% of our oceans by alleviating nitrogen limitation, contributing to carbon (C) sequestration through the N2-primed Prokaryotic C Pump (PCP). Yet we do not know how much diazotroph-derived organic C (OC) is exported to the deep ocean, which prevents robust predictions of how the ocean contributes to CO2 sequestration and climate change mitigation. This knowledge gap is due to the multiple and complex pathways by which diazotrophs are exported to the deep ocean, which quantification and drivers of variability are impossible to capture with current methods. HOPE will bridge this gap thanks to a new isotopic technique I developed and to a coupling between lab and in situ approaches examining processes occurring at different spatiotemporal scales, and capable of capturing both transient and seasonal features of the PCP. HOPE will:
1. Determine how various diazotrophs aggregate, sink and are remineralized by using an automated experimental water column I designed for this proposal,
2. Decipher by which pathways diazotroph-derived OC is exported to the deep ocean thanks to a pioneer approach combining single-cell isotopic analyses, in-depth microbiological characterization of sinking particles and geochemical budgets,
3. Investigate how environmental drivers control the whole process, from the surface diazotroph community up to their eventual export to the deep ocean, by deploying a cutting-edge autonomous smart buoy, unique as it performs synoptic measurements both in and below the euphotic zone at high resolution (hourly/daily).
In its final stage, HOPE will use the generated data to provide global, spatially resolved estimates of the contribution of diazotrophs to overall OC export. Based on my expertise at the interface between microbial oceanography and geochemistry, HOPE has the potential to deliver a multidisciplinary and ground-breaking knowledge leading to potential scientific-based recommendations to fight climate change.
1. Determine how various diazotrophs aggregate, sink and are remineralized by using an automated experimental water column I designed for this proposal,
2. Decipher by which pathways diazotroph-derived OC is exported to the deep ocean thanks to a pioneer approach combining single-cell isotopic analyses, in-depth microbiological characterization of sinking particles and geochemical budgets,
3. Investigate how environmental drivers control the whole process, from the surface diazotroph community up to their eventual export to the deep ocean, by deploying a cutting-edge autonomous smart buoy, unique as it performs synoptic measurements both in and below the euphotic zone at high resolution (hourly/daily).
In its final stage, HOPE will use the generated data to provide global, spatially resolved estimates of the contribution of diazotrophs to overall OC export. Based on my expertise at the interface between microbial oceanography and geochemistry, HOPE has the potential to deliver a multidisciplinary and ground-breaking knowledge leading to potential scientific-based recommendations to fight climate change.
HOPE uses an ambitious approach at the interface between microbial oceanography, geochemistry and autonomous sensor technology. The general approach combines both lab (WP1) and in situ (WP2, WP3) approaches, studying processes occurring at different spatiotemporal scales. At this stage of the project (2 years), the 3 WPs have been started.
-In WP1, we designed and realized a specific prototype device able to study the dynamics of diazotroph-derived particles in the lab, i.e. the SOCRATE experimental column (Simulated OCean wateR column with AutomaTEd sampling). Briefly, SOCRATE is an illuminated and temperature-controlled experimental 6 m-high water column fitted with a wave simulator in surface, and three sediment traps at the base to collect particles at desired time intervals. Sinking particles can be automatically and simultaneously sampled at desired time intervals without perturbation at every meter along the column, and their sinking velocity is measured by cameras fitted in the column. This column makes it possible to acquire metrics that are not accessible in the natural ocean (size, shape descriptors of particles, aggregation rates, sinking velocities, remineralization rates of specific diazotrophs, etc.) that are essential to be integrated in the marine components of the climate models used by the IPCC.
-In WP2, we performed 3 oceanographic expeditions (32 days at sea in 2024) onboard the Research Vessel ANTEA in the subtropical South Pacific Ocean. We used a combined strategy aiming at deploying 1000 m-long drifting mooring lines fitted with sinking particle traps in the deep ocean and studying carbon trophic flows in surface, to shed light on the complex carbon export pathways to the deep ocean.
-In WP3, we deployed in the South Pacific Ocean a smart profiling buoy (HOPE'ORTUNITY) equipped with high-tech sensors and automated devices, some of which were developed exclusively for the HOPE project. This 8.5 meters high and 5 meters in diameter platform, is completely self-sufficient in energy, powered by solar panels and wind turbines. The buoy works in tandem with six 700m-long instrumented fixed mooring lines, forming an oceanographic measurement system covering an area of over 2km2 in the ocean - one of the most extensively instrumented oceanographic sites to date. The combined system measures the high-frequency variability (hourly and daily for several years) of plankton biodiversity, associated carbon fluxes and currents, simultaneously at the surface and in the deep ocean. This data will shed light on the complex processes involved in carbon sequestration by the ocean.
-In WP1, we designed and realized a specific prototype device able to study the dynamics of diazotroph-derived particles in the lab, i.e. the SOCRATE experimental column (Simulated OCean wateR column with AutomaTEd sampling). Briefly, SOCRATE is an illuminated and temperature-controlled experimental 6 m-high water column fitted with a wave simulator in surface, and three sediment traps at the base to collect particles at desired time intervals. Sinking particles can be automatically and simultaneously sampled at desired time intervals without perturbation at every meter along the column, and their sinking velocity is measured by cameras fitted in the column. This column makes it possible to acquire metrics that are not accessible in the natural ocean (size, shape descriptors of particles, aggregation rates, sinking velocities, remineralization rates of specific diazotrophs, etc.) that are essential to be integrated in the marine components of the climate models used by the IPCC.
-In WP2, we performed 3 oceanographic expeditions (32 days at sea in 2024) onboard the Research Vessel ANTEA in the subtropical South Pacific Ocean. We used a combined strategy aiming at deploying 1000 m-long drifting mooring lines fitted with sinking particle traps in the deep ocean and studying carbon trophic flows in surface, to shed light on the complex carbon export pathways to the deep ocean.
-In WP3, we deployed in the South Pacific Ocean a smart profiling buoy (HOPE'ORTUNITY) equipped with high-tech sensors and automated devices, some of which were developed exclusively for the HOPE project. This 8.5 meters high and 5 meters in diameter platform, is completely self-sufficient in energy, powered by solar panels and wind turbines. The buoy works in tandem with six 700m-long instrumented fixed mooring lines, forming an oceanographic measurement system covering an area of over 2km2 in the ocean - one of the most extensively instrumented oceanographic sites to date. The combined system measures the high-frequency variability (hourly and daily for several years) of plankton biodiversity, associated carbon fluxes and currents, simultaneously at the surface and in the deep ocean. This data will shed light on the complex processes involved in carbon sequestration by the ocean.
HOPE's greatest innovation/novel methodology is without doubt the HOPE smart buoy. It represents an unprecedented technological innovation in oceanography. Indeed, one of the major obstacles to the study of carbon sequestration in the ocean stems from the fact that we are unable to observe the ocean on the time scale required to understand the processes involved. Indeed, plankton organisms (which largely control carbon sequestration) have life spans of hours to days, and carbon flows through this ecosystem are very rapid and fluctuate on an hourly/daily scale (in addition to seasonal and interannual scales). Current techniques cannot capture this high-frequency variability, at least not with the degree of microbiological and chemical/physical complexity needed to understand the processes. Today, it is impossible for oceanographers to perform complex sampling every 3 or 4 hours for several years in the ocean. That is why the HOPE smart buoy has been designed to take hourly complex measurements and samples in the middle of the ocean for several years, and send the data ashore to scientists in real time. The HOPE buoy does not work alone, but in tandem with fixed mooring lines installed around the buoy, i.e. 6 cables stretched from the ocean floor to the surface, also equipped with dozens of sensors and automated collectors, some of which are also prototypes. To date, the HOPE site in New Caledonia is probably one of the most instrumented sites in the world ocean.
The project is underway and data are being generated, beginning to reveal dynamics and a degree of complexity never seen before in the control of carbon fluxes to the deep ocean.
The project is underway and data are being generated, beginning to reveal dynamics and a degree of complexity never seen before in the control of carbon fluxes to the deep ocean.