The Hope Project

Summary

General scientific objective Understanding the process of diazotrophs Understanding the process of diazotrophs

Why this project

Tropical and subtropical oceans cover ~60% of the global ocean surface. Until recently, they were considered inefficient for CO₂ sequestration because they are nutrient-poor zones. However, these vast regions host a particular type of plankton known as "diazotrophs", which fertilize the surface ocean with nutrients. These microorganisms stimulate the marine food chain and CO₂ sequestration via an alternative biological carbon pump, the importance of which was highlighted in a recent study.

How strong is this alternative carbon pump? Could these marine micro-organisms absorb more CO₂ than previously thought? And thus help mitigate climate change? This is what the HOPE project "How do diazotrophs shape the ocean biological carbon pump?" will explore over 5 years, combining approaches at the interface between microbial oceanography, geochemistry and autonomous sensor technology, thanks to the intelligent profiling buoy.
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Today's oceans are becoming "tropicalized", and the role of this alternative pump could become predominant in the oceans of the future. The results of the HOPE project could thus modify the climate models taken into account by the IPCC (Intergovernmental Panel on Climate Change) experts, and shape the ocean of tomorrow.

Our 3 specific goals

Determine how various diazotrophs aggregate, sink and are remineralized by using the SOCRATE automated experimental water column designed for this project,

Decipher by which pathways diazotroph-derived organic C 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 during seasonal oceanographic campaigns,

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 platform, unique as it performs synoptic measurements both in and below the euphotic zone at high resolution (hourly/daily).

Our biogeochemical strategy

Understanding particle formation, sinking & Remineralisation

In Work Package 1 (WP1), we will use the innovative SOCRATE column (Simulated OCean wateR column with AutomaTEd sampling) designed for the project, to study formation, sinking velocity and fate of particles resulting from different diazotrophs.

Quantifying export pathways - in situ

In WP2, we will perform seasonal seagoing expeditions at our slected study sites. We will use the device developed in WP1 to expand our recently-developed isotopic method to trace the Diazotroph-Derived Nitrogen in the food web. Concomitantly, we will deploy sediment traps fitted with polyacrylamide gels to examine the flux composition, particle origin and eventually decipher diazotroph export pathways,

Identifying the drivers of export

In WP3, we will deploy an autonomous moored Smart buoy, which is unique in that performs synoptic measurements of processes occurring both in and below the euphotic zone at hour/day resolution. In its final stage, HOPE will integrate the generated data into a global database to infer the impact of diazotrophs on the Biological Carbon Pump at the global scale.

Our smart buoy

The automated profiling smart buoy biut by our partner MOBILIS and its mooring network stand out for their ability to remotely conduct comprehensive measurements of processes both within and beneath the euphotic zone, providing high-resolution data on an hourly/daily basis. The profiling buoy is an autonomous floating platform, 8.5 meters tall and 5 meters in diameter, complete with a meteorological station and powered by solar panels and wind turbines.

What makes it innovative?

This innovative buoy boasts a winch that conducts vertical profiles up to 100 meters deep at intervals we choose. Fitted with sensors measuring temperature, salinity, depth, dissolved oxygen, chlorophyll fluorescence (indicating phytoplankton concentration), turbidity, nitrate, and phosphate concentrations, as well as particulate optical backscattering at 700 nm and particle concentrations >100 µm (Underwater Vision Profiler UVP6), it completes profiles within the euphotic zone (~0-100 m) at approximately 6 dive cycles per day.

In a second groundbreaking feature, when raised, the sensors are cleaned and stored in a garage out of the water, preventing biofouling and ensuring high-quality data without human intervention. Moreover, the generated data are transmitted to the shore in real-time.

The phytoplankton community composition is closely examined using an automated submersible flow cytometer with a camera and an automated filtration device. This device concentrates and fixes plankton biomass at desired intervals, preserving DNA and RNA on-site for about one month, allowing for further molecular biology analyses. Near the buoy, a fixed mooring line is anchored. It comes equipped with 8 programmable sediment traps collecting sinking particles at a high frequency at the base of the euphotic zone (~100 m depth) and near the sea floor. The line is also furnished with IODAs (In situ Oxygen Dynamic Auto-sampler), current meters, CTDs, fluorometers, turbidimeters, UVP6 sensors, and trace metal autonomous sensors. As part of the HOPE-VV partner project, five additional physical moorings will be strategically placed around the buoy.

Technicals details : outside the buoy

Crown equipment support

Solar panel (x12)

Lifting point

Float

Ballast

Sacrificial anode

Shackle DN 50

Mooring point

Technicals details : inside the buoy

Power compartment

Winch

Batteries

Current meter

Technical compartment

Fuelcell

Tank

Acoustic modem

Our physical strategy : HOPE‑VV

Physical processes also contribute to carbon storage

In addition to the gravitational settling of OC, physical mechanisms also transport suspended and sinking particles to depth. They include several processes acting on different spatial and timescales (1). The mechanism that we anticipate the most in tropical/subtropical waters where diazotrophs thrive is the subduction by mesoscale (10–100 km) to submesoscale (1–10 km) frontal circulation, termed the eddy-subduction pump. This physical pump subducts particle-rich surface waters on timescales of days, driven by Vertical Velocities (VV) associated with fronts and eddies.

However, unlike horizontal components of 3D-oceanic currents that are generally well known, their vertical ones (vertical velocities, VV) are still largely uncharacterized, although they influence the carbon transfer from the surface to the deep ocean. VV have long been simply parametrized or neglected because considered as not measurable, being often short-lived and one to two orders of magnitude smaller than horizontal velocities. Consequently, direct in situ measurement of VV is currently one of the biggest challenges in physical oceanography.

To bridge this knowledge gap, our physical team has recently developed innovative technologies to directly measure VV in situ. In the framework of the HOPE-VV project (PI: Dr Anne Petrenko), we propose to deploy and apply these new methods to add a physical component to the biogeochemical HOPE project (see above).

Recent technological developments

The in situ VVProfiler (VVP), able to directly measure VV throughout the water column with high precision. The VVP is a novel instrument, composed by a set of floats associated with a friction disc and an electric propeller that drives the profiler down to a predefined depth. Once the depth is reached, the profiler rises slowly towards the surface under the sole effect of its slightly positive buoyancy.
The mechanical balance between buoyancy and drag results in a theoretical vertical speed of ascent which depends on the water density, the drag coefficient and the mass and volume of the profiler. Any deviation from this theoretical speed is then interpreted as the effect of a vertical current,

New analyses to derive VV with accuracy from Acoustic Doppler Current Profilers (ADCPs) In our recent study Comby et al. (2022), we were able to make, for the first time, VV measurement in small scale/variable VV regions of the order of mm s-1, and standard deviations of a few mm s-1. Such accuracy has never been reached in previous studies.
Comby et al. (13) have also shown that the fifth beam of new generation ADCPs exhibits a better accuracy than conventional 4 beams ADCPs, and that the free-fall technique (sensor tethered to the ship but allowed to fall freely with gravitation) provides a more accurate measurement compared to the carousel technique (sensor attached to the ship by a descending cable, more impacted by ship motions). Hence, the best recommended method is based on a 5-beams ADCP deployed in a free-fall technique.

Scientific objectives of HOPE-VV

The ambition of the HOPE-VV project is to take advantage of these recent methodological and technological developments to add a physical component to the HOPE project that only considers biogeochemical processes leading to OC export to the deep ocean. The scientific objectives of the project are :

To quantify the intensity of VV and their high-frequency (hour) temporal variability along several annual cycles at a fixed site, and determine their contribution to OC transfer to the deep ocean,

To investigate the spatial variability of VV across fine scale physical structures (e.g., eddies’ core and edges, fronts), and estimate how they influence OC transport to the deep ocean across such structures.

Scientific strategy of HOPE-VV

HOPE-VV is structured around 2 parts working in conjunction with HOPE's WP2 and WP3.

High frequency temporal variability

First, we will quantify the intensity of VV and their high-frequency (hour) temporal variability thanks to the deployment of a set of 5 physical mooring lines (including a 5-beam ADCP) around the automated profiling smart buoy for 3 annual cycles in 2 contrasted regions (see study sites below),

Spatial variability across physical structures

Second, we will investigate the spatial variability of VV across physical structures (e.g., eddies’ core and edges, fronts) by making profiles with out 5-beam FreeFall ADCP and deploying the newly developed VVProfiler during seasonal ocean expeditions performed around the chosen contrasted environments.

Our study site

The HOPE and HOPE-VV projects will unfold across two carefully selected study sites distinguished by their diverse diazotroph communities, trophic regimes, and physical conditions.

Tropical Site: The Western tropical South Pacific Ocean, identified as an atmospheric CO₂ sink due to intense N₂ fixation. This region hosts a myriad of diazotrophs, with Trichodesmium forming extensive blooms detectable from space. UCYN-B dominates, accompanied by UCYN-A, UCYN-C, DDAs, and NCDs at lower abundances. The composition of the diazotroph community varies across seasons.

Temperate Study Site: The Western temperate South Pacific, to be explored for the first time, exhibits intense diazotroph activity, although the communities differ from those in the tropics. Here, UCYN-A and NCDs are the predominant species.

Consequently, the South West Pacific Ocean emerges as an ideal location to explore how diazotrophs influence the BCP under diverse diazotroph communities and trophic regimes.

The HOPE project