How do diazotrophs shape the ocean biological carbon pump? A global approach, from the single cell to the ecosystem

Introduction

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 don’t 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.

Knowledge Gap

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.

Project Overview

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.

Objectives

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

Expected Outcomes

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.

Significance

Based on my expertise at the interface between microbial oceanography and geochemistry, HOPE has the potential to deliver multidisciplinary and groundbreaking knowledge leading to potential science-based recommendations to fight climate change.