Sophie Bonnet, explorer of oceanic deserts

Could marine microorganisms absorb more CO2 than we think? And thus help reduce global warming? This is what oceanographer Sophie Bonnet thinks. She has just been awarded a European ERC Consolidator grant to finance her research project on this subject, HOPE. Meet a woman who is passionate about tropical ocean deserts, these nutrient-poor spaces, and their hidden riches.
Sophie Bonnet grew up sailing in a family that loved sailing. As a child, she often drew herself on a boat, surrounded by blue expanses and marine creatures. An obsession that would quickly become a professional project for the young woman. As a teenager, she already knew what she wanted to be: an oceanographer. I discovered this word at the age of 13 and it crystallized my dreams," she recalls. After graduating from high school, I undertook long studies up to the doctorate. But before that, I went around the world for a year on a sailing boat, sailing to West Africa and Madagascar. With the navigator Michel Huchet, we wanted to "sail for a purpose", and offer the possibility to other navigators to do the same. So we created the humanitarian association Voiles sans frontières which brings medical and educational aid to isolated populations, only accessible by sea and river with small boats. I was then president of this association for ten years.

Tiny organizations...

After her gap year, Sophie Bonnet studied at the University of Paris Sorbonne where she obtained a master's degree in biological oceanography and then began a thesis supervised by the oceanographer Cécile Guieu. The subject: "Iron of atmospheric origin in oligotrophic oceanic environments and its role in ocean fertilization". Although she is a biogeochemist by training, the student wishes to work on subjects of societal interest, particularly the ocean's capacity to absorb carbon dioxide (CO2).

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Could marine microorganisms absorb more CO2 than we think? And thus help reduce global warming? This is what oceanographer Sophie Bonnet thinks. She has just been awarded a European ERC Consolidator grant to finance her research project on this subject, HOPE. Meet a woman who is passionate about tropical ocean deserts, these nutrient-poor spaces, and their hidden riches.
Sophie Bonnet grew up sailing in a family that loved sailing. As a child, she often drew herself on a boat, surrounded by blue expanses and marine creatures. An obsession that would quickly become a professional project for the young woman. As a teenager, she already knew what she wanted to be: an oceanographer. I discovered this word at the age of 13 and it crystallized my dreams," she recalls. After graduating from high school, I undertook long studies up to the doctorate. But before that, I went around the world for a year on a sailing boat, sailing to West Africa and Madagascar. With the navigator Michel Huchet, we wanted to "sail for a purpose", and offer the possibility to other navigators to do the same. So we created the humanitarian association Voiles sans frontières which brings medical and educational aid to isolated populations, only accessible by sea and river with small boats. I was then president of this association for ten years.

Tiny organizations...

"I became fascinated with diazotrophs, plankton microorganisms that provide nitrogen to the entire food chain. Under certain conditions, they can create real oases of life in areas considered to be 'oceanic deserts', which represent 60% of the surface of the global ocean," she explains. There was little study of this in France, so I joined the laboratory of Douglas Capone, a specialist in the subject, at the University of Southern California in Los Angeles. He introduced me to this world!

Upon her return from the United States in 2007, the post-doctoral researcher was recruited by the IRD within the UMR MIO and moved to Noumea a few years later, where she was able to study diazotrophs, which are mainly present in the tropics, and their interactions with the carbon cycle. The young oceanographer went on a series of projects and expeditions. She then noticed that these microorganisms are not only found in the surface layer but that they are also present in the mesopelagic zone, located between 200 and 1,000 meters below the surface.

... so useful

This calls into question the consensus that these microorganisms are recycled in the surface ocean, and therefore do not participate in the biological pump, the series of processes that lead to the transport of carbon from the surface zone to the sea floor. Through this pump, the ocean absorbs and sequesters part of the anthropogenic CO2 emitted by the combustion of fossil fuels. "Diazotrophs absorb CO2 from the atmosphere via photosynthesis and transform it into organic matter that then sinks to the depths and is stored in the sediments for millennia. Scientists previously thought that the carbon ingested by diazotrophs was essentially recycled at the surface and then returned to the atmosphere," continues the researcher. These microorganisms would thus play an essential role in the biological pump in these vast desert areas. It is important to study this process because these so-called desert areas are expanding with global warming and will probably be part of our future oceans.

For highlighting this process, Sophie Bonnet was awarded the 2019 "Christian Le Provost" grand prize for oceanography. She then set out to solve two obstacles to understanding the influence of diazotrophs on the biological pump. It remains extremely complex to quantify trophic flows within the food chain, and thus to quantify and model the proportion of carbon derived from diazotrophs that falls to the deep ocean. On the other hand, current microbiological observation methods - which measure the ocean on a weekly or monthly scale - do not allow to capture with enough finesse the processes involved, which are done on an hourly or daily scale.

Imagining the ocean of tomorrow

The oceanographer is developing the HOPE project, for which she will receive an ERC Consolidator grant (see box) in 2022. This project includes the deployment of an intelligent profiling buoy equipped with high-tech sensors, some of which are developed on this occasion. It will allow us to scan the surface and bottom oceans simultaneously, at high frequency (hourly and daily), and to resolve the complexity of the microbiological processes involved and the factors of their variability with a finesse never before achieved.

The oceanographer hopes to deploy this buoy for three years to obtain 6,000 measurement points, i.e. measurements every four hours between 0 and 100 meters deep, in both tropical and temperate zones. These data will then be used to develop global maps of deep-sea carbon export attributed to these microorganisms.

IPCC modellers are beginning to take an interest in diazotrophs, which have just been identified as key players in maintaining the productivity of the warmer, more stratified ocean of the future," adds Sophie Bonnet. The component developed in the ERC focuses on what happens next, what happens to these diazotrophs in the ocean, and their ability to sequester CO2. Given their likely future importance, we need to understand their ability to sequester carbon dioxide in today's ocean, to understand and model their role in tomorrow's ocean."

Thanks to the 2.5 million euro funding, Sophie Bonnet will also be able to develop an automated water column named SOCRATE (for Simulated OCean wateR column with AutomaTEd sampling) designed for the project with engineers Jean-Michel Grisoni and Julien Vincenti. She will be able to study the sedimentation of diazotrophs and trophic fluxes both in the laboratory in Marseille and in the South Pacific, where SOCRATE will embark on oceanographic expeditions planned from 2024. In the meantime, Sophie Bonnet is enthusiastically preparing her new departure from the coasts of Marseille to those of the Pacific, in the company of her family, including her six year old twin daughters.