Could marine micro-organisms absorb more CO2 than we think? And so help reduce global warming? That's the view of oceanographer Sophie Bonnet, who has just been awarded a European ERC Consolidator grant to fund HOPE, her research project on this subject. Meet a woman who is passionate about tropical ocean deserts, these nutrient-poor areas, and their hidden riches.
Sophie Bonnet grew up in a sailing family. When she was very young, she often drew herself on a boat, surrounded by blue waters and sea creatures. It was an obsession that was soon to 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 crystallised my dreams", she recalls. After my baccalaureate, I embarked on a long course of studies right through to a doctorate. But before that, I went around the world for a year on a sailing boat, sailing to West Africa and Madagascar. With navigator Michel Huchet, we wanted to 'sail for a cause', and give other sailors the chance to do the same. So we set up the humanitarian association Voiles sans frontières, which provides medical and educational aid to isolated populations that are only accessible by sea and river in small boats. I was then president of this association for ten years.
Tiny organisms...
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 fertilisation". Although she trained as a biogeochemist, the student wanted to work on subjects of societal interest, in particular the ocean's capacity to absorb carbon dioxide (CO2).
"I became fascinated by diazotrophs, plankton microorganisms that supply nitrogen to the entire food chain. Under certain conditions, they can create veritable oases of life in areas considered to be 'ocean deserts', which represent 60 % of the global ocean surface," she explains. There wasn't much 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!
On her return from the United States in 2007, the post-doctoral researcher was recruited by the IRD's UMR MIO and moved to Nouméa a few years later, where she was free to study diazotrophs, which are mainly found in the tropics, and their interactions with the carbon cycle. The young oceanographer embarked on a series of projects and expeditions. She then noticed that these micro-organisms are not only found in the surface layer but that they are also present in the meso-pelagic zone, at a depth of between 200 and 1,000 metres.
... so useful
This calls into question the consensus that these micro-organisms are recycled in the surface ocean and therefore do not participate in the biological pump, the series of processes that transport carbon from the surface zone to the seabed. Thanks to this pump, the ocean absorbs and sequesters some 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, which then sinks to the depths and is stored in sediments for thousands of years. 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 micro-organisms therefore 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 zones are expanding with global warming and will probably form part of our oceans of the future.
Sophie Bonnet was awarded the Christian Le Provost Grand Prize in Oceanography in 2019 for her work on this process. She then set about solving two obstacles to understanding the influence of diazotrophs on the biological pump. On the one hand, it remains extremely complex to quantify trophic flows within the food chain, and therefore 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 capture with sufficient precision the processes involved, which take place 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 being developed as part of the project. This will enable the surface and bottom oceans to be scanned simultaneously, at high frequency (on an hourly and daily scale), and to resolve the complexity of the microbiological processes involved and the factors contributing to their variability with a degree of precision never before achieved.
The oceanographer hopes to deploy this buoy over a three-year period to obtain 6,000 measurement points, i.e. measurements every four hours between 0 and 100 metres depth, in both tropical and temperate zones. This data will then be used to draw up global maps of carbon exports to the deep sea attributed to these micro-organisms.
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 part developed in the ERC focuses on what happens afterwards, the fate of these diazotrophs in the ocean, and their capacity to sequester CO2. Given their likely future importance, we need to understand their capacity to sequester carbon dioxide in today's ocean, in order to understand and model their role in tomorrow's ocean.
Thanks to the 2.5 million euros in funding, Sophie Bonnet will also be able to develop an automated water column called 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 flows both in the laboratory in Marseilles and in the South Pacific, where SOCRATE will be on board oceanographic expeditions planned from 2024. In the meantime, Sophie Bonnet is enthusiastically preparing for 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.