The ocean is a carbon sink, with major implications for climate change. Among the phenomena involved in CO2 sequestration by the ocean, plant plankton (or phytoplankton) absorb CO2 through photosynthesis and produce organic matter made up of carbon, which is transferred along the marine food chain. When the organisms die, some of this carbon sediments to the bottom of the oceans in the form of marine snow, thus subtracting CO2 from the atmosphere. This is known as the biological carbon pump. New studies show that this pump could capture more carbon than expected.
The (sub)tropical ocean (~50% of the global ocean surface) is considered to be inefficient at trapping CO2, as it is low in nitrogen (an essential nutrient), which limits the growth of phytoplankton. Yet these vast regions are home to organisms known as 'diazotrophs', which fix CO2 and also fertilise surface waters with nitrogen, thereby supporting the food chain. However, it is now accepted that diazotrophs do not sink to the deep ocean, but are recycled in the surface layer, returning their CO2 to the atmosphere. Studies carried out as part of the TONGA project (South Pacific) are challenging this paradigm: by taking measurements in the deep ocean using a combination of tools that collect marine snow, a research team from an INSU field (see box) has demonstrated that diazotrophs fall towards the deep ocean, contributing to most of the carbon export flow. The study also revealed that the organisms are little degraded, or even virtually intact, at this depth, suggesting a rapid fall and therefore little recycling of CO2 during the descent. A complementary study reveals that some of these organisms (Trichodesmium) are still alive at a depth of 1000 m, confirming their rapid descent (several hundred metres per day) towards the deep ocean, where this carbon will be trapped over the long term. This work calls for a detailed exploration of the role of diazotrophs in the biological carbon pump, in order to understand their potential role in the ocean of the future, which will be carried out as part of the HOPE1 project.
CNRS laboratory involved
Mediterranean Institute of Oceanology (MIO / Osu Pythéas)
Supervisory bodies: CNRS / IRD / AMU / Univ. of Toulon
To find out more
References
Bonnet, S., Benavides, M., Le Moigne, F.A.C. et al. Diazotrophs are overlooked contributors to carbon and nitrogen export to the deep ocean. ISME J (2022).
Benavides, M., Bonnet, S., Le Moigne, F.A.C. et al. Sinking Trichodesmium fixes nitrogen in the dark ocean. ISME J 16, 2398-2405 (2022).
Notes
HOPE project 'How do diazotroph shape the ocean biological carbon pump': A global approach from the single-cell to the ecosystem. ERC CoG 2023-2028 project coordinated by Sophie Bonnet.
Contact
Sophie Bonnet
IRD Research Director at the Mediterranean Institute of Oceanology (MIO)
sophie.bonnet@mio.osupytheas.fr