A study into marine micro-organisms known as diazotrophs show they capture CO2 from the atmosphere and take it with them to the ocean floor when they die

A new study has revealed that diazotrophs, a group of marine cyanobacteria that are able to convert nitrogen gas (N2) from the atmosphere into nutrients for primary producers in the ocean, contribute directly to the carbon export and sequestration on the seafloor.

The results of the work lead by the French Mediterranean Institute of Oceanography (MIO), carried out as part of the TONGA ocean expedition, represent a major paradigm shift, since until now, while it was known that these micro-organisms ‘recycled’ the CO2 captured from the atmosphere, experts were unaware that they also took the CO2 with them when they died and sank to the ocean floor. It is exactly what plant plankton (phytoplankton) also does.

Francisco Cornejo is a researcher at the Institut de Ciències del Mar (ICM-CSIC) and of the authors of the study.

He said: “This process is known as the biological carbon pump, and until now was attributed mainly to phytoplankton, which converts CO2 into organic matter during the photosynthesis.

“When it dies, the carbon sinks with these micro-organisms, storing on the seabed approximately twice as much carbon as is currently found in the atmosphere.”

In turn, organisms at higher levels of the marine food web use the same organic matter to survive, thus ensuring the functioning of the entire oceanic system.

In fact, thanks to the biological carbon pump, the ocean is considered a carbon sink.

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accurate picture

The results of the study have important implications for science because, at present, global biogeochemical models, i.e. the tools used to make predictions about the evolution and fluxes of carbon on the planet, do not take into account the direct contribution of diazotrophs in this process.

MIO researcher Sophie Bonnet initiated the collaborative study.

She said: “Our results will provide us with a more accurate picture of carbon fluxes in the ocean, which is particularly relevant at a time when climate models predict an expansion of nitrogen-poor zones, where diazotrophs thrive.”

To carry it out, researchers collected hundreds of samples with sediment traps installed at different depths during a campaign in the south Pacific, which were later analysed using microscopy, sequencing and DNA quantification techniques.

They noticed that the particles that sink from the surface to the seabed, in addition to phytoplankton organisms, contain a great quantity and diversity of these diazotrophs.

All this has made it possible to quantify, for the first time, the role of these micro-organisms in the global biological carbon pump.

For future research, experts will try to delve deeper into the role of diazotrophs in the biological carbon pump, paying special attention to the routes that these micro-organisms undergo during their sinking in different oceanic regions.

The research is published in ISME journal.

Image: Colony of the filamentous diazotroph Trichodesmium sp. in surface waters of the South Pacific Ocean. © Sophie Bonnet, IRD.