According to a new study published in The ISME journal, diazotrophs, a group of marine cyanobacteria capable of converting nitrogen gas (N2) from the atmosphere into nutrients for primary producers in the ocean, directly contribute to carbon export and sequestration on the seafloor.
The findings of this research, led by the French Mediterranean Institute of Oceanography (MIO) and carried out as part of the TONGA ocean expedition, represent a major paradigm shift, because it was previously thought that these microorganisms “recycled” the CO2 captured from the atmosphere, but experts were unaware that they also took the CO2 with them when they died and sank, as do the rest of the phytoplankton (i.e., plant plankton).
“This process is known as the biological carbon pump, and it was previously attributed primarily to phytoplankton, which converts CO2 into organic matter during photosynthesis, and when it dies, the carbon sinks with these microorganisms, storing approximately twice as much carbon as is currently found in the atmosphere,” explains Francisco Cornejo, researcher at the Institut de Ciències del Mar (ICM-CSIC) and one of the study’s authors.
In turn, higher levels of the marine food web use the same organic matter to survive, ensuring the overall functioning of the oceanic system. In fact, the ocean is considered a carbon sink due to the biological carbon pump.
The paradigm shift
The study’s findings have important implications for science because global biogeochemical models (the tools used to predict the evolution and fluxes of carbon on the planet) do not currently account for the direct contribution of diazotrophs in this process.
“Our findings will give us a more accurate picture of carbon fluxes in the ocean, which is especially important given that climate models predict an expansion of nitrogen-poor zones, where diazotrophs thrive,” says MIO researcher Sophie Bonnet, who initiated this collaborative study.
During a campaign in the South Pacific, researchers collected hundreds of samples using sediment traps installed at various depths, which were then analysed using microscopy, sequencing, and DNA quantification techniques.
They discovered that, in addition to phytoplankton organisms, the particles that sink from the surface to the seabed contain a large number and diversity of these diazotrophs. All of this has enabled researchers to quantify the role of these microorganisms in the global biological carbon pump for the first time. Experts will try to delve deeper into the role of diazotrophs in the biological carbon pump in the future, paying special attention to the routes that these microorganisms take while sinking in different oceanic regions.