A New Role for Nitrogen Fixers in Oceanic Carbon Sequestration

Paper: Diazotrophs are overlooked contributors to carbon and nitrogen export to the deep ocean

Authors: Sophie Bonnet, Mar Benavides, Frédéric A. C. Le Moigne, Mercedes Camps, Antoine Torremocha, Olivier Grosso, Céline Dimier, Dina Spungin, Ilana Berman-Frank, Laurence Garczarek, and Francisco M. Cornejo-Castillo

You swell up and down with each salty wave, until suddenly, the thought of lunch triggers a pang of hunger. A ray of sunshine stifles the starvation, and you take a satiating bite of carbon dioxide with a side of nitrogen. As a nitrogen-fixing, photosynthetic bacterium you use energy from sunlight to convert nitrogen gas to necessary nutrients for yourself and your neighbors.

Photosynthesizers in the ocean play a crucial role in regulating the earth’s climate in the face of climate change. Through a process called the biological carbon pump, the carbon dioxide that these microbes incorporate into their cells can sink to the bottom of the ocean, keeping that carbon out of the atmosphere for decades to millions of years. However, more than carbon dioxide and sunlight are necessary to prime this pump. These cells need nitrogen, and a category of microbe called a diazotroph converts nitrogen gas in the atmosphere to the nitrogen currency that most cells can use: ammonia.

For a long time, these nitrogen-converting microbes – diazotrophs –  were thought to only play an indirect role in carbon sequestration by marinemicrobes. Scientists thought that diazotrophs supplied the oceanic food web with nitrogen but wouldn’t themselves be the ones to sink down and store carbon in the deep ocean. A new open access paper, first authored by Sophie Bonnet at Université de Toulon in France, calls this paradigm, and its implications for climate change, into question.

On a ship in the Pacific Ocean, Bonnet and the team collected samples of water in an enormous 1.5-meter-tall sampler. After retrieving the water, they let it sit on board the ship for 2 hours. This allowed the microbes that were sinking toward the deep ocean to separate. They then collected water at the very bottom of the sampler to represent the quickly sinking cells, the water just above the bottom of the sampler to represent the slower-sinking cells, and the water from the top to represent cells that didn’t sink. Sinking can be a result of cells being eaten by zooplankton and excreted in fecal pellets, of cells sticking together into larger agglomerates, or of metabolic processes decreasing buoyancy. By repeating this process at different locations and different depths, Bonnet was able to determine which diazotrophs were present and which ones were actively sinking.

To identify the various diazotrophs present, the team amplified and sequenced a gene called nifH, which codes for a part of the nitrogenase enzyme. This enzyme catalyzes the conversion of di-nitrogen gas to ammonia – a defining characteristic of diazotrophs. This single gene amplification and sequencing can be done qualitatively as a survey to determine which genes are present in that environment, or can be performed quantitatively, to understand the relative number of copies of the gene in that environment. This paper employed both techniques.

Using the nifH sequences, the authors identified two main groups of diazotrophs in their samples – photosynthetic (cells that can photosynthesize and fix nitrogen) and non-photosynthetic. The photosynthetic diazotrophs primarily consisted of Trichodesmium, a large filamentous cyanobacteria, and a diverse group of unicellular cyanobacteria abbreviated UCYN. Both of these groups are common marine diazotrophs. Surprisingly, in the qualitative surveys, the majority of the non-photosynthetic diazotrophs were novel and unidentified. The paper disclaims that their quantitative analyses would not pick up these unidentified nifH sequences, and therefore no claims about their abundance could be made.

The team found that the same diazotrophs present in the surface water (identified by their nifH sequences) were also present at depths of 1000m, indicating that these nitrogen-fixing cells do sink to the deep ocean. While Trichodesmium and UCYN were both abundant in shallow water, samples of the settled material from the 1.5m water sampler indicated that the smaller UCYN cells were sinking more efficiently. This seemingly contradicts a previously-held idea that large cells sink faster than small ones, and therefore contribute more to carbon sequestration. To unravel this apparent paradox, the team looked at material that was settling toward the deep ocean under the microscope. They saw UCYN cells clumped into large aggregates of hundreds of cells, this large particle size likely explaining their efficient sinking. Aggregation of organic material is crucial in facilitating the transport of carbon to the deep ocean. The fact that ubiquitous diazotrophs, such as UCYN cells, are clumping together and sinking was unexpected and indicates that these organisms contribute more than anticipated to the biological carbon pump

Bonnet and the team’s findings lay the groundwork to redefine the role that marine diazotrophs play, both in the marine food web and in sequestering carbon. UCYN cells are some of the most abundant diazotrophs in the ocean and this research indicates that they are efficiently exported from the upper ocean to depth. While Trichodesmium sinks less efficiently, they were also found sinking and at depths that indicate that this ubiquitous diazotroph is also directly involved in the biological carbon pump. Additionally, the discovery of unidentified nifH sequences implies that there may be other major diazotrophs at play. Once thought to only ‘prime’ the biological carbon pump by providing nutrients to other photosynthesizers, diazotrophs themselves may be sinking, bringing carbon to the deep ocean, and helping to counteract climate change!

A New Role for Nitrogen Fixers in Oceanic Carbon Sequestration © 2024 by  William Christian is licensed under CC BY-SA 4.0