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 

Impacts of global warming on soil carbon storage, biodiversity, and crop yields

Image credit: Public Domain (Pexels)

Paper: Soil organic carbon loss decreases biodiversity but stimulates multitrophic interactions that promote belowground metabolism.

Authors: Ye Li, Zengming Chen, Cameron Wagg, Michael J. Castellano, Nan Zhang, Weixin Ding.

Few issues are as pressing and relevant for the future of our own species as climate change. We may think first about glaciers and polar bears when we consider its devastating impacts. However, new research brings our attention to much smaller organisms, microbes, as major players in stabilizing soils and preventing agriculture from collapsing under global warming.

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Making Mountains Out of Molehills? Long-Term Geomorphic Surface Impacts of Mountaintop Removal Mining

Featured Image:  Mountaintop removal mining site in Appalachia. Copyright: CC BY-SA 4.0 via. wikimedia commons.

Report: Peripheral gully and landslide erosion on an extreme anthropogenic landscape produced by mountaintop removal coal mining (2020)

Authors: Miles Reed & Dr. Steve Kite

There’s a general consensus that coal mining is ‘bad’ for the environment, but beyond carbon emissions, what is its visible, physical impact on our surroundings? What lasting damage does mining create on the Earth’s surface? The answer is that it has a tremendous impact; specifically, mining in Appalachia is linked to distorting the natural flow of water on the landscape, which creates ripple (no pun intended) effects on the greater environment. A recent study by Reed and Kite details those effects on Appalachian landscapes, directly linking mountaintop mining to erosion and landslides. Now, as worries about access to safe, clean water being jeopardized by fossil fuel production abound nationwide, exploring the impacts of mountaintop mining on Appalachian freshwater becomes incredibly important with immediate and personal impacts.

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How Much is War Fuelling the Climate Crisis?

Featured Image: Global militaries are a major contributor to climate change, however, we face many challenges when assessing their environmental footprint. Copyright: CC BY-SA 4.0 via. wikimedia commons.

Report: Estimating the Military’s Global Greenhouse Gas Emissions (2022)

Authors: Dr. Stuart Parkinson & Linsey Cottrell

Organisations: Scientists for Global Responsibility & Conflict and Environment Observatory

War is likely to worsen in the near-future as climate change forces more disasters, political instability, and poverty onto the planet and strains resource supplies. Yet war is not just a product of climate change: it is also a major cause. In addition to the societal devastation it creates, militarism is a major emitter of greenhouse gases and contributor to environmental degradation. Politicking from the worst emitters has ensured that military emissions are shielded from the same type of accountability seen across other sectors such as agriculture, transport, land use, technology, and waste. For example, the latest installment of the IPCC report barely mentioned military emissions despite its immensely detailed analysis of other sectors. A recent report from Stuart Parkinson (Scientists for Global Responsibility) and Linsey Cottrell (Conflict and Environment Observatory) helps correct this oversight and unpacks the impact of war on climate change.

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How sediments can save drowning river deltas

Featured image: A satellite image of the Ganges – Brahmaputra delta along the Bangladesh coastline.captured by the Envisat satellite of the European Space Agency (ESA). The image also shows sediment plumes in the coastal area. (Image credit: ESA CC BY-SA 3.0 IGO)

Paper: Sediment delivery to sustain the Ganges- Brahmaputra delta under climate change and anthropogenic impacts

Authors: Jessica L. Raff, Steven L. Goodbred Jr., Jennifer L. Pickering, Ryan S. Sincavage, John C. Ayers, Md. Saddam Hossain, Carol A. Wilson, Chris Paola, Michael S. Steckler, Dhiman R. Mondal, Jean-Louis Grimaud, Celine Jo Grall, Kimberly G. Rogers, Kazi Matin Ahmed, Syed Humayun Akhter, Brandee N. Carlson, Elizabeth L. Chamberlain, Meagan Dejter, Jonathan M. Gilligan, Richard P. Hale, Mahfuzur R. Khan, Md. Golam Muktadir, Md. Munsur Rahman, Lauren A. Williams

The Ganges – Brahmaputra delta is the largest river delta in the world, covering an area of 1,00,000 sq. km. About two-thirds of the delta lies in Bangladesh, and the rest in the Indian state of West Bengal. Today, sea level rise due to climate change poses a massive challenge to the delta region which more than 200 million people call home!

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Nature’s Secret Weapon: How Nature-based Solutions Can Tackle Climate Change and More

Featured Image: Two striking illustrations of the river Culm catchment in the UK. Created by local artist Richard Carman, the left image shows the existing (degraded) situation, while the right image depicts a co-created nature-based solutions scenario developed in collaboration with local stakeholders, including farmers and landowners, as part of the Co-Adapt project. These illustrations provide a clear visual representation of how nature-based solutions can be used to address environmental challenges in the area.

Papers: Soil carbon sequestration impacts on global climate change and food security; Climate-smart Soils; IPCC (2014) Report on Mitigation of Climate Change; Synthesizing US River Restoration Efforts; Limited potential of no-till agriculture for climate change mitigation; Sequestering carbon in soils of agro-ecosystems; Crop Residue Removal Impacts on Soil Productivity and Environmental Quality; Towards an EU research and innovation policy agenda for nature-based solutions & re-naturing cities.

Authors: Rattan Lal, Keith Paustian, Johannes Lehmann, Stephen Ogle, David Reay, Philip G. Robertson, Pete Smith, Humberto Blanco-Canqui and more.

Are you worried about the impact of climate change on our planet and wondering what you can do to help? Look no further than nature itself, because nature-based solutions may just hold the key to mitigating its effects through soil carbon sequestration.

Climate change is an ongoing problem that poses a significant threat to our planet. Many strategies have been proposed to mitigate climate change, including renewable energy, carbon capture and storage, and nature-based solutions (NbS). Among these, NbS have gained considerable attention because they offer a range of benefits, including reducing greenhouse gas emissions, mitigating the impact of natural disasters such as floods and droughts, and improving biodiversity.

But what are NbS, and how can they help in mitigating climate change? Nature-based solutions are interventions that work with nature to address environmental challenges. These solutions involve restoring, protecting, and managing ecosystems such as forests, wetlands, and grasslands. One of the significant benefits of NbS is soil carbon sequestration, which refers to the process of capturing carbon dioxide from the atmosphere and storing it in soil.

Soil carbon sequestration is a powerful tool to mitigate climate change because it can store carbon for decades or even centuries. According to the Intergovernmental Panel on Climate Change (IPCC), soil carbon sequestration can reduce atmospheric carbon dioxide concentrations by up to 15% by 2050. This approach has gained traction in Europe, where various projects have been implemented to sequester carbon in soils.

For example, in the UK, the Farm Carbon Cutting Toolkit is a non-profit organization that works with farmers to adopt practices that increase soil carbon levels. One such practice is the use of cover crops, which are planted between cash crops to prevent soil erosion, improve soil health, and increase carbon sequestration. According to the organization’s website, “the planting of cover crops, such as clover, can increase soil organic matter and carbon content by up to 15% over ten years.”

Similarly, in France, the 4 per 1000 initiative aims to increase soil carbon content by 0.4% per year. This initiative focuses on a range of NbS, such as agroforestry, conservation agriculture, and the use of biochar. According to a study published in the journal Nature, increasing soil carbon by 0.4% per year could offset around 3.5 billion tonnes of carbon dioxide emissions.

Soil carbon sequestration through NbS not only helps mitigate climate change but also has several co-benefits. For example, it can improve soil health, increase agricultural productivity, and reduce the risk of natural disasters such as floods and droughts. As Dr. Pauline Chivenge, a soil scientist at the University of Zimbabwe, explains:

”If we improve soil health, we can improve crop yields, and that translates into better nutrition and food security for communities”

However, it’s important to note that soil carbon sequestration alone cannot solve the climate crisis. We also need to reduce our reliance on fossil fuels, promote renewable energy, involve the local community and implement other sustainable practices. Nonetheless, soil carbon sequestration is an important piece of the puzzle and should be considered as part of a comprehensive climate action plan.

In conclusion, nature-based solutions such as soil carbon sequestration offer a promising strategy for mitigating climate change while providing multiple benefits. By implementing NbS practices such as agroforestry, cover crops, and conservation agriculture, we can increase soil carbon levels, improve soil health, and enhance biodiversity. By implementing NbS practices, we can all contribute to mitigating the impacts of climate change and promoting sustainable development. Here are some ways you can get involved:

  1. Educate yourself: Learn about the benefits and potential of nature-based solutions in addressing environmental challenges. Read about case studies, best practices, and research on nature-based solutions.
  2. Advocate for nature-based solutions: Speak up about the benefits of nature-based solutions in conversations with family, friends, colleagues, and community members. Encourage local leaders to consider nature-based solutions in planning and decision-making.
  3. Support conservation efforts: Donate to conservation organizations or volunteer for conservation efforts in your community. Protecting natural areas can support nature-based solutions and the ecosystem services they provide.
  4. Plant trees and native plants: Trees and native plants play an important role in sequestering carbon, improving air and water quality, and supporting biodiversity. Planting trees and native plants in your yard or community can support nature-based solutions.
  5. Support sustainable agriculture: Sustainable agriculture practices, such as agroforestry and regenerative agriculture, can support nature-based solutions by promoting soil health, biodiversity, and carbon sequestration.
  6. Participate in citizen science: Citizen science projects can provide valuable data for understanding environmental challenges and the effectiveness of nature-based solutions. Participate in citizen science projects in your community or online
  7. Support green infrastructure: Green infrastructure, such as green roofs and bioswales, can support nature-based solutions by reducing stormwater runoff and improving air quality. Encourage your community to invest in green infrastructure or start from your own garden by removing paved surfaces and replacing them with greenery, make your own compost etc..
  8. Support policies and funding for nature-based solutions: Policy changes and funding can help support the uptake of nature-based solutions at local and national levels. Support policies and funding initiatives that promote nature-based solutions.

By taking action and supporting NbS practices, we can all make a difference in the fight against climate change. As Dr. Bedford, a climate change expert, reminds us:

”We all have a role to play in addressing the challenges of climate change, and implementing nature-based solutions is one of the most effective ways to do so.”


Nature’s Secret Weapon: How Nature-based Solutions Can Tackle Climate Change and More by Borjana Bogatinoska is licensed under a Creative Commons Attribution 4.0 International License.

Eunice Foote, the original founder of climate change dynamics

Featured Image: Artist rendition of Eunice Foote conducting research on compressed gasses. Image courtesy Carlyn Iverson, NOAA.  Featured image courtesy GNU Free Documentation License

Papers: Circumstances affecting the heat of the Sun’s rays; Understanding Eunice Foote’s 1856 experiments: heat absorption by atmospheric gases

Authors: Eunice Foote; Joseph Ortiz and Ronald Jackson

“An atmosphere of [carbon dioxide] would give our Earth a high temperature.”

These words were spoken out loud in August of 1856 at the 10th annual meeting of AAAS, though not by their author. The speaker continues on to suggest that, “[if] at one period of its history the air had mixed with [carbon dioxide] a larger proportion than at present, an increased temperature…must have necessarily resulted.” This paper was the first recorded finding of the link between carbon dioxide and global warming, and was discovered by the female physicist and scientist, Eunice Foote. While these findings were remarkable on their own, she synthesized the implications to correctly state that carbon dioxide concentrations in the atmosphere both increase global warming and can explain Earth’s geologic history, specifically regarding the Devonian period1,2.  Despite being on the sidelines of science at the time because of her gender, Eunice Foote provided fundamental and groundbreaking knowledge in the field of gaseous physics. 

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One Lake, Two Lake; Green Lake, Blue Lake

A large lake divided by a shallow spit of land, water to the left of the spit appears green and murky, the right side clear and blue.

Paper: Shallow lakes under alternative states differ in the dominant
greenhouse gas emission pathways

Authors: Sofia Baliña, María Laura Sánchez, Irina Izaguirre, Paul A. del Giorgio (2023)

Imagine some of the most dynamic, ecologically important lakes in the world…. you are picturing a deep, wide lake, not something knee deep and murky, or so full of aquatic plants you can’t see the bottom, right? Well, perhaps you should; while they don’t always make the most inviting swimming holes, small, shallow lakes have an outsized importance in the cycling of carbon and other nutrients through the landscape. 

Shallow depths tend to lead to warmer temperatures and more concentrated growth of algae and aquatic plants, not always the most desirable features for recreation.  But what these lakes might lack aesthetically, they make up for with a massive contribution to the global carbon cycle. Combine the abundance of small lakes with a tendency for frequent mixing of the water column, and high rates of organic input from the surrounding watershed and small lakes pack a big punch in terms of cycling nutrients, including carbon, through pathways in both the water and lake bottom sediments. 

These carbon cycling power houses are tricky to pin down because they can operate in what scientists call two different ‘stable states’: a murky, turbid state, dominated by algal growth that blocks the sunlight from reaching the bottom, and a clearwater state where plants anchored in the lake bottom sediments are dominant. A number of natural events, including floods, droughts, or changes in surrounding vegetation can lead to a ‘flip’ between states. Human activity can lead to a ‘flip’ as well, for example, in the Pampean Plains of Argentina, agricultural practices have added excess nutrients to the system, which tends to push lakes toward the murky, turbid state. The two lake states not only look different from the surface, but also have important differences in rates of photosynthesis, burial of organic material, and circulation in the water.

Knowing the importance of small lakes to global carbon cycling, a team in Argentina did a detailed investigation on how the different states impact carbon cycling and green house gas emissions.  By monitoring sets of turbid and clear shallow lakes in the Pampean Plains over the course of a year, they found important seasonal differences in rates of carbon dioxide (CO2) diffusion into and out of water column, and in the flux of methane (CH4) from lake bottom sediments.

Through monitoring instrumentation suspended in the air above the lakes, as well as measurements taken in the water and sediments, researchers were able to observe weather-driven seasonal changes. The biggest differences were between winter and spring: cold, clear lakes tended to act as CO2 source. When the lakes warmed up, they started to move gas from the water into the atmosphere and became carbon sinks, while turbid lakes did the opposite. 

Figure 3 from Baliña et al. (2022) showing the different pathways and relative ratios for carbon flow in clear-water, vegetated lakes (on the left) compared to more green, or turbid, lakes with heavy algal growth on the right. In total, the total greenhouse gas emissions (or CO2 equivalents) for both lake states was similar, but came from different pathways in the lake.

Over an annual cycle, clear lakes had as much as 5 times the CO2 emissions to the atmosphere as compared to turbid lakes, mainly attributed to the vegetation. Turbid lakes, however, had a higher annual emission of CH4. On balance, the two groups of lakes had roughly the same total contribution to green house gas fluxes, but the seasonal variability and differences in carbon pathway are important to understand as we continue to learn more about these dynamic ecosystems and how they change over time.


One Lake, Two Lake; Green Lake, Blue Lake by Avery Shinneman is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Forests under (mega)fire in the Pacific Northwest

Accompaniment to the Third Pod from the Sun Episode

Featured Image: “Forests under fire” original artwork by Jace Steiner. Used with permission.

Paper: Cascadia Burning: The historic, but not historically unprecedented, 2020 wildfires in the Pacific Northwest, USA

Authors: Matthew Reilly, Aaron Zuspan, Joshua Halofsky, Crystal Raymond, Andy McEvoy, Alex Dye, Daniel Donato, John Kim, Brian Potter, Nathan Walker, Raymond Davis, Christopher Dunn, David Bell, Matthew Gregory, James Johnston, Brian Harvey, Jessica Halofsky, Becky Kerns

The natural legacy of fire in the Pacific Northwest (PNW) is complex.  The variable geography of the wet, westside temperate rain forests, to the dry, high elevation forests beyond the Cascade crest make it difficult to find a “catch-all” description of PNW forest fires.  For instance, drier forests of ponderosa pines in eastern Washington experience more frequent, low-severity fires while the temperate rain forests of western Oregon rarely see fires.  However, scientists can reconstruct historical fire regimes and identify centuries-long patterns of burning related to precipitation, temperature, and ignition frequency to define what are historical patterns and what is modern climate change.  In 2020, multiple megafires (a wildfire that burnt more than 100,000 acres of land) broke out in the typically wet parts of Oregon and Washington, burning more than 700,000 acres combined.  This event is called the 2020 Labor Day Fires, and Matthew Reilly and colleagues have revealed these fires were likely part of historical regimes and not a product of accelerated climate change.

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Getting to Grips With the Sixth Mass Extinction

Featured Image: It is well-understood that the Earth’s biodiversity is in severe decline. However, it is less clear if this decline can now be called a mass extinction. Public domain image via. The Wilderness Society.

Paper: The Sixth Mass Extinction: fact, fiction, or speculation?

Authors: Robert H Cowie, Philippe Bouchet & Benoît Fontaine

Human-driven emissions and land use changes have impacted Earth’s biosphere greatly, causing global extinction rates to climb fast. However, does the current undeniable biodiversity crisis meet the requirements to be called a mass extinction? 

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