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Using a mathematical model, an international team of researchers has been able to show that heterotrophic bacteria inside sinking particles can fix nitrogen in nearly every part of the global ocean. The scientists from Bremen (Germany) and Copenhagen (Denmark) estimate that these types of specialised bacteria account for 10% of global marine nitrogen fixation. Their findings have just been published in the high-impact journal Science Advances.
Nitrogen is essential for life, enabling marine organisms to survive, grow, and reproduce. Although abundant in seawater, most marine species cannot use nitrogen gas (N₂) directly. Specialised bacteria convert it into ammonium, a form of nitrogen that is accessible to marine photosynthesizing organisms. This process is called nitrogen fixation. It supports the marine food web by supplying nitrogen and plays an important role in the ocean’s ability to store carbon dioxide.
Historically, photosynthetic cyanobacteria living in warm, sunlit, low-latitude waters were thought to be the primary organism responsible for nitrogen fixation. Recent discoveries have identified non-photosynthetic heterotrophic bacteria that also carry out nitrogen fixation. These bacteria are attached to sinking particles, or marine snow, and use organic material for energy and nitrogen fixation.
Unlike cyanobacteria, heterotrophic bacteria are found in diverse environments, from tropical to polar waters and from surface to deep ocean waters. This widespread presence has puzzled scientists for some time.
Despite their ecological importance, heterotrophic nitrogen fixers are still poorly understood as they are difficult to grow and study in the lab. An international team of researchers from the Leibniz Centre for Tropical Marine Research (ZMT) in Germany, the University of Copenhagen and the Technical University of Denmark worked together to understand how heterotrophic bacteria adapt to survive and carry out nitrogen fixation across different oceanic conditions.
“We asked ourselves how different environmental factors come together to regulate the nitrogen fixation by these heterotrophic bacteria,” says ZMT researcher Subhendu Chakraborty, a mathematical modeller specializing in marine ecosystems and lead author of the publication. “We wanted to find out how widespread their nitrogen-fixing activity is and how much these bacteria contribute to the overall nitrogen fixation in the global ocean.”
+++Modelling shows the contribution of heterotrophic bacteria to the global budget of nitrogen fixation+++
Since it was not an option to study nitrogen-fixing heterotrophic bacteria in the lab, the researchers relied on mathematic modelling to find answers to their questions.
“Our new model captures nitrogen-fixing heterotrophic bacteria living inside sinking particles, showing how they regulate nitrogen fixation to support growth. The model reveals how these mechanisms drive the global distribution of nitrogen fixation in sinking particles,” explains co-author Agostino Merico, a systems ecologist and modeller at ZMT.
Through their calculations, the scientists made some exciting discoveries summarised by ZMT’s Subhendu Chakraborty: “For the first time we can now estimate that heterotrophic bacteria are responsible for about 10% of the nitrogen fixation in the world's oceans. This is a large contribution to the total global budget of nitrogen fixation. The model also shows us that about 85% of this contribution occurs in tropical oxygen minimum zones.”
Co-author Lasse Riemann, a marine biologist from the University of Copenhagen, adds: “Nitrogen-fixing cyanobacteria are generally not active in tropical oxygen minimum zones. As these zones are expanding due to climate change, understanding the role of heterotrophic nitrogen fixers can help us to predict how nutrient cycles and ocean ecosystems might respond to a warming planet.”
“While the presence of their genetic material in the oceans can only give us a general idea of how common nitrogen-fixing heterotrophic bacteria are, it doesn't provide detailed information about how many of them are actively present or how much nitrogen they are fixing. Using our model, we were able to overcome these limitations and gain a better understanding of the nitrogen-fixing heterotrophic bacteria and their activity,” says Ken Andersen, an expert in theoretical marine ecology at the Technical University of Denmark, who also contributed to the study.
Agostino Merico explains this further: “Our model shows that heterotrophic bacteria can fix nitrogen across a wide range of temperatures, spanning from -2 to 24 °C. In comparison, the temperature range of cyanobacterial nitrogen fixers was previously found to be approximately 18 to 38 °C. This helps to explain why heterotrophic nitrogen fixers are found throughout the oceans whereas cyanobacterial nitrogen fixers are only limited to the warm tropical and subtropical surface waters.”
+++Implications for nitrogen fixation processes and nutrient cycling+++
The researchers suggest that their new findings call for a re-assessment of the nitrogen fixation patterns and the biogeochemical cycling of nitrogen in the global ocean. They argue that their results refine the understanding of the global nitrogen budget, revealing that heterotrophic bacteria play a more significant role in maintaining nitrogen availability for marine ecosystems than previously thought.
“This study provides a basis for better predicting how nutrient cycles might shift in response to environmental changes, such as global warming and ocean deoxygenation, which have major implications for marine biodiversity and ecosystem services,” states Agostino Merico.
“Our research contributes directly to ZMT's goal of advancing knowledge about vital oceanic processes such as nutrient cycling, which are fundamental to the health and productivity of tropical marine ecosystems. We expect that the high rate of heterotrophic nitrogen fixation in tropical and subtropical oxygen minimum zones, as shown by our model, is significant for ecosystem functioning and resilience,” concludes Subhendu Chakraborty.
Dr. Subhendu Chakraborty | AG Systemökologie | Leibniz-Zentrum für Marine Tropenforschung (ZMT)
Email: subhendu.chkraborty@leibniz-zmt.de
Prof. Dr. Agostino Merico | AG Systemökologie | Leibniz-Zentrum für Marine Tropenforschung (ZMT)
Email: agostino.merico@leibniz-zmt.de
Subhendu Chakraborty, Ken H. Andersen, Agostino Merico, and Lasse Riemann. Particle-associated N2 fixation by heterotrophic bacteria in the global ocean. Science Advances 11, eadq4693 (2025). DOI: 10.1126/sciadv.adq4693
Link: https://www.science.org/doi/10.1126/sciadv.adq4693
Marine snow consists of debris from diverse organisms in the water column. The photo shows marine sn ...
L. Riemann, University of Copenhagen
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