Ancient Microbe Reveals a Surprising New Route to Natural Fertilizer

One of the oldest lineages of life on Earth has just rewritten a chapter of one of biology’s most important textbooks. Researchers at Saint Louis University School of Medicine, working with collaborators at the University of Arkansas, have determined the structure of a nitrogen-fixing enzyme from a methane-producing microbe and found that it is assembled in a way no one had ever seen before. The discovery, published in Nature, upends decades of assumptions about biological nitrogen fixation and opens a path toward engineering more sustainable ways to produce ammonia, the essential building block of fertilizer.

Nitrogen fixation is the process by which certain microbes convert atmospheric nitrogen gas into ammonia, the form of nitrogen that plants can actually use. In agriculture, this chemistry underpins everything from cereal crops to soybeans, yet humanity currently relies on the energy-hungry Haber-Bosch process to make industrial fertilizer, a method that consumes vast quantities of natural gas and generates significant greenhouse gas emissions. Biological nitrogen fixation, carried out by an enzyme called nitrogenase, performs the same reaction at ambient temperature and pressure. For decades, scientists believed they understood how nitrogenase was built and how it worked, because the model was derived from studies of common bacteria. The new study shows that this model was only part of the story.

The Saint Louis University team, led by Edwin Antony, professor of biochemistry and molecular biology, focused on a methanogen, an oxygen-averse microbe belonging to an ancient lineage that predates many familiar forms of life. Methanogens live in environments devoid of oxygen, from wetlands to the guts of cattle, and they carry out nitrogen fixation with molecular machinery that evolved independently of the bacterial systems studied for the past half century. When the researchers imaged the methanogen’s nitrogenase, they found a previously unknown structure: a nitrogenase supercomplex, an assembly that contradicts the long-standing assumptions derived from bacterial systems.

For decades, scientists have relied on a model of nitrogenase based on what was observed in common bacteria, Antony explained. This discovery demonstrates that nature has evolved alternative ways to accomplish the same chemistry, giving researchers an entirely new framework for understanding biological nitrogen fixation. In other words, the enzyme that makes ammonia is not a single, universal design but a family of solutions, and the methanogen version represents a blueprint that had remained hidden until now.

Reaching that blueprint required overcoming formidable technical obstacles. Nitrogenase is exquisitely sensitive to oxygen, which destroys the enzyme on contact, so every step of the work had to exclude air. The University of Arkansas team, led by Daniel Lessner, spent several years engineering the oxygen-averse microbe and isolating the native protein before sending samples to Antony’s laboratory in St. Louis. There, researchers developed specialized cryo-electron microscopy workflows that maintained strict anaerobic conditions during grid preparation and imaging. Preserving the oxygen-sensitive protein in its native state demanded complex engineering and meticulous sample handling at every stage, and only by clearing those hurdles could the team determine the protein’s structure and obtain an unprecedented view of its molecular architecture.

Cryo-electron microscopy, the technique that made the discovery possible, works something like a freeze frame of a dancer captured from thousands of angles at a single instant, according to Rajnandani Kashyap, a postdoctoral fellow at Saint Louis University School of Medicine and the study’s first author. Each frozen particle image records the molecule from a different orientation, and computational methods align and average those views to reconstruct a three-dimensional model. Using this approach, Kashyap reconstructed the architecture of the methanogen nitrogenase and its associated PII protein, revealing the supercomplex that had never been observed before. Looking at the result, she said, it was the coolest discovery of her life.

The significance of the structure extends well beyond basic biochemistry. Understanding how an alternative nitrogenase is organized gives synthetic biologists a second template to work from as they attempt to engineer nitrogen fixation into new contexts. If researchers can borrow design principles from the methanogen supercomplex, they may be able to reduce the costs and pollution associated with industrial agriculture by helping crops or soil microbes supply more of their own nitrogen. The implications also reach into green energy, since nitrogenase chemistry and hydrogen metabolism are closely intertwined, and even into space exploration, where compact biological systems for producing ammonia and fertilizer could support long-duration missions, including travel to Mars.

The achievement was funded by the Department of Energy and represents the culmination of a multi-institution collaboration spanning Missouri and Arkansas. It was also made possible by a deliberate institutional investment in imaging infrastructure. Under a 15-year agreement established in 2019 with Washington University in St. Louis, the Department of Biochemistry and Molecular Biology at Saint Louis University contributed $2.5 million from its Doisy Fund toward the purchase of a new $5 million cryo-electron microscope. In exchange, SLU researchers gain access to the instrument and to the Washington University Center of Cellular Imaging, one of the leading facilities of its kind in the nation.

That investment has paid dividends well beyond a single paper. Enrico Di Cera, chair of the Department of Biochemistry and Molecular Biology at SLU, credits the cryo-EM capability with generating more than $22 million in federal grant funding, dozens of publications in top-tier journals and key new hires for the department. He described the arrangement as an exemplary success story, one that continues to strengthen the research enterprise at the school. For a department founded in a medical school, the ability to visualize molecular machines at near-atomic resolution has become a cornerstone of both basic discovery and translational ambition.

Behind the instrumentation and the funding, the researchers emphasize, is a culture that empowers young scientists to take on risky, demanding projects. Antony and Kashyap point to the combination of world-class infrastructure and personalized mentorship at SLU, which allows undergraduate and graduate students to contribute to leading-edge research at an uncommon level. As young investigators, Kashyap noted, they have the energy to carry challenging projects forward and to tackle difficult questions, even when the work demands years of patience and persistence, because such efforts reward those who refuse to be scared away. The methanogen nitrogenase supercomplex now stands as proof of that persistence: a structure hidden inside one of Earth’s oldest microbes, finally visible, and pointing the way toward a more sustainable future for agriculture, energy and perhaps even life beyond this planet.

Subject of Research: Cryo-EM structure of a methanogen nitrogenase–PII protein supercomplex and its implications for biological nitrogen fixation

Article Title: Saint Louis University researchers uncover blueprint for more sustainable agriculture

Article References: Saint Louis University researchers uncover blueprint for more sustainable agriculture. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: nitrogenase, nitrogen fixation, cryo-electron microscopy, methanogen, sustainable agriculture, ammonia, fertilizer, biochemistry, structural biology, Saint Louis University, Nature, green energy