A deep-sea microbe named Methanocaldococcus infernus uses a highly heat-resistant enzyme called nitrogenase to convert atmospheric nitrogen into ammonia, even at temperatures that would typically destroy most proteins. This discovery, made by researchers at the Max Planck Institute for Marine Microbiology, highlights the enzyme’s unique ability to function under extreme conditions. Some parts of the enzyme remained intact even at 98°C, which is significantly hotter than the boiling point of water. This finding could lead to new methods in biotechnology and sustainable fertilizer production, as the enzyme’s resilience might inspire more efficient industrial processes. Nitrogen fixation is the process by which certain microbes convert nitrogen gas (N₂), which makes up about 78% of Earth’s atmosphere, into ammonia (NH₃). Although nitrogen is abundant, it cannot be used directly by most living organisms because of the strong bond between the two nitrogen atoms in N₂. The nitrogenase enzyme, crucial for this process, contains a complex metallocofactor—a structure that includes metals like molybdenum, vanadium, or iron. Most studied nitrogenases use molybdenum, but others rely on vanadium or iron alone. The nitrogenase found in Methanocaldococcus infernus is unique because it combines features of all three types of nitrogenases, suggesting it might represent an ancient, shared form of the enzyme. Researchers isolated the enzyme directly from the microbe and found it remarkably stable at high temperatures. The enzyme began to break down only at 90°C, and even at 98°C, parts of it remained intact. This stability made it possible to study the enzyme in states that are usually hard to observe. To analyze the enzyme, the researchers used a combination of techniques, including microbial physiology, enzyme purification, and structural biology. All steps had to be carried out in an oxygen-free environment, as oxygen can permanently damage the enzyme’s metallocofactor. The team crystallized the enzyme and used a synchrotron at the Institut de Biologie Structurale in Grenoble, France, to determine its molecular structure at near-atomic resolution. This process revealed that the enzyme is the simplest known example of nitrogenase and contains structural elements from all three major nitrogenase families. The study also confirmed the presence of a molybdenum-based metallocofactor in the enzyme, which was difficult to detect. Researchers observed a molecular state previously seen only in vanadium and iron-only nitrogenases, suggesting that all nitrogenase forms might share a common mechanism for breaking apart nitrogen molecules. These findings could have broader implications beyond the deep sea, as nitrogen-fixing microbes like M. infernus play a key role in Earth’s carbon cycle and contribute to atmospheric methane. In the future, scientists might explore these organisms for sustainable ways to produce useful compounds like ammonia and methane, potentially using green hydrogen as an energy source.