Nitrogen is abundant in our atmosphere, but plants and animals can’t use it directly. Enter nitrogenases, special enzymes that convert nitrogen into usable forms. Understanding how these enzymes worked billions of years ago has been a tricky puzzle for scientists. Recently, though, researchers have made exciting progress.
A study in Nature Communications revealed this new approach. Funded by NASA, the project brought together teams from Utah State University and the University of Wisconsin-Madison. Instead of relying on fossilized remains, they created ancient nitrogenase genes using synthetic biology. They inserted these genes into living bacteria, allowing direct observation rather than just educated guesses.
The team built four ancient nitrogenase versions, called Anc1 to Anc4, tracing back about 700 million to 2.3 billion years. By engineering these genes into a nitrogen-fixing bacterium called Azotobacter vinelandii, they could see how these ancient enzymes performed.
Biochemist Lance Seefeldt, who has studied nitrogenases for over 30 years, highlighted its importance. He noted that prior studies relied heavily on fossils, which assumed ancient enzymes worked like modern ones. This research tests that idea directly.
The study found that all four ancestral strains could fix nitrogen, supporting growth, though they worked slower than today’s enzymes. Interestingly, the oldest variant, Anc4, functioned despite being quite different from modern counterparts.
A key finding revolves around nitrogen isotope fractionation. This process shows how nitrogenase enzymes differentiate between heavy and light nitrogen atoms, leaving a unique chemical signature in biological materials. Geologists have long used these signatures in ancient rocks as markers of life, but earlier methods depended on unproven assumptions. The new study demonstrated that ancient enzymes produced similar isotopic signatures as modern ones, bringing clarity to earlier research.
The researchers measured fractionation values from -0.9 to -2.9‰, closely matching values found in contemporary nitrogen-fixing microbes and ancient rocks dating back 3.2 billion years. This consistency suggests that nitrogenases have maintained their functionality over billions of years.
Betül Kaçar, a professor of bacteriology involved in the study, emphasized the significance of understanding life’s history. “The search for life starts here at home, and our home is four billion years old. We need to understand life before us if we want to understand life ahead of us and life elsewhere,” she stated.
The isotopic findings also hint that the origin of molybdenum-dependent nitrogenase could be older than the previously accepted age of 3.2 billion years. The similarities between ancient rock values and ancestral enzyme signatures suggest that current estimates may be conservative. New evidence showing higher levels of dissolved molybdenum in Archean seawater supports this idea.
This study challenges an earlier hypothesis that lower oxygen levels in the ancient atmosphere significantly altered isotopic fractionation. The results indicate that nitrogenase stability didn’t rely heavily on atmospheric oxygen fluctuations, suggesting these enzymes developed early and maintained their efficiency.
Understanding ancient and modern nitrogenases is crucial today. Seefeldt noted that these insights can help tackle modern agricultural challenges, especially in areas facing drought or limited access to fertilizers. The same enzymes central to a two-billion-year-old mystery might offer solutions to feed a growing population amid uncertain climate conditions.
