When studying Earth’s past, researchers want to understand how nutrients like nitrogen cycled and the impact biology had on them. Scientists rely on ancient rock, soil, fossils, and marine sediment to understand the world that used to be.
Approximately 2.5 billion years ago, oxygenic photosynthesis, the form of photosynthesis that produces oxygen as a byproduct, evolved. This caused what is called the Great Oxidation Event. Before this event, the Earth was largely oxygen free, and all life had evolved with that in mind. This rapid introduction of oxygen into Earth’s early atmosphere proved toxic to many organisms. In turn, many of the ancient Earth’s nutrient cycles, including the nitrogen cycle, had to adjust as well.
Nitrogen is a critical element to life on Earth, as life cannot live without it. Unfortunately, not all forms of nitrogen are usable. Only ammonia (NH4+/NH3), nitrate (NO3–), and nitrite (NO2–) can be used. The most abundant form of nitrogen in our atmosphere is N2 gas. Since this isn’t a usable form, life relies on a series of chemical reactions performed by microorganisms to convert it. The process of turning nitrogen gas into a biologically useful form of nitrogen is called nitrogen fixation. This is highly sensitive to oxygen, so a massive injection of oxygen would affect biological nitrogen fixation and would limit the locations where it could occur. This is important because nitrogen fixation is important for all life as we know it.
Currently, scientists have not been able to clearly “observe” the Great Oxidation Event with enough precision. To remedy this, researchers from the University of St Andrews studied a newly discovered set of 2-billion-year old shale (sedimentary rock) from South Africa. Shale is made of fine silty sediment, which can be deposited in a variety of environments on Earth, like river deltas or in the ocean. This specific shale from South Africa represents part of an ancient ocean. The scientists studied the nitrogen and oxygen deep within the rock to better understand what conditions were like when the silty material was deposited all those years ago.
They first examined the rock’s minerals to see if any oxygen was present. Any oxygen detected could mean that oxygen was around way back when the rock formed. They then analyzed the form of nitrogen in the rock to see how it had been impacted by the presence (or absence) of oxygen. The authors combined this technique with another analysis that allowed them to see how microorganisms used the nitrogen. Each time a microbe uses nitrogen and changes its form (for example, from NO3– to NO2– ), it alters its isotopic ratio, or the ratio of different varieties of nitrogen that scientists can measure. By looking at the changes between ratios of nitrogen varieties, the researchers interpreted what type of biological activity created each form of nitrogen.
What they observed was a layering effect in the ancient ocean: a surface oxygen-rich layer and a deeper oxygen- free layer. This split in oxygen concentration within the water column results in what the authors call a “redox boundary”. The boundary causes high levels of ammonia and nitrate to be formed in the deeper oxygen free layer via nitrogen fixation. This provides nutrients to the shallower oxygenic layer through ocean upwelling. Add sunshine to the mix and you get a lot of photosynthesizers taking up nitrogen and driving the addition of more nutrients from the depths. The photosynthesizers eventual death and decay would return large amounts of nitrogen to the deep, resetting the cycle. These analyses allowed the scientists to fill a 400 million-year gap in the Earth’s nitrogen record! This helped them to understand how the ancient ocean’s nitrogen cycle changed with the introduction of oxygen.
All of this together results in a nitrogen cycle that looks shockingly similar to our modern day one, which makes sense as this is probably when and how our cycle developed. As this process continued, it would allow for the evolution of many new organisms that could use this newly cycling nutrient. These observations suggest that before Earth’s Great Oxidation Event, nitrogen fixation could take place anywhere in the ocean, so there was no real “cycle”, but more like chemical chaos. With the introduction of oxygen, it resulted in the separation of nitrogen fixation and oxygenic photosynthesis. This created a global cycle and allowed both to take place in their respective locations as they do today. Additionally, since this observed boundary in the shale dates so close to the time of the Great Oxidation Event, it shows just how quickly our biosphere can adjust to major environmental changes, providing some hope for how our planet could face the ongoing climate change.