Starved for Nitrogen
|To illustrate the importance of biological nitrogen fixation, the image above shows part of the Lower Sonoran desert in Arizona. Every plant that we see in this scene depends ultimately on biological nitrogen fixation.
Credit: University of Edinburgh
A team of researchers, including a NASA scientist, reports that an early-life nitrogen crisis may have triggered a critical evolutionary leap about 2 billion years ago.
The team, from Universidad Nacional Autonoma de Mexico (UNAM) and NASA’s Ames Research Center in the heart of California’s Silicon Valley, published its results in the July 5 issue of the journal Nature. Their paper is entitled "A Possible Nitrogen Crisis for Archaean Life Due to Reduced Nitrogen Fixation by Lightning." The researchers, who simulated early Earth atmospheric conditions in a laboratory, postulate that the bacteria that existed then began producing their own nitrogen "fertilizer" in order to survive.
"Our findings show how life on early Earth had to adapt to major changes in the environment," said Dr. Chris McKay, a team member from NASA Ames. "Our results indicate that a couple of billion years ago, life had to invent a way to make its own nitrogen fertilizer because the amount being produced by lightning dropped to almost zero."
|Cyanobacteria filaments. Cyanobacteria can fix atmospheric nitrogen (N2) gas into amino acids and thus progressively enrich the soil with nitrogen for plant growth.
The UNAM-Ames researchers cite evidence that carbon dioxide was much more abundant on early Earth than it is now, and suggest that ancient lightning bolts made nitrate (in a form usable by early life) by combining oxygen from carbon dioxide with nitrogen in the atmosphere. This nitrate acted as a natural "fertilizer" for early life, providing nourishment and spurring growth, they suggest. However, because making nitrate from atmospheric nitrogen is energy intensive, the bacteria did not develop this capability on their own until the lightning source became inadequate for their needs, the research team concluded.
In the lab, the team simulated conditions on early Earth over a wide range of atmospheric compositions, from mostly carbon dioxide to mostly complex nitrogen atmospheres, but always without oxygen. They used a high-power laser to simulate lightning and measured the nitrate produced.
Although the temporary lapse in nitric oxide production may have lasted for only 100 million years, a relatively short period on the geologic time scale, researchers believe this was long enough to cause the ecological crisis that triggered early life’s ability to "fix" its own nitrogen.
"We are used to thinking of the environment and life as steady and unchanging," said McKay, "but the early Earth was quite different. Major changes in the atmosphere occurred and life had to adapt. To me, the interesting follow-up question is: Did it happen on other planets, too?"
McKay said the process was triggered because, over time, carbon dioxide levels on Earth dropped and the production of atmospheric nitrate by lightning was greatly reduced. This left bacteria literally starving for nitrogen. The UNAM-NASA Ames team estimates that this environmental crisis occurred between 2 and 2.2 billion years ago.
In addition to McKay, other team members included Dr. Rafael Navarro-Gonzalez and Dr. Delphine Mvondo, UNAM. NASA’s Astrobiology Division in Washington, D.C. funded McKay’s and NASA Ames’ participation in the research project.