Lightning Was Important In Early Life, But Probably Not For Long

One constant throughout Earth's history has been a nitrogen-rich atmosphere. The majority of life on Earth, however, cannot use the N2 molecules in the atmosphere because it needs accessible nitrogen to make proteins. Nitrogen dioxide (N2) is broken down by lightning into oxides of nitrogen, which then react with water to make nitrates and nitrites, which are then used by living things to create more complicated molecules.

Today, however, the majority of the accessible nitrogen comes from bacteria that play a similar role, notably in nodules on the roots of certain plants, or from ammonia created by the Haber process. Although the time of the biological origins was unknown previous to a report released this week, we now know that commercial nitrogen-fixing has only been around for a little over a century.

Researchers put water and gases that represent the Earth's atmosphere at various points throughout its history into flasks. He subjected each to 50,000 volts before measuring the amount of nitric oxide, nitrite, and nitrate produced. According to the study's first author, Ph.D. student at the University of St. Andrews Patrick Barth, "Our results demonstrate that lightning can effectively produce nitrogen oxides in the CO2-rich atmosphere that likely existed on early Earth." This gives life at that period and on planets outside of our Solar System a possible supply of nutrition.

Despite earlier indications that the early atmosphere didn't transmit lightning as effectively as it does now, this much was predicted. After that, the fixed nitrogen utilized by the first life must have come from a non-biological source if organisms couldn't manufacture it before they existed, and lightning is the most likely possibility.

The researchers subsequently discovered something, nevertheless, that was less expected. The nitrogen mixture created in this manner did not correspond to the nitrogen found in prehistoric rock formations. The Isua Greenstone Belt in Greenland may provide an exception, since its rocks may be among the planets oldest at about 3.8 billion years old.

In particular, the outcomes of the trials had a lower ratio of nitrogen-15 to nitrogen-14 than is typical of rocks older than 3.2 billion years. The more recent, though still extremely old, deposits are considerably more compatible with having been created by life forms than by lightning because they preferentially consume nitrogen-15. The rocks of the Isua Belt were murkier, but they had the ability to produce lightning. Dr. Eva Stüeken, the study's author, stated that "This suggests that lightning may have supported the earliest life on Earth."

However, it appears that bacteria had replaced plants as the main nitrogen fixers by the time the somewhat younger rocks the researchers analyzed were deposited. They showed that living creatures could do tasks formerly performed only by the spark of heaven, acting as a smaller version of Prometheus.

The team has gone above and beyond to make their efforts known to the public. A science fiction novel based on the findings was also donated to an anthology where authors were teamed with St. Andrews researchers to produce fiction about their work in addition to making the article open-access. Another sculpture by Barth is included in an exhibition of works inspired by atmospheric science. The research is published in the open-access journal Nature Geoscience.