Lightning’s role in nitrogen availability for early earth life found to be short-lived, reveals study

A recent study led by researchers from the University of St Andrews Centre for Exoplanet Science suggests that lightning’s role in making nitrogen available for life on Earth may have been relatively short-lived. The study, published in the journal Nature Geoscience, indicates that our planet’s biosphere quickly became independent from lightning as a nutrient source.

In the past, lightning has been considered a significant supplier of bioavailable nitrogen for early life on Earth. However, the new research challenges this notion and reveals that our biosphere adapted to alternative nitrogen sources relatively quickly.

Furthermore, these findings have implications for understanding the origin of nitrate deposits not only on Mars but also on other celestial bodies within our solar system.

Nitrogen is a crucial element for the emergence and development of life as we currently understand it. During the early Earth’s existence, nitrogen mainly existed in the form of unreactive N2 molecules, making it inaccessible to organisms.

Certain microorganisms have the capability to convert N2 gas into bioavailable forms such as ammonium. However, prior to the evolution of this metabolism, energetic processes like lightning were likely responsible for breaking down N2 molecules.

To investigate how lightning contributes to nitrogen availability for life, the researchers conducted a series of spark discharge experiments. They filled glass flasks with water and various gas mixtures that resembled both modern and early Earth atmospheres. Subsequently, they subjected these mixtures to an electric discharge of approximately 50,000 volts. Following the experiments, the scientists analyzed the gas mixture and water composition and observed elevated concentrations of nitric oxide, nitrite, and nitrate.

Patrick Barth, the study’s first author and a Ph.D. student at the St Andrews Centre for Exoplanet Science and the Space Research Institute (IWF) of the Austrian Academy of Sciences, stated, “Our results demonstrate that lightning can effectively produce nitrogen oxides in the CO2-rich atmosphere that likely existed on early Earth. This could have provided a potential source of nutrients for life during that time and even on planets beyond our solar system.”

Measurements of δ15N in sedimentary rocks over geologic time, separated by metamorphic grade. Credit: Nature Geoscience (2023). DOI: 10.1038/s41561-023-01187-2

However, the researchers discovered that the isotopic composition observed in their spark experiments does not match the nitrogen isotopes preserved in the early Earth’s rock record. This disparity indicates that lightning was not a significant nitrogen source during the evolution of microbial life.

Instead, these findings provide additional evidence that microorganisms have been responsible for converting N2 gas into bioavailable forms for over three billion years.

Nevertheless, there are a few rock samples from the Isua Greenstone Belt in Greenland, dating back nearly 3.8 billion years, whose isotopic composition could potentially be explained by nitrogen contributions from lightning.

Dr. Eva Stüeken, a member of the St Andrews Centre for Exoplanet Science and the School of Earth and Environmental Sciences at the University of St Andrews, noted, “This suggests that lightning might have played a role in supporting early life on Earth. The established isotope signature of lightning could now aid in investigating the origin of nitrate deposits on Mars.”

In order to convey the significance of these findings, the research team collaborated with artists to create an exhibition. This exhibition includes a video by Patrick Barth discussing the role of lightning on exoplanets. Additionally, the research inspired a short story titled “A Spark in a Flask,” which was published in the science fiction anthology “Around Distant Suns.” The story narrates the experiences of a robot conducting spark experiments on the Moon.

Professor Christiane Helling, co-founder of the St Andrews Centre for Exoplanet Science and director of the IWF, emphasized the importance of communicating their scientific discoveries by showcasing the exceptional nature of Earth’s atmosphere within the broader astronomical context.

Source: University of St Andrews

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