Dec. 12, 2018
Research Highlight
Markers for Atmospheric Nitrogen in Earth's History
A recent study examines potential biological indicators for dinitrogen gas (N2) in Earth’s ancient atmosphere. The atmosphere of Earth has changed dramatically throughout the history of our planet. Some of the gases that make up the atmosphere are easy to track through geological time, particularly those that were actively involved in chemical reactions that left behind markers of their atmospheric concentrations in the rock record. Other gases, such as N2 are inert, and are much more difficult to trace.
The Moon through the atmosphere of Earth. This astronaut photograph (ISS013-E-54329) was acquired July 20, 2006.Image credit: NASA JSC.
Researchers examined two different biological indicators that could be used to track N2 concentrations through history. They include morphological and isotopic signatures found in heterocystous cyanobacteria. These microscopic organisms use an oxygen-sensitive enzyme called nitrogenase to fix nitrogen. What’s unique is that this fixation occurs in special ‘microoxic’ cells (heterocysts), and nitrogen fixation leaves behind a measureable isotopic effect. This creates two biosignatures, which have previously been studied under conditions similar to the atmosphere on Earth today.
The new study focuses on how and if the biosignatures change when the partial pressure of N2 is varied to simulate other periods in Earth’s history, when the concentration of N2 in the atmosphere was different than today. The data could be compared to the geological record, providing a method for measuring the partial pressure of N2 in Earth’s past, thereby uncovering new information about the evolution of our planet’s atmosphere.
Life on Earth is intimately tied to the planet’s atmosphere and environment, and tracking the composition of Earth’s atmosphere through time can help astrobiologists better understand the conditions that led to (and continue to maintain) Earth’s habitability.
The study, “Morphological and isotopic changes of heterocystous cyanobacteria in response to N2 partial pressure,” was published in the journal Geobiology. The work was supported by NASA Astrobiology through the Exobiology Program.