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Project Progress

Rocco Mancinelli & Amos Banin- Nitrogen is an essential element for life. Understanding the planetary nitrogen cycle is critical to understanding the origin and evolution of life. Of particular significance to prebiotic and biotic chemistry is “fixed N” (NH₃, NH₄+, NOx, organic-N). Without fixed nitrogen life would not have originated on earth and would most likely not occur on any other planet.

The Atacama Desert in Chile is an enigma in that it contains vast nitrate deposits. Elsewhere on earth, nitrate is either denitrified through the activity of microorganisms, or is dissolved and leached from the system. The low water supply (1-2 mm rainfall yr^-1) in the Atacama limits biological activity, including denitrification, and leaching of salts from the soil, and is hypothesized as the primary reason for the observed stability of nitrate. In some parts of the Atacama, such as Yungay (24°S, 69°W), the soil parent material is volcanic in origin. As a result, these soils contain high concentrations of oxidized transition metal minerals, particularly chromium and manganese, as well as perchlorate. The presence of these oxidized metals stabilizes the soil redox potential at high values, which may inhibit denitrification and also contribute to nitrate stability.

Data from measurements of salinity in the Atacama soils and other extreme deserts were compiled and analyzed as a possible close model for Mars soils. The aforementioned presence of perchlorate (ClO4-), a highly oxidized soluble salt of chloride is well established, but its ratio to the ion Cl- has not been fully studied and is not well understood. The recent detection of perchlorate in the Polar Plains of Mars by the Phoenix Lander, if corroborated, is intriguing and its astrobiological ramifications may be assessed using results of previous chemical and biological tests conducted on the Atacama soils.
Denitrifiers in most soil environments are ubiquitous and should be in the atmosphere and falling to the desert surface. The results of experiments to isolate and culture denitrifying organisms from Yungay soils were negative. However, fragments of the nir-s gene could be amplified from the soil at a low level, suggesting that denitrifiers occur in the soil. It has been shown that very low concentrations of organic-C and organic-N exist in the surface soil, i.e. stable soil organic matter, which indicates very limited biological soil activity.

Rates of denitrification were determined using acetylene to block the reduction of N₂O to N₂. The soil samples were placed into vials fitted with syringe septa, and incubated for at least 10 hours, dry, wet, anaerobic, and aerobic. Controls were run with no acetylene. Periodically, gas samples were collected and analyzed via gas chromatography. Rates of nitrogen fixation were determined using the acetylene reduction method. No nitrogen fixation was detected, which is not unexpected due to the abundance of NO₃. Denitrification tests were negative, including those from the wet test, suggesting that low water activity alone cannot account for the lack of denitrification in this system. However, when an organic matter was mixed with the soil and incubated nitrite was produced. This suggests that denitrifiers are present in the soil, but the lack of organics (a potential electron donor) may account for the lack of denitrication in the system. Additionally, it is known that O₂ inhibits denitrification and raises the cell’s redox level. Following from this we hypothesize that the high soil redox potential is also responsible for the lack of denitrification. To date, the Yungay region represents the only place in the world where anaerobic denitrification cannot be detected in the presence of nitrate.