Nitrogen Cycling

Abiotic Fixation of Nitrogen Nitrogen on Early Terrestrial Planets

by David Summers

The prebiotic fixation of nitrogen for organic compounds is important for the origin of life and loss of nitrogen can result in loss of atmospheric pressure and loss of planetary habitability. Also, if nitrogen fixation processes depend on the presence of liquid water, the presence or absence of fixation products can be a marker for such water. Current theories suggest the nitrogen can be fixed by a series of photochemical and thermal reactions, in the gas phase and in liquid water, to convert the products of shock heating (NO and CO) into nitrite and nitrate. However, except for the formation of NO by shock heating, this process is theoretical. As our results demonstrate, the fixation of nitrogen, to nitrate and nitrite does occur. It appears there are two mechanisms for the such fixation. One appears to follow the experimentally predicted path through HNO. An alternate path appears to go through NO2.

n-cycling
Nitrogen Cycling

Project Progress: A paper  is currently under review in the journal Astrobiology.  Effort has partially turned to study of the stable isotope fractionation and the effects of water layers on mineral surfaces.  (This data could then be combined with future isotopic composition work from the Atacama where nitrates may be of abiotic origin.)  This included designing and constructing a new irradiation apparatus (now finished).  Work also contributed to the study of the isotope fractionation in next step in the reaction sequence, reduction of nitrite to ammonia FeS + NO2- at pH ~5, shows an average fractionation of +6 per mil; FeCl2 + NO2- at pH 8.2 shows an average fractionation of -4 per mil).  This work will also contributes to experiments to include the action of Fe(II) in the aqueous phase on the fixation processes.  (2006 NAI Annual Report)

Nitrogen Cycling at Yungay in the Atacama Desert

by Rocco L. Mancinelli, Kimberley Warren-Rhodes, Amos Banin

n-cycling
Biotic and Abiotic Nitrogen Cycling

Nitrogen is an essential element for life. Consequently, understanding the nitrogen cycle is critical to understanding the origin and evolution of life. The Atacama desert in Chile is a large NO3- salt deposit that is stable through geologic time. This deposit is of particular interest as an analog and model for planets, or niches of limited biological activity. Although hypotheses have been presented for the NO3- stability, i.e., low rainfall and little biological activity, none have been proven. We analyzed Yungay, the driest area of the Atacama. Rates of denitrification were determined using acetylene to block the reduction of N2O to N2. Soil samples were placed into Whirl-pak® bags fitted with syringe septa, and incubated for at least 10 hours, dry, wet, anaerobic, or aerobic. Controls were run similarly with no acetylene. Periodically, gas samples were collected and analyzed via gas chromatography. Rates of nitrogen fixation were determined similarly using the acetylene reduction method. No nitrogen fixation was detected, which is not surprising due to the abundance of NO3-. The denitrification tests were negative. The negative result from the anaerobic wet test suggests that low water activity alone cannot account for the lack of denitrification in this system. This is the only place in the world where denitrification cannot be detected in the presence of nitrate anaerobically. Denitrifiers are ubiquitous and should be in the atmosphere and falling to the surface of the desert. We are attempting to do determine why no biological denitrification occurs in this system.

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Atacama Desert, Chile.

Project Progress: In a set of soil samples from the Yungay region of the Atacama desert we  have conducted detailed analyses of organic and inorganic C and N concentrations. Organic carbon (OC) and organic nitrogen (ON) were low, especially in the soils from the most extreme arid region.  OC/ON ratio was in the range typical for biotically synthesized organic matter.  Comparison to estimates of C content in the Mars soil analyzed by the Viking Landers show that the Atacama soils, even in the hard-core extreme desert sites, have very low biological activity as terrestrial soils are concerned, but still have higher concentrations of total organic carbon compared to the Mars soils analyzed by the Viking Pyrolytic experiment. 

When soil samples collected from the Yungay region of Atacama desert were analyzed  for DNA encoding the genes for the nitrite reductase S gene the gene encoding for the key denitrification enzyme It was not found.  These data combinded with last years data where no trace of nitrogen cycling was detected in the field, even under wet conditions suggests that that either there are no organisms capable of N-cyclein in the soil, or the soil contains somethin inhibiting their activity.
(2006 NAI Annual Report)

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