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Research at St Andrews

Jianxun Shen

Person

Research overview

I am an astrobiologist and biogeochemist. I am studying the nutrient cycling and bacteria in an extremely dry desert, the Atacama Desert in Chile. The Atacama Desert has long been considered as a good Mars model on Earth. My project goal is to understand what Atacama bacteria eat, what trace do they leave in sands, and who they are. Answers of these questions may contribute to the life detection on Mars.

The Atacama Desert receives 0 to 40 mm of rainfall every year across a latitudinal dryness gradient. Tons of salts form in the ozone layer and fall on the ground of the Atacama Desert. These salts cannot be washed away by such a small amount of rain there, so Atacama bacteria are literally drowned into these salts, among which nitrate and phosphate are the two most important members. I found a positive relationship between soil organics and nitrate or phosphate, and found Atacama bacteria actually ate these salts to build up their necessary organic compounds.

To study the trace left by biological activity, I choose to focus on unharmful stable isotopes of nitrate and phosphate. Nitrogen and oxygen elements have a few isotopes of different weights. There are dominantly abundant isotopes of nitrogen and oxygen that takes over more than 99.5% of their respective elements. When bacteria eat nitrate and phosphate, the percentage of the less abundant isotopes will change, which can be used to detect life signals. I studied the biological activity-driven changes in the percentage of these less abundant isotopes to identify possible Martian life markers.

To answer who the Atacama bacteria are, I applied DNA technology and phospholipid analysis to study the compositions and functions of different bacterial species.

I used more than 10 different analytical approaches for a comprehensive in-depth investigation of the Atacama Desert ecosystem. These approaches broadly cover chemistry, geology, microbiology, molecular biology, cell culture, and statistics. My research is the first time to apply the phosphate isotope technology to soil samples as dry as the Atacama Desert. It is also the first time to holistically mingle the studies of bacterial compositions, bacterial functions, nutrient cycling, and changes in isotopes across a latitudinal dryness gradient in the Atacama Desert.

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