Using Soil Trace Gas Flux as a Signature for Life: A Terrestrial Case Study in Martian Life Detection
Abstract
The search for life on Mars is a challenging endeavor due to the low potential biomass and degraded chemical biosignatures of putative microorganisms. It is vital to evaluate the possible history of martian life and the metabolisms that could drive it for improving effective search strategies. To assess the metabolic mechanisms that could enable survival under the harsh conditions of Mars, we sampled the soils surrounding the Polloquere Hot Spring in the Salar de Surire of the Andean Altiplano. Polloquere offers the unique opportunity to study conditions analogous to Mars throughout its history. While the spring-adjacent soils represent an ancient martian environment in which hot springs were present on the surface and supplied a consistent source of reduced gases to the surrounding region, the more distant soils reflect the development of modern martian surface conditions including elevated salinity, aridity, and UV flux. The pore water geochemistry of the soils confirmed the distinct zonation of the soil environments relative to spring proximity, displaying a decline in soil moisture along with increasing chloride and sulfate concentrations to values equivalent to 3x and 20x average seawater at 20 meters distance from the spring, respectively. DNA concentrations revealed a steep biomass gradient as a function of distance, with relatively high DNA recovery adjacent to the spring (1,300 ng/gdw soil) and a severe drop to < 10 ng/gdw soil beyond 10 meters distance. Distal samples displayed unusually broad size distributions suggestive of extensive DNA degradation. Despite the extremely low biomass and poor DNA quality, gas microcosm experiments showed biological uptake of reduced gas species (i.e., H2 and CO) occurring at atmospheric concentrations with significantly higher rates as distance increased. Reconstruction of genomes via metagenomic analyses further indicated the presence of distinct microbes with high-affinity gas metabolisms in the harsher "modern Mars" conditions away from the spring. This discrepancy between apparent biomass and evidence of functional/active metabolism suggests that detectable geochemical signatures of potential microbial activity such as soil gas flux can be used as a guide for identifying life where organic indicators of biology are sparse.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMP024.0010G
- Keywords:
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- 0406 Astrobiology and extraterrestrial materials;
- BIOGEOSCIENCES;
- 0424 Biosignatures and proxies;
- BIOGEOSCIENCES;
- 5225 Early environment of Earth;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6297 Instruments and techniques;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS