High-Affinity Trace Gas Consumption by Soil Microbial Communities Around Hot Springs in the Andean Altiplano with Implications for Early Mars

Whether searching for extant life or for biomarkers of extinct life, one of the fundamental questions driving astrobiological studies of Mars is how putative Martian microorganisms could derive energy for maintenance and growth. One possible mechanism of energy acquisition is the utilization of trace gases (e.g., CH₄, H₂, and CO) obtained directly from the atmosphere via expression of high affinity enzymes, as has been observed in bacteria on Earth. To study the potential for trace gas metabolisms to support Martian life, we sampled soil surrounding the gas-emitting Polloquere Hot Springs in the Salar de Surire of northern Chile. The site represents the high elevation, high precipitation, low temperature, and high UV flux end member of an environmental gradient spanning the Arica-Tarapacá region. These conditions along with the highly saline and sulfurous geothermal waters serve as a compelling analog for the surface environment of early Mars where thermal springs were likely widespread.

Microcosm experiments were performed for soils along the 3 sampled transects to characterize the microbial gas consumption and production via gas chromatography. Upon exposure to typical Earth atmospheric mixing ratios (2 ppmv CH­₄, 500 ppbv H­₂, and 100 ppbv CO), soils from transects perpendicular to and downwind of the wind direction exhibited significant depletion of H­₂ and CO after 1 week. Nearly complete to total consumption of these gases was observed for soils at 20 to 30 m distance from the spring. Additional short-term microcosms revealed particularly rapid uptake of H­₂, achieving total consumption of 500 ppbv within 24 hours. Conversely, samples from the upwind transect displayed limited to no trace gas consumption except for H₂ depletion at 20 and 30 m. The darkest soil along the shore was the only sample to emit gas (6.0 nmol CH₄ and 1.1 nmol H₂) after 1 week. The observed consumption of H₂ and CO in the direction of the wind supports the plausibility of geothermal springs as sources for trace gas metabolisms in Mars-like environments. The increased gas consumption with distance from the spring suggests a community shift toward microbes with higher affinity enzymes as the emitted gases are depleted. Sequencing of the samples is in progress for correlation of gas uptake with community changes and abundance of trace gas metabolisms.