Few organisms can survive in the upper reaches of the windswept McMurdo Dry Valleys of Antarctica. In the high, desiccated, “ultraxerous zone,” ice rarely if ever melts, and life consists mostly of lichens, moss, fungi, and bacteria. A new study finds the first evidence that some microbes perform cellular respiration in this inhospitable zone, which many scientists consider the terrestrial environment closest to that of Mars.
To track the source and distribution of organic matter—possible chemical signatures of life—in the dry, ultraxerous zone, Faucher et al. collected 16 icy permafrost cores from the floor of a small glacial valley in Antarctica’s Quartermain Mountains. They cut the ice cores into 2-centimeter-thick slices and sealed them in plastic bags in which the samples were allowed to thaw. Then they measured the soils’ carbonate, organic carbon, and total nitrogen content.
Organic carbon was extremely scant in the permafrost soils, ranging from 0 to 313 micrograms per gram of soil. Most of the organic carbon and carbonates in the soils were the product of the weathering of higher-elevation sandstone that contains endoliths, organisms that live inside rocks or in pores between mineral grains. However, a very small fraction of the soil carbon content, the team concluded, was likely derived from active microbial respiration in the location where the samples were collected.
The soils the team analyzed have been accreting for the past 200,000 years, yet the chemical composition has not changed much over time, the team found. Although life in the ultraxerous zone has been scant for as long as scientists can measure, the trace amounts of non-endolith-derived carbon reveal that some microbes do respire in those soils. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG004006, 2017)
—Emily Underwood, Freelance Writer