The Mars Rover in the Atacama Desert.

What should we be looking for when we’re looking for life on Mars? Given the harsh environment that includes radiation, extreme temperatures and drought, life might look quite different from what it looks like here on earth. Some spots on our planet come close to Mars-like conditions, though. The Atacama Desert in Chile is one of them. Stephen Pointing from Yale NUS college and his team sent a Nasa rover on a test run in this desert. In their recent publication in Frontiers , they describe the strange bacteria they found strewn beneath the desert surface in patches, and what it tells us about life in space.There is nowhere on Earth quite like the surface of Mars. The surface has potentially lethal levels of radiation and very little liquid water. A summer’s day on Mars might see equatorial temperatures rise to comfortable 20C but at night they would plummet to below -100C. Just beneath the surface, however, where the mineral soil and rock provide shelter from the extreme conditions there is a potential habitat for life.Some of the most Mars-like soils on Earth are in the Atacama Desert in Chile. There is very little water input to the desert and soils have become very nutrient-poor and extremely salty over time, and chemically they resemble the soils on Mars in several ways. In preparation for future missions to Mars, we use places such as the Atacama Desert to test theories about the distribution of life and new technologies to search for life.Our study was the first to demonstrate that in near-surface soil horizons there is a clear zonation of bacteria. The surface supports a ubiquitous and unremarkable community dominated by phototrophic Chloroflexi and these have been widely reported before. Just below the surface is where it starts to get interesting.We saw that with increasing depth the bacterial community became dominated by halotolerant alkalophilic bacteria that can thrive in the extremely salty and alkaline soils. They in turn were replaced at depths down to 80cm by a methylotrophic group of bacteria that survive by metabolizing methane and other C1 compounds.This is very exciting because it demonstrates that the subsurface of the Atacama supports highly specialized microbes that can thrive in the salty Mars-like soils and the recent measurements of significant methane emission from Mars surface suggest methylotrophic bacteria could also thrive there.Overall the bacterial distribution in soil horizons was extremely patchy and this was correlated with increased salt levels that limited the bio-availability of water. These bacteria clearly survive right at the limit of habitability.The Rover was deployed robotically in a simulation of the actual mission conditions, complete with time delays to communications that will occur when controlling a rover on Mars. The locations were selected based on satellite mapping of geology in the harsh desert terrain, and the rover was directed to these pre-selected locations of interest.The bacteria are not physiologically different from those already known, although several are likely to be new species. What is novel is the way they were found to colonize a substrate previously viewed by many as incapable of sustaining life.The results are a cause for optimism that bacterial life could tolerate the conditions of the Martian subsurface. There are, however, two major issues that may hinder the actual recovery of biosignatures for life on Mars: The patchy nature of the colonization suggest that a rover would be faced with a “needle in a hay stack” scenario in the search for Martian bacteria. Also, our study showed that paleo-soil features formed long ago when water was abundant but with no recent history of water input had no recoverable bacteria, and Martian soils are likely very similar to these in many places.Now that we know bacteria occur in these subsurface habitats, we need to start trying to understand how they survive. Ongoing studies are attempting to unravel the metabolic and stress tolerance strategies bacteria employ in this extreme habitat.Feature image: Nasa