Microorganisms of the Atacama Desert

The Atacama Desert, located in northern Chile, is the driest and oldest Desert on Earth. Research aimed at the understanding of this unique habitat and its diverse microbial ecosystems begun only a few decades ago, mainly driven by NASA’s astrobiology program. A milestone in these efforts was a paper published in 2003, when the Atacama was shown to be a proper model of Mars. From then on, studies have been focused to examine every possible niche suitable for microbial life in this extreme environment. Habitats as different as the underside of quartz rocks, fumaroles at the Andes Mountains, the inside of halite evaporates and caves of the Coastal Range, among others, have shown that life has found ingenious ways to adapt to extreme conditions such as low water availability, high salt concentration and intense UV radiation. 

The Atacama has been proposed to be the oldest desert on Earth. Based on palaeomagnetic data, Hartley et al. found no significant latitudinal movement since the late Jurassic, 150 million years ago. This property, along with the location of the Atacama within the dry subtropical climate belt and the presence offshore of a cold, upwelling current dating from at least the early Cenozoic, resulted in conditions promoting climatic stability and the development of this arid desert.

For a long time it was thought that regions of the Atacama could not uphold any type of life forms. However, recent culture-independent methods (metagenomics, transcriptomics, in situ hybridization, etc.) have improved the sensitivity for life detection. Thus, microorganisms have been found even in the driest areas of this desert, which makes scientists wonder about the true limit of water availability needed to sustain life as we know it. In this review, we summarize the efforts devoted to the characterization of microbial life in the Atacama Desert. For previous reviews on the subject, see Gómez-Silva et al. and Gómez-Silva.

The early stages: laying the groundwork for the Vikings

Studies on the microbiology of the Atacama Desert in modern times have been intrinsically tied to the nascent field of Astrobiology, as it will become evident below. One of the first descriptions of microorganisms in this desert was published in 1966 by Cameron et al. . Working by that time at the Jet Propulsion Laboratory (JPL) and commissioned by NASA, they approached the Atacama as a way to obtain basic information on terrestrial desert environments and its microbiota. This program was undertaken in order to develop and test the life detection instruments that were to be taken to Mars ten years later by the Viking Mission. Thus, samples were collected near Uriba (apparently the name of this location is misspelled in the manuscript and most probably refers to the Uribe train station located about 15 km south east of Antofagasta), where soils were physicochemically characterized. Using trypticase soy agar plates, aerobic and anaerobic bacteria, fungi and algae were barely detected. However, growth obtained in dilution tubes of thioglycollate medium indicated the presence of 106–107 microorganisms per gram of soil. Subsequently, making use of an array of different growth media, a higher abundance of bacterial and Streptomyces colonies were additionally detected. Also commissioned by JPL and NASA, W.B. Bollen further worked on these isolates, identifying Bacillus subtilis aterrimus, Bacillus brevis, Bacillus cereus and Micrococcus caseolyticus.