Life can adapt to extreme environments but are there limits? Can life adapt to places where there are essentially no water or nutrients? The Atacama Desert in Northern Chile is the oldest and driest desert on Earth. Its hyper-arid core has extremely low levels of organic carbon and rain events are sparse, occurring once every 10 to 50 years. While microorganisms have been shown to inhabit this unique environment, no large-scale study has been conducted to characterize the microbial community in this extreme environment. In this project, we will use a metagenomic approach to survey and characterize the structure, metabolic diversity, and genomic heterogeneity of the microbial soil community of the Atacama Desert. The Atacama Desert is well suited for this investigation because of the long-established extreme conditions (>15 million years) with low water and nutrient availability.
This project will also provide insights on the diversity of resilient microorganisms that we might find when large-scale desertification takes place, which can occur with climate change. The project will train a PhD student in genomics and bioinformatics, and several undergraduate students will be involved through the NSF Research Experience for Undergraduates program. The PIs will continue their commitment to K-12 education through NSF programs and direct contributions to the community. The metagenomic data generated by this project will be integrated into a microbial genomic class at Johns Hopkins University.
". Jocelyne DiRuggiero Johns Hopkins University In this project, we characterized the microbial inhabitants of one of the oldest and driest desert in the world, the Atacama Desert in Chile (Figure 1). We collected soil and rock samples from the desert and used molecular methods to answer (1) what microorganisms are there? (2) How do they make a living? And (2) what environmental factors impact their distribution? Arid systems are important to study because they are highly vulnerable to disturbance and therefore to the effect of global climate change. The Atacama Desert is extremely well suited for such studies given its unique physical geography, with a range of hyper-arid conditions and geology, and its easy access for fieldwork. The Atacama Desert hyper-arid region is described as "the most barren region imaginable". In this central area, the long-term mean annual rainfall is only a few millimeters, with rain events typically occurring once per decade. Not surprisingly, we found very few microorganisms in the soil of the Atacama Desert. The microbial diversity increased with atmospheric moisture and with diminishing soil conductivity (a measure of salinity). While these microorganisms might be dormant most of the time, we showed that they become metabolically active when water was added to the soil, such as during rainfall events. In contrast, we found flourishing microbial communities inside a variety of translucent rocks across the desert. We call these endolithic (within rock) communities our "Islands of Life" (Figure 2). Substrates were composed of sedimentary rocks, volcanic rocks, and evaporitic deposits such as gypsum and halites (NaCl). These rocks provided the microorganisms that colonized them with enough light for photosynthesis (they are translucent), water collected during rain or fog events, and protection against harmful ultraviolet light. Photosynthetic cyanobacteria, and in one instance a novel type of algae, provided the carbon necessary to support a diverse community of heterotrophic bacteria and archaea. We found that their distribution was correlated to the amount of water available within the rocks, which was directly linked to the chemical and physical properties of the substrate. Calcite rocks from Valle de la Luna displayed networks of deep cracks and fractures that allowed better water retention than the gypsum rocks from Tilocalar (Figure 3). The high thermal conductivity of the calcite also favored water condensation at the surface, providing another water input into the system. The Valle de la Luna rocks were colonized by a more diverse community of cyanobacteria and heterotrophic bacteria than the Tilocalar rocks providing a direct correlation between water availability and microbial diversity (Figure 4). In the Atacama Desert, halite deposits are shaped in the form of small pinnacles (Figure 5) and are colonized by endolithic communities of photosynthetic cyanobacteria and heterotrophic bacteria and archaea. Within the halite pinnacles, condensation of water vapour at relative humidity corresponding to the deliquescence point of NaCl (melting point) provided a potential source of water for microorganisms. The communities were dominated by cyanobacteria and members of the Archaea but in places where the air relative humidity was a bit higher - due to fog activity - we found a completely novel type of algae (Figure 5). Here again, the diversity of the microbial communities was correlated with the amount of water available inside the rocks. We did not find any algae in the halites of the hyper-arid region of the Atacama, which environmental conditions might represent the dry limits for eukaryotic life. Our next challenge is to understand the functioning of these unique ecosystems, the interactions between community members, their response to stress and disturbances, and their contributions to biogeochemical processes. Endolithic communities are among the simplest microbial communities known, they are ubiquitous, and as such provide tractable model systems for testing ecological hypotheses. Characterizing the metabolic activities supporting microbial life in a desert environment may reveal novel adaptations of life. This project will inform us on the diversity of resilient microorganisms that we might expect to find when desertification takes place and help determine the recovery potential of an area after long periods of desertification, which may occur with climate change.
