Life at the dry limit: Cyanobacteria inside halite pinnacles in the Atacama desert

Grant #: NNX12AD61G
Senior Scientist: Alfonso Davila

The goal of this project is to understand the survival mechanism of cyanobacteria inhabiting the interior of halite pinnacles in the Yungay region of the Atacama desert. The Yungay region is so inhospitable that even the organisms better adapted to water stress are absent. For decades it was thought that life was not possible in this region. The discovery of endolithic colonies of cyanobacteria inside halite pinnacles was a surprise. The halite pinnacles themselves are unique, and have not been described in any other desert on earth. The pinnacles form at the edges of desiccation polygons. At their centers, the polygons contain a mixture of halite, sand and other debris, are very hard, non-porous, and opaque, whereas the pinnacles are almost pure halite, porous and translucent. We think that the particular properties of the pinnacles allow for life to exist. 

We hypothesize that the colonization of the pinnacles is primarily due to a property of halite named Deliquescence. Deliquescence is the spontaneous condensation of liquid water on a mineral, when relative humidity (RH) reaches a critical value. As RH increases, water vapor diffuses into the pinnacle and on reaching 75%, deliquescence causes water vapor to condense in the pore space. This forms microscopic brines inside the pinnacles, in which the cyanobacteria can thrive. In essence, halite converts vapor into liquid water, which cyanobacteria can use. Further, we think that deliquescence also controls the formation of the pinnacles. Slow dissolution/precipitation cycles due to deliquescence, drive the migration of salt from the center of the polygons towards its borders, and as a result pinnacles form and become a long-lived habitat that supports life. This slow process is controlled by changes in atmospheric RH, and as such it can only occur in extremely dry environments, where ground water, surface runoff and rainfall do not interfere. To test our hypothesis we have formulated 3 science tasks: 

Task 1. Photosynthesis of halite endoliths as a function of RH: we will measure the photosynthetic activity continuously along RH cycles, by chlorophyll fluorescence using Pulse Amplitude Modulated (PAM) fluorometry. We predict that cyanobacteria will start to photosynthesize at RH=75%, the deliquescence point. 

Task 2. Formation and evolution of halite pinnacles: We will study four different salt flats in the Central Atacama to map the morphology of the halite deposits. We will conduct morphologic, mineralogic and crystallographic studies in order to reconstruct the sequence of events that lead to the formation of halite pinnacles. 

Task 3. Water dynamics and biomass in halite pinnacles: We will record long term RH, T, and conductivity data inside and outside halite pinnacles from four different salars using miniature sensing devices. We will also measure a suite of photopigments as indicators of the total phototroph biomass. This measurements will allow us to tie the environmental and the biological aspects of the work. 

At the end of this project we will have a clear understanding of the environmental processes that control the formation of halite pinnacles, and the biological activity of the cyanobacteria inside them. The proposed research is highly relevant to Astrobiology in that it will provide new insights on the dry limit of life on Earth, and the potential for life in other extremely dry environments, such as the surface of Mars.