Primary Productivity in Extreme Dry Environments on Earth and Mars Part II
I propose to study the primary productivity of endoevaporitic cyanobacteria from the Atacama Desert, as well as to quantify the habitability of evaporitic deposits as a function of water activity and temperature. Although many factors can limit the growth and reproduction of organisms, the main constrain for the occurrence of life on the surface of Mars is the presence of liquid water.
While the surface of present day Mars is extremely dry and cold, rendering it practically inhabitable, conditions in the past where seemingly milder, and sufficient to allow liquid water stability on the surface. The surface of Mars transitioned from a relatively wet to an extremely hyper-arid environment during the first 1.5 Ga (Bibring et al., 2006). If any putative martian biota was present on the surface early in the planet history, these organisms would have needed to adapt to the increasingly arid conditions, and to the increasing salinity of the surface waters, perhaps using strategies similar to organisms inhabiting hyper-arid regions on Earth. During this climatic evolution the planet necessarily had to cross through environmental conditions that we encounter today in extreme hyper-arid environments on Earth, both in terms of aridity and salinity. Therefore, the study of terrestrial life in hyper-arid deserts provides a first approximation to assessing the potential for life on Mars, and to understand the evolution and survival strategies of a putative Martian biosphere.
By means of long-term climate monitoring, McKay et al. (2003) showed that the Atacama Desert, ranks as the driest desert on Earth. Mean precipitation rates in the hyper-arid core of the Atacama Desert are <1mm over periods of several years (McKay et al., 2003). The only sources of liquid water are dew and occasionally fog. This has resulted in the formation of vast evaporitic deposits, the so-called Salares, and in soils with properties similar to martian soils, including extremely low levels of organic compounds, and a dominant non-biological oxidizing chemistry (Navarro-Gonzalez et al., 2003). Both in terms of soil chemistry and liquid water availability, the Atacama Desert represents the best terrestrial analogue to the extreme arid conditions on Mars (McKay et al., 2003).
It has recently been shown that primary productivity in the Atacama Desert occurs almost exclusively within hygroscopic salts (Wierzchos et al., 2006), and that this is likely due to mineral deliquescence, which provides liquid water at relative humidity well below atmospheric condensation levels (Davila et al., 2008). Endoevapotitic cyanobacteria from the Atacama Desert therefore represent a previously unrecognized analogue to a potential ecosystem on Mars. The metabolism and physiology and these endoevaporitic microorganisms remains largely unexplored, as well as the habitability of hygroscopic salts under current and past martian conditions. Both types of studies are fundamental in order to assess whether the observed adaptations of microorganisms in the Atacama Desert could have occurred on Mars.