Master of Science (MS)


Biological Sciences

Document Type



The Atacama Desert in Chile extends from latitudes 17º S to 28º S between longitudes 69º W and 71º W. It has been reported that surface soils in the hyper-arid region contain low numbers of culturable heterotrophic bacteria. These soils are considered Mars-like and offer an ideal setting to investigate the application of life detection systems. Thirty-three surface sites were sampled to examine further the extent of the hyper-arid region. We also excavated four soil pits, 40 cm to 90 cm in depth, to explore subsurface microbial communities. One pit was dug in a southern region of the desert compared to the location of the pits in the hyper-arid region. Samples were examined using culture-dependent techniques, including serial dilution plating methods on five media for the cultivation of heterotrophic bacteria. Using 16S rRNA gene sequence comparisons, 1,260 organisms have been recovered and identified. Fifty-four percent of the samples obtained from within the hyper-arid region show numbers of culturable bacteria above the detection limit, yet there is evidence of microbial patchiness in surface and subsurface soils. In some samples, no bacterial colonies were retrieved; in the majority, less than ten were recovered. One pit showed an increase in CFUs/g at 40 cm then dropped back to levels near and below the detection limit. A second pit showed an increase at 10 cm with numbers near and below the detection limit at further depths. The southern soil pit had CFUs/g up to four orders of magnitude greater than those in the hyper-arid region. Direct count methods employing DAPI epifluorescence microscopy were applied to samples, but proved suitable only for a sample having the highest CFUs/g of soil (7.4 x 105) due to the determined detection limit for the technique utilized on the minimal-life containing soils. A chemical composition analysis was performed on all soil samples and showed that elevated ion concentrations may correlate with low numbers of culturable bacteria. The data obtained for the desert samples point to the importance of developing surface and subsurface sampling protocols for future missions to Mars searching for evidence of past or present life.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Fred A. Rainey