HYPER-ARID AND PERCHLORATE-RICH REGIONS OF THE ATACAMA DESERT CHILE REPRESENT SOME OF THE BEST TERRESTRIAL ANALOGS OF MARS. PRIOR STUDIES SUPPORT THE PRESENCE OF INDIGENOUS MICROBIAL COMMUNITIES THAT ARE PERIODICALLY ACTIVE DURING FAVORABLE CONDITIONS. THE CHILEAN COST RANGE AND ANDES MOUNTAINS CREATE A RAIN SHADOW WITH ARIDITY LEVELS ALONG ONE TRANSECT VARYING FROM ARID TO HYPER-ARID (20 MM/YR TO<1 MM/YR). SUCH PRECIPITATION DIFFERENCES EXERT A STRONG CONTROL OVER SURFACE SOIL AND BIOMASS; IN THE DRIEST REGIONS THE RELATIVE HUMIDITY AT 1 M DEPTH IS AROUND 14% ON PAR WITH MEASUREMENTS MADE BY THE MARS SCIENCE LABORATORY ON MARS. DESICCATION TOLERANCE OR XEROTOLERANCE IS CLOSELY RELATED TO RESISTANCE TO IONIZING RADIATION (IR) AND INDEED THE LATTER MAY RESULT FROM SELECTION FOR THE FORMER. HOWEVER ARCHAEAL AND BACTERIAL RESPONSES TO DESICCATION ARE DIVERSE UNDERSTUDIED AND NOT ALWAYS CONCORDANT WITH IR RESISTANCE. ADAPTATION MECHANISMS INCLUDE DORMANCY FORMATION OF PROTECTIVE BARRIERS OR CELL MEMBRANE ALTERATIONS MOLECULAR MEANS TO PRESERVE ACTIVITY OR CONVERSELY INDUCE VITRIFICATION OR MECHANISMS TO REDUCE REACTIVE OXYGEN SPECIES (ROS) PRODUCTION OR IMPROVE ROS SCAVENGING. HYPOTHESIS: WE HYPOTHESIZE THAT ADAPTATION TO ARIDITY AND IR IMPOSES COSTS THAT SELECT FOR A SLOW-GROWTH LIFESTYLE. THIS MAY LIMIT SUCH ORGANISMS ABILITY TO RESPOND TO BENIGN CONDITIONS WHILE ALSO HELPING MATCH GROWTH RATES TO LIMITED NUTRIENT SUPPLY IN HARSH ENVIRONMENTS. OBJECTIVES: WE PROPOSE TO BUILD ON PRIOR SYSTEMATIC SAMPLING ALONG VARIABLE ARIDITY TRANSECTS IN THE ATACAMA INCLUDING EXTENSIVE PRIOR CHARACTERIZATION OF SOIL PARAMETERS RIBOSOMAL (16S) COMMUNITY CHARACTERIZATION AND CULTIVATION WORK. FIRST (1A) WE PROPOSE TO PROVIDE A COMPREHENSIVE VIEW OF MICROBIAL COMMUNITY STRUCTURE FUNCTION AND ACTIVITY ALONG THESE TRANSECTS. SECOND (1B) WE WILL CONFIRM ARIDITY-ASSOCIATED RESPONSES AND IN SITU ACTIVITY VIA A FOCUSED FIELD STUDY. WE WILL (2A) REFINE MECHANISMS OF XEROTOLERANCE USING ISOLATE STUDIES AND (2B) DISENTANGLE XEROTOLERANCE AND IR RESISTANCE INTEGRATING GENOMIC TRANSCRIPTOMIC PHYSICAL CHEMICAL AND PHYSIOLOGICAL RESPONSES. METHODOLOGY: A DIVERSE ARRAY OF SOIL ANALYSES AND PARAMETERS HAVE ALREADY BEEN OBTAINED INCLUDING ORGANIC CARBON AND NITROGEN DELTA-13C WATER CONTENT CARBONATE CONTENT DISSOLVED ORGANIC CARBON AND NITROGEN GRAIN SIZE PORE SPACE CATION CONTENT (ICPOES) ANION CONTENT (ION CHROMATOGRAPHY) PH BULK DENSITY ELECTRICAL CONDUCTIVITY (SALINITY) AROMATIC COMPOUNDS (UV-VIS) DITHIONITE-SOLUBLE METAL-OXIDE AND -HYDROXIDE COMPOUNDS AND ELECTRON MICROSCOPY. WE PROPOSE TO COMPLEMENT THESE MEASUREMENTS WITH METAGENOMIC AND METATRANSCRIPTOMIC SEQUENCING TO CHARACTERIZE TAXONOMY FUNCTION AND ACTIVITY FROM EXISTING SAMPLES. THE CONFIRMATORY STUDY WILL UTILIZE IN SITU METAGENOMIC (DNA) AND RNA SEQUENCING TO CHARACTERIZE COMMUNITY WITHOUT POTENTIAL STORAGE BIAS. WE WILL ASSESS XEROTOLERANCE OF EXISTING AND NEWLY GENERATED ATACAMA ISOLATES AND CONTROLS THROUGH SURVIVAL ASSAYS TRANSCRIPTOMICS NANOSCALE IMAGING (FIB/SEM) CHEMICAL CHARACTERIZATION (INCLUDING EPR) AND IR RESISTANCE. SIGNIFICANCE AND RELEVANCE: OUR WORK WILL HELP TO DEFINE IN ONE OF THE BEST ANALOGS OF MARS ON EARTH THE TRADE-OFFS MICROBIAL COMMUNITIES MAKE TO SURVIVE IN INCREASINGLY ARID ENVIRONMENTS. OUR COMPREHENSIVE APPROACH WILL LINK MICROBIAL COMMUNITY STRUCTURE FUNCTION AND ACTIVITY TO ENVIRONMENTAL AND GEOLOGICAL PARAMETERS OF RELEVANCE TO HABITABILITY AND THE SEARCH FOR LIFE ON MARS AND ELSEWHERE. OUR WORK IS SIGNIFICANT TO NASA S SEARCH FOR LIFE IN THE UNIVERSE (BIOSIGNATURES AND LIFE ELSEWHERE THEME) IN THAT IT INVOLVES BIOSIGNATURE STUDIES OF SAMPLES FROM EARTH SITES THOUGHT TO BE ANALOGUES OF OTHER PLANETARY ENVIRONMENTS THAT MIGHT POTENTIALLY HARBOR LIFE.