FOR BILLIONS OF YEARS BACTERIA AND ARCHAEA HAVE FORMED ORGANIZED COMMUNITIES IN MICROBIAL MATS. SUCH COOPERATION AMONG MICROBES HAS PLAYED DETERMINATIVE ROLES IN THE PERSISTENCE OF LIFE AND PROVIDES INSIGHT INTO HOW LIFE ON EARTH AND ELSEWHERE MAY RESPOND TO AND EVOLVE IN EXTREME CONDITIONS. A KEY COOPERATIVE MECHANISM AMONG CELLS IN HIGH DENSITY HABITATS IS QUORUM SENSING (QS) USING CELL-TO-CELL SIGNALS. GENETIC EVIDENCE SUGGESTS QS EVOLVED 3.4 GYA AND WAS LIKELY IMPORTANT DURING LIFES EARLY EVOLUTION WHEN EXTREME ENVIRONMENTS PREVAILED. HOWEVER QS HAS NOT BEEN EXPLORED IN AN ASTROBIOLOGY CONTEXT NOR IN ASTROBIOLOGICALLY RELEVANT EXTREME ENVIRONMENTS WHERE DEEP BRANCHING MICROBIAL LINEAGES THRIVE. THIS PAUCITY OF DATA PRESENTS A CRITICAL KNOWLEDGE GAP. WE PROPOSE TO INVESTIGATE QS IN MICROBIAL MATS IN HYPERSALINE SYSTEMS AND LAVA CAVES BOTH ANALOGS OF HABITATS BELIEVED TO HAVE BEEN PRESENT ON EARLY EARTH AND MARS BOTH ALSO HOST DEEP-BRANCHING ARCHAEA AND BACTERIA. OUR MAIN HYPOTHESIS IS THAT QS SIGNALING CONTRIBUTES TO MICROBIAL-SURVIVAL AND RESILIENCY UNDER MULTIPLE EXTREME CONDITIONS. WE WILL FOCUS SPECIFICALLY ON GENETIC COMPONENTS OF QS AND HOW THEY DRIVE GENE EXPRESSION DURING CHANGING EXTREME CONDITIONS. DOING SO WILL ESTABLISH A SOLID FOUNDATION FOR UNDERSTANDING QS IN MATS IN EXTREME ENVIRONMENTS ANALOGS OF WHICH THRIVED IN EARLY LIFE ON EARTH. QS IS A FUNDAMENTAL LIFE PROCESS LEADING TO HIGHER ORGANIZATION AND ALSO A CRITICAL PATHWAY THROUGH WHICH MICROBES TOLERATE AND ADAPT TO ENVIRONMENTAL EXTREMES SIMILAR TO THOSE THAT PREVAILED ON EARLY EARTH AND MARS. OBJECTIVE 1: QS OCCURS IN EXTREME ENVIRONMENTS: HYPERSALINE MATS AND HAWAIIAN LAVA CAVES. H1: QS OCCURS AMONG PHYLOGENETICALLY DIVERSE BACTERIA AND ARCHAEA IN EXTREME ENVIRONMENTS. ACYLHOMOSERINE LACTONES (AHLS) ARE WELL-STUDIED SIGNALING MOLECULES THAT CONFER IMPORTANT METABOLIC PROPERTIES ON MICROBES BUT THEIR DIVERSITY AND ACTIVITIES IN MICROBES IN EXTREME ENVIRONMENTS ARE POORLY UNDERSTOOD. AHL SIGNALING COMPRISES A CONSERVED AHL AND A RECEPTOR PROTEIN (I.E. LUXR HOMOLOG). OUR PRELIMINARY DATA SHOW THE PRESENCE OF AHLS IN HYPERSALINE MATS. HOWEVER ADDITIONAL WORK IS NEEDED TO DETERMINE IF AHL-BASED QS SYSTEMS ARE COMMON IN MICROBIAL COMMUNITIES IN EXTREME ENVIRONMENTS. WE WILL INVESTIGATE GENETIC BASES OF AHL-QS AND PHYLOGENETIC DIVERSITIES OF LUXR HOMOLOGS AS POSSIBLE MECHANISMS OF ADAPTABILITY IN THESE COMMUNITIES. OBJECTIVE 2: QS CONFERS AND ENHANCES SURVIVABILITY UNDER CHANGING EXTREMES IN EARLY EARTH- AND EARLY MARS-LIKE ENVIRONMENTS. H2: BOTH THE TYPES OF SIGNALS USED BY ARCHAEA AND BACTERIA AND RESULTING GENE EXPRESSION CHANGE AS CONDITIONS (SALINITY DESICCATION UV) BECOME MORE EXTREME. WE POSIT FROM OUR PRELIMINARY RESULTS THAT CERTAIN SIGNALS ARE LESS SUSCEPTIBLE TO DEGRADATION THAN SHORTER-CHAIN COUNTERPARTS. WE THUS PREDICT THAT CELLS WILL UTILIZE MORE-RESILIENT SIGNALS UNDER HARSH ENVIRONMENTAL CONDITIONS. THIS IN TURN WILL TEST THE POTENTIAL FOR LIFE TO ADAPT TO CHANGING ENVIRONMENTAL EXTREMES AND ITS IMPLICATIONS FOR LIFE ELSEWHERE. TO DO SO WE WILL DETERMINE CHANGES IN LEVELS OF GENE EXPRESSION CONCENTRATIONS AND TYPES OF AHLS IN NATURAL MULTI-SPECIES MATS AND IN ISOLATES EXPOSED TO VARYING EXTREME ENVIRONMENTAL CONDITIONS (HIGH UV CO2 DESICCATION SALINITY) IN BOTH A PLANETARY ENVIRONMENTAL LIQUID SIMULATOR (PELS) AND CONTROLLED ATMOSPHERE CHAMBER. OBJECTIVE 3: AHL SIGNALS PERSIST OVER TIME THROUGH EXTREME ENVIRONMENTAL CHANGES. H3: QS SIGNALS ARE PRESERVED UNDER EXTREME CONDITIONS (HIGH SALINITY DESICCATION UV). THIS CONTRIBUTES TO RECOVERY AND PERSISTENCE OF COOPERATIVE PROCESSES IN MATS. OUR PRELIMINARY STUDIES SHOW THAT EXTRACELLULAR OSMOLYTES PROTECT AHLS IN NATURAL HYPERSALINE MATS DURING DESICCATION AND UV EXPOSURE. WHEN MORE FAVORABLE CONDITIONS RETURN THIS MAY ELICIT COMMUNITY-WIDE RECOVERY RATHER THAN JUST INDIVIDUAL CELL SURVIVAL AND ENHANCE COMMUNITY SURVIVAL OVER LONG PERIODS.