THIS PROPOSAL WILL TEST THE HYPOTHESIS THAT EXTREME RADIATION RESISTANCE IN DESERT MICROORGANISMS IS FUNCTIONALLY RELATED TO PHOTOCHEMICAL OXIDATION OF DESICCATED CELLS. AEROBIC ORGANISMS POSSESS ANTIOXIDANT ENZYMES (SUPEROXIDE DISMUTASE CATALASE PEROXIDASE) THAT PROTECT METABOLICALLY ACTIVE CELLS AGAINST ENDOGENOUS REACTIVE OXYGEN SPECIES (ROS) GENERATED FROM AEROBIC RESPIRATION. THE ENZYMES ARE USELESS DURING DRY PERIODS AGAINST EXTERNAL UV-INDUCED ROS. SURVIVAL THROUGH DRY PERIODS REQUIRES NONENZYMATIC ANTIOXIDANTS SUCH AS MN AND TREHALOSE. THE SAME ANTIOXIDANTS WOULD ALSO BE PROTECTIVE AGAINST IONIZING RADIATION. WE WILL TEST THIS HYPOTHESIS BY STUDYING PHOTOCHEMICAL OXIDATION AND MICROBIAL RESPONSE IN THE MOJAVE DESERT. WE PREDICT THAT: 1) THERE IS A DEPTH LIMIT BEYOND WHICH PHOTOCHEMICAL OXIDANTS NO LONGER REACH 2) TOPSOIL BACTERIA CAN SURVIVE HIGHER HYDROGEN PEROXIDE CONCENTRATIONS AND HIGHER IONIZING RADIATION DOSES THAN BOTTOM-SOIL BACTERIA 3) NONENZYMATIC ANTIPEROXIDE ACTIVITY BUT NOT CATALASE ACTIVITY DIFFERS BETWEEN RADIATION RESISTANT AND SENSITIVE BACTERIA AND 4) BOTH RESISTANT AND SENSITIVE ISOLATES CAN SURVIVE BRIEF DEHYDRATION ESPECIALLY WHEN PROTECTED FROM OXIDATION. THIS INVESTIGATION WOULD SHED LIGHT ON AN EVOLUTIONARY BIOLOGY ENIGMA: THE WIDESPREAD PRESENCE OF RADIATION RESISTANT ORGANISMS ON A PLANET WHERE NATURAL BACKGROUND RADIATION IS LOW.