DESERT CYANOBACTERIA LICHENS AND BRYOPHYTES (MOSSES) POSSESS THREE ADAPTATIONS CRITICAL TO THEIR EXISTENCE NEAR THE DRY LIMIT OF HABITABILITY. THESE INCLUDE 1) POIKILOHYDRY THE ABILITY TO SURVIVE DEHYDRATION IN A VEGETATIVE STATE 2) SUN SCREENS THE ABILITY TO FORTIFY THEIR THALLUS OR CELL SURFACE WITH PIGMENTS THAT ARE OPTICALLY OPAQUE TO ULTRAVIOLET RAYS (UV) BUT RELATIVELY TRANSPARENT TO PHOTOSYNTHETICALLY ACTIVE RADIATION AND 3) ANTIOXIDANTS WHICH ARE NECESSARY DURING LONG PERIODS OF DEHYDRATION AND METABOLIC INACTIVITY. IN A PRELIMINARY STUDY INITIATED TO DETERMINE WHETHER OR NOT THESE ADAPTATIONS HAVE A LIMIT WE CHALLENGED DRIED CYANOBACTERIA AND LICHENS TO WET-DRY CYCLES UVC (254 NM 4.7 WATTS/M2) AND 5% O3. AFTERWARD WE REHYDRATED THE ORGANISMS AND MONITORED THEIR PHOTOSYNTHETIC RECOVERY USING A PULSE-AMPLITUDE-MODULATED CHLOROPHYLL FLUOROMETER (PAM). RESULTS THUS FAR INDICATE THAT THE ORGANISMS ARE EFFECTIVELY INSENSITIVE TO WET-DRY CYCLES AND UVC BUT THEY ARE SUSCEPTIBLE TO EXTREME OXIDATION. IN OXIDATION RESISTANCE LICHENS OUTPERFORMED CYANOBACTERIA PRESUMABLY BECAUSE THEY HAVE A MORE SOPHISTICATED THALLUS WITH A MULTI-LAYERED ANTIOXIDANT DEFENSE. THIS PROPOSAL SEEKS TO EVALUATE THESE INSIGHTS WITH THREE SPECIES FROM EACH GROUP BY DETERMINING THEIR TOLERANCE UNDER MORE CONTROLLED CONDITIONS INCLUDING 1) A RAPID SEQUENCE OF WET-DRY CYCLES IN AN ANAEROBIC GLOVEBOX IN THE DARK WITHOUT OPPORTUNITY FOR REPAIR 2) HIGH-INTENSITY UVC IRRADIATION (9 WATTS/M2) IN AMBIENT AIR 3) HIGH-INTENSITY UVC IRRADIATION IN CO2 TO KEEP OUT O3 AND 4) 5% O3. PHOTOBIONT VIABILITY WILL BE ASSESSED USING PAM CHLOROPHYLL FLUOROMETRY AND 14CO2 LABELED UPTAKE. IN O3-KILLED SPECIMENS OXIDATION BIOMARKERS SPECIFIC TO POLYSACCHARIDES LIPIDS AND PROTEINS SEPARATING CYTOSOLIC AND DNA/NUCLEUSASSOCIATED PROTEINS WILL BE QUANTIFIED TO UNDERSTAND DEATH BY OXIDATION AT THE MOLECULAR LEVEL. RESULTS ARE EXPECTED TO CHALLENGE THE NOTION THAT A UVC-ATTENUATING ATMOSPHERE IS A NECESSARY CONDITION FOR SURFACE BIOTA TO SHOW THAT DRY LIMITS ARE BECAUSE OF OXIDATION RATHER THAN DESICCATION AND TO REVEAL THE POSSIBILITY THAT ANAEROBIC WORLDS ARE WITHOUT DRY LIMITS.