EVALUATING THE COEVOLUTION OF MICROBES AND MINERALS AND THE STRATEGIES USED BY MICROBES TO ACCESS INSOLUBLE ELECTRON ACCEPTORS IS CRITICAL TO UNDERSTANDING EARLY LIFE ON EARTH AND THE POTENTIAL FOR LIFE ELSEWHERE IN THE UNIVERSE. RELEVANT IN THIS REGARD IS ONE OF THE EARLIEST METABOLISMS ON EARTH DISSIMILATORY IRON REDUCTION OR THE EXTRACELLULAR REDUCTION OF FE(III) TO FE(II). THE TERMINAL ELECTRON ACCEPTORS IN THIS PROCESS FE(III) OXIDES ARE ABUNDANT IN TERRESTRIAL AND MARINE ENVIRONMENTS ON EARTH AND HAVE BEEN FOUND IN EXTRATERRESTRIAL ENVIRONMENTS PARTICULARLY ON MARS. MANY MICROBES ARE KNOWN TO REDUCE AND TRANSFORM FE(III) OXIDES; HOWEVER THE BEST CANDIDATES FOR STUDYING THIS PROCESS ARE HYPERTHERMOPHILIC ARCHAEA DUE TO THEIR PRIMITIVE NATURE AND WIDESPREAD PREVALENCE IN EXTRATERRESTRIAL ANALOG ENVIRONMENTS ON EARTH. PREVIOUS STUDIES ON MICROBIAL FE(III) REDUCTION HAVE PRIMARILY FOCUSED ON MESOPHILES PARTICULARLY PROTEOBACTERIA; LITTLE IS KNOWN ABOUT THE PHYSIOLOGY OF FE(III) REDUCTION IN HYPERTHERMOPHILIC ARCHAEA. TO ADDRESS THIS GAP IN KNOWLEDGE THIS PROJECT EXAMINES THE PHYSIOLOGICAL MECHANISM OF IRON REDUCTION IN PYRODICTIUM DELANEYI A HYPERTHERMOPHILIC ARCHAEON ISOLATED FROM A DEEP-SEA HYDROTHERMAL CHIMNEY. WE WILL PERFORM MRNA TRANSCRIPT AND PROTEIN ANALYSES TO IDENTIFY THE RESPIRATORY COMPLEX(ES) USED BY THE ORGANISM TO REDUCE THE IRON OXIDE MINERAL FERRIHYDRITE TO MAGNETITE. FURTHER WE WILL EVALUATE THE EXTENT OF MICROBE-MINERAL INTERACTION AND THE PHYSICAL MECHANISM FOR FE(III) REDUCTION IN THE ORGANISM. ADDRESSING THESE GOALS WILL PROVIDE FIRST-PRINCIPLES ON HOW PRIMITIVE ORGANISMS SUCH AS HYPERTHERMOPHILIC ARCHAEA PAST AND PRESENT COULD AND CAN GAIN METABOLIC ENERGY THOUGH THE DISSIMILATORY REDUCTION OF A MINERAL. THIS KNOWLEDGE WILL FILL A CRUCIAL GAP IN UNDERSTANDING AN EARLY MICROBIAL PROCESS FOR EARTH AND ONE THAT MAY REPRESENT PARADIGMS FOR LIFE ON OTHER PLANETS.