BIOLOGICAL SULFATE REDUCTION COULD BE A QUANTITATIVELY IMPORTANT METABOLISM IN THE CONTEXT OF MODERN MARS EUROPA AND POSSIBLY ENCELADUS BUT PRESENTLY AVAILABLE DATA POORLY CONSTRAIN THIS POTENTIAL. IN PARTICULAR EXTENSIVE REACTIVITY OF SULFATE DURING WATER-ROCK REACTIONS (E.G. SERPENTINIZATION) WOULD SERVE TO DEPLETE ITS AVAILABILITY AS A METABOLIC OXIDANT FOR SUBSURFACE ECOSYSTEMS EVEN ON WORLDS WHERE SULFATE IS PREVALENT IN THE SURFACE CHEMISTRY BUT THE DEGREE OF REACTIVITY IS LARGELY UNCERTAIN FOR THE ROCK TYPES AND CONDITIONS MOST RELEVANT TO THE SUBSURFACE OF MARS AND THE ICY SATELLITES. MOREOVER THOSE SAME WATER-ROCK REACTIONS YIELD A RANGE OF FLUID CHEMISTRIES IN WHICH THE POTENTIAL FOR BIOLOGICAL SULFATE REDUCTION IS NOT WELL EXPLORED. THE WORK PROPOSED HERE WILL ADDRESS BOTH AREAS OF UNCERTAINTY WITH FIELD AND EXPERIMENTAL DATA THAT CONSTRAIN THE POTENTIAL FOR BOTH ABIOTIC AND BIOLOGICAL SULFATE REDUCTION IN THE CONTEXT OF FLUIDROCK INTERACTION. RESULTS OF THIS WORK WILL IMPROVE OUR UNDERSTANDING OF SULFUR CYCLING IN FLUID CHEMICAL EVOLUTION AND THE HABITABILITY OF MARS AND THE OCEAN WORLDS.