The oxidation of sulfide to sulfate via anoxygenic photosynthesis is a process that may have had significant importance to the formation of sulfate in the Earth's early ocean.  Investigating how modern sulfide oxidizers affect the isotopic composition of sulfate may provide us clues as to how sulfate was formed in the geological past.  For the first time, we measured the d³⁴S and d¹⁸O values of sulfate produced during a series of sulfide oxidation experiments in the laboratory with a pure culture of the photosynthetic purple sulfur bacterium, Allochromatium vinosum.  The e¹⁸O_SO4-H2O and e³⁴S_SO4-H2S values measured from these experiments will be used to more accurately interpret the d³⁴S_SO4 and d¹⁸O_SO4 values of sulfates preserved in the geological record and specifically their relationships to the evolving redox state of the early ocean and atmosphere.  Biomarkers from purple sulfur bacteria are known to extend back more than 1.6 billion years, and phototrophic sulfide oxidation may have been a major source of sulfate in the Archean ocean.  Consequently, diagnostic isotopic signatures may allow us to fingerprint these processes in the rock record.  Our data also provide an essential experimental context for our ongoing studies of modern sulfidic systems dominated by purple sulfur bacteria, such as Mahoney Lake in British Columbia.