Modern microbial mats have been used as analogs for early life because of the longevity of microbial life on Earth. Mats collected from hypersaline salterns in Baja California were maintained for over a year and a half under both normal (85 ppt salinity, 50 mM SO42-) and reduced salinity (35 ppt salinity; near modern seawater) and sulfate concentrations (20 mM, ≤1 mM SO42-) to assess carbon processing under sulfate conditions more similar to the Archean oceans. As sulfate was removed from the mats by diffusion out into the overlying water, methane concentrations within the mats increased. Highest methane concentrations occurred in mats with reduced salinity and little sulfate. The δ13C values of bulk particulate organic matter in all of the mats averaged -11?, similar to what had been observed previously for these Microcoleus mats. In mats with sulfate, pore water concentrations of dissolved inorganic carbon (DIC) δ13C values averaged about -3?. However, in the mats with ≤1 mM sulfate concentrations, the DIC δ13C values increased substantially with depth, from about -1? in the overlying water to +10? by 20 mm depth. Rates of methanogenesis, calculated from pore water dissolved methane concentration profiles, were too low to account for the total increase in DIC δ13C values. These positive isotopic signatures, however, are also consistent with the occurrence of acetogenesis, as are the higher acetate concentrations in the low-sulfate mats. Acetogens may be poised to become successful competitors for substrates in these mats, given the right environmental conditions.