Although hypersaline environments are often thought of as terminal habitats, they play an active role in the interconversion of organic and inorganic carbon, a process that is interfaced microbiologically with the reduction of sulfate to sulfide. In both normal marine and hypersaline marine environments, carbon cycle transformations result in changing pools of dissolved Σ CO2 (CO2 + HCO 3 - + CO 3 2- ), solid carbonates, and organic carbon. In the marine environment, sulfur cycle transformations result in changing pools of dissolved sulfate and sulfide, solid sulfides and sulfur, and organic sulfur, while in marine-derived hypersaline environments, solid sulfate (gypsum) may provide an additional large pool of potential oxidant of organic matter. Given the simplicity of food chains in hypersaline environments relative to those in normal marine habitats, the high concentrations of sulfate, and the possibly limited activity of anaerobic bacteria in extremely saline sediments, it might be expected that rates and kinds of interconversions between reduced and oxidized pools of carbon and sulfur in hypersaline environments may differ from those in normal marine sediments. The following discussion outlines aspects of carbon and sulfur cycles from sediment and porewaters of hyper-saline environments, using evidence from rates and types of transformations, organic and inorganic geochemistry, and stable isotope fractionations.
