The biogeochemical transformation and remineralization of organic matter (OM) in marine sediments is largely driven by the activity of complex and diverse microbial communities. A major unknown in carbon biogeochemistry is determining what controls the reactivity or lability of OM to microorganisms. Recent studies have found that the ability to use different carbon sources varies among microorganisms, suggesting that the reactivity of specific carbon pools can be specific to the taxa that utilize the pool. This project investigated the extent to which the reactivity and transformation of OM is species-specific. Using a novel bioreactor system (IsoCaRB), we carried out time-series incubations using bacterial isolates and sterilized organic-rich sediment collected from Guaymas Basin. The IsoCaRB system allows us to measure the production rate and natural isotopic (Δ14C and δ13C) signature of microbially-respired CO2 to constrain the type and age of organic matter that is accessible to each species. Separate incubations using marine bacterial isolates (Vibrio sp. andPseudoalteromonas sp.) and sterilized sediment under oxic conditions showed that the rate and total quantity of organic matter metabolized by these two species differs. Approximately twice as much respired CO2 was collected during the Pseudoalteromonas sp. incubations compared to the Vibrio sp. incubations. Δ14C and δ13C signatures of respired CO2 show selective utilization of different carbon pools by the two species. These differences in OM degradation may be due to the gene copy number and substrate specificity of degradative enzymes. The results of this work indicate that organic matter transformation in marine sediments may depend on the metabolic capabilities of the microbial populations that are present and active.