Marine sediments cover almost ~75% of the Earth’s surface and contain a large fraction of the Earth’s living biomass. The metabolic activities of sedimentary microbes largely depend on the availability of electron donors and electron acceptors. In most marine sediments, organic matter is the predominant electron source due to its high redox potential and ubiquity. However, the biotic and abiotic factors that mediate organic matter availability and degradation are not well understood. The rate of organic matter degradation is thought to be dependent on the quantity and quality (or lability) of organic matter. However, the capacity to use various carbon sources appears to vary among marine microorganisms such that the lability of certain pools of carbon can be specific to the microbial taxa that utilize the pool. Using a combination of genomic, biochemical and isotopic approaches, I propose to elucidate the mechanisms that influence the lability, and ultimately availability, of sedimentary organic matter to microorganisms. Specifically, I will test whether specific classes of extracellular enzymes lead to differential mobilization of sedimentary carbon. Characterizing the nature and availability of organic matter is imperative for elucidating the mechanisms controlling geochemical gradients and the corresponding microbial activities in these ecosystems.