In subsurface environments, heterotrophic microbial communities access complex organic matter in order to survive. The rates and means by which they remineralize deeply buried organic carbon are still topics of speculation, due to a lack of methods for measuring specific carbon-degrading activities of these communities. The paucity of information, in turn, is related to technical limitations of standard methods, including methods for measuring microbial enzymatic activity, the initial step in degradation of high molecular weight organic matter. To date, there are only 2 publications (Coolen et al., 2000; 2002) on microbial enzyme activities in sediments at depths greater than ca. 20 cm. The proposed project will adapt a technique developed by the P.I. to measure microbial enzyme activities in surficial sediments so that it can be used routinely (also aboard ship) in deep subsurface sediments. Initial experiments suggest that using fluorescently labeled macromolecules, enzymatic hydrolysis rates and substrate structural specificities can be determined in deep subsurface sediments. Once the method has been thoroughly tested and standardized, it can be combined in the future with targeted investigations of heterotrophic metabolism and gene expression in order to obtain a more complete picture of the controls on heterotrophic life in subsurface environments.