In marine sediments, the organic matter pool is largely dominated by organic detritus mostly composed of high-molecular-weight compounds, recycling of which is largely dependent upon prokaryotic-mediated degradation processes (Deming and Baross 1993). Degradation processes are mainly dependent upon extracellular enzymatic hydrolysis, which allows the breakdown of high-molecularweight compounds into bioavailable monomers/oligomers suitable for prokaryotic uptake (Hoppe 1991). Degradation rates of organic matter pools in the sediment are inuenced by their biochemical composition (Arnosti and Repeta 1994; Fabiano and Danovaro 1998). Organic compounds such as humic and fulvic acids and black carbon, being resistant to decomposition, tend to accumulate in the sediment (Zegouagh et al. 1999; Middelburg et al. 1999), whereas biopolymers (i.e., proteins, carbohydrates, lipids, and nucleic acids), being more labile, are efciently degraded and recycled by extracellular enzymatic activities (Poremba 1995; Keith and Arnosti 2001). Therefore, the determination of the enzymatic hydrolytic potential of the main biopolymeric component of organic matter in the sediment represents a key functional variable for a better understanding of organic matter cycling and diagenesis and microbial loop functioning (Hoppe 1991; Meyer-Reil 1991; Meyer-Reil and Koster 1992). Enzymatic activity in the sediment can be measured by the use of uorogenic analogs (Hendel and Marxen 1997). The cleavage of the substrates, releasing the uorogenic components, provides a measure of the potential degradation rates. The degradation activities of l-aminopeptidase, β-d-glucosidase, and alkaline-phosphatase can be quantied uorometrically using, respectively, l-Leucine-4-methylcoumarinyl-7-amide (Leu-MCA), 4-methylumbelliferone β-d-glucopyranoside (MUF-Glu), and 4-methylumbelliferone phosphate (MUF-P) as substrates (Hoppe 1993; Figure 30.1).