Limnol. Oceanogr., 44(7), 1999, 1809–1814

Examination of amino acids in particulate samples from a variety of marine environments (fresh phytoplankton to deep-sea sediments) revealed systematic compositional changes upon progressive degradation. These consistent trends have been used to derive a quantitative degradation index (DI) that is directly related to the reactivity of the organic material, as indicated by its lability to enzymatic decay and its firstorder degradation rate constant. This direct link between molecular composition and degradation rate allows us to quantify the quality of organic matter based solely on its chemical composition. Decomposition of particulate organic matter (POM) is responsible for oxygen consumption in the ocean and its sediments, for the recycling of essential nutrients, and for most early diagenetic processes. The heterogeneous composition of POM leads to selective preservation of more stable (or less available) molecular compounds and to the loss of labile compounds, resulting in a continuously altered biochemical composition of the material during diagenesis (Tegelaar et al. 1989; Cowie and Hedges 1994; Wakeham et al. 1997). These compositional changes in POM are probably the reason for a decreasing first-order degradation rate (Middelburg 1989) and a reduced nutritional value toward heterotrophic consumers (Tenore et al. 1984) as degradation proceeds. Although intrinsic differences in molecular structure (de Leeuw and Largeau 1993) and differences in physiochemical association with the sediment matrix (Keil et al. 1994; Mayer 1994) are documented factors acting on early diagenesis of POM, there are few studies that directly link the resulting shifts in biochemical composition to the degradation state (Cowie and Hedges 1994; Wakeham et al. 1997; Dauwe and Middelburg 1998). Moreover, compositional characteristics of organic matter have not yet been linked to its biological availability or its degradation dynamics. A series of molecular diagenetic maturity indicators have been used to estimate the relative degradation state of the organic matter (Cowie and Hedges 1994; Wakeham et al. 1997), varying from short-term (e.g., chlorophyll) to longer term (e.g., nonprotein amino acids) indicators. Broadly applicable degradation state indicators should be based on major components that are widely distributed geographically and that are omnipresent in organisms so that variability in sources of organic matter is minimized. Moreover, they should ideally be sensitive to all stages of alteration. Proteins are ubiquitous components of all source organisms and degradation mixtures (Cowie and Hedges 1992). Although there is some dissimilarity in amino acid composition of the ultimate source organisms (e.g., diatoms, coccolithophorids, and bacteria) (Cowie and Hedges 1992), these differences are minor compared to the alteration of the spectra upon degradation (Dauwe and Middelburg 1998).