136
C. Lee
branched-chain fatty acids can serve as specific markers of bacteria (Lee et al. 1983;
de Baar et al. 1983; Wakeham and Canuel 1988).
5.4.2
Diagenetic Indicators
Although phytoplankton producers initially dominate the organic composition of sinking particles, consumers can substantially alter this composition through degradation
and alteration reactions. Not only can organic compounds be consumed, but waste
products that are a structural portion of the original compound can be left behind.
Or, bacteria colonizing a particle can synthesize new bacterial biomass either from
the organic matter within the particle or by consuming DOC. A number of individual
compounds indicate the freshness or diagenetic state of organic matter. For example,
certain labile phytoplankton constituents, such as polyunsaturated fatty acids, are
readily degraded in the environment and/or herbivore guts, and thus are depleted in
more degraded particles (de Baar et al.1983; Wakeham and Canuel 1988). Preferential
loss of labile algal fatty acids resulting in the enrichment of more stable components
in the products of heterotrophic metabolism has been observed both in field studies
and laboratory feeding experiments (Prahl et al. 1985; Wakeham and Canuel 1988;
Harvey et al. 1987). Similarly, high relative percentages of nitrogen and carbon in the
form of chromatographically-measured amino acids (Whelan 1977; Lee and Cronin
1984; Cowie and Hedges 1992) and carbohydrates (Cowie et al.1992; Cowie and Hedges
1992) are also indicative of relatively undegraded organic remains. Conversely, high
relative concentrations of characteristic diagenetic products, such as the non-protein
amino acids ornithine and f3-alanine can indicate the presence of bacterially degraded
material (Lee and Cronin 1982,1984; Ittekkot et al. 1984a,b). Use of multiple compounds
that indicate the stage of diagenesis provides a sensitive and consistent means of comparing the quality of organic matter in aquatic environments. For example, using a
combination of only sixteen lipid, amino acid, pigment and carbohydrate compounds
or groups of compounds, Wakeham et al. (1997) divided organic matter collected in
the water column and sediment of the equatorial Pacific into four clearly-defined diagenetic classes that could be traced from sea surface to sediment (Fig. 5.7).
Freshness of organic matter can also be influenced by an intimately associated
mineral or organic matrix. Resistant organic matrices may protect otherwise labile
lipids. For example, waxy coatings common to land plants apparently protect higher
plant alkanes, fatty acids, and fatty alcohols from degradation. Thus, terrestrial
biomarkers are abundant in abyssal sediments, even though they are minor components of the particles produced in the overlying waters (Wakeham et al. 1984; Volkman
et al. 1983; Gagosian et al. 1983). Marine-derived compounds seem to lack such a protective matrix and thus are preferentially degraded in the water column.
Several studies have reported that the adsorption of organic compounds serves to protect them from microbial degradation (Christensen and Blackburn 1980; Marshman
and Marshall 1981; Gordon and Millero 1985). Mayer (1994) has recently postulated that
adsorption of organic matter into micropores of inorganic sedimentary material might
physically remove it from the action of hydrolyzing enzymes, which cannot function
within the tiny pores. This mechanism may be responsible for the almost universal correlation between organic carbon content and mineral grain size observed in marine sedi-
C. Lee
branched-chain fatty acids can serve as specific markers of bacteria (Lee et al. 1983;
de Baar et al. 1983; Wakeham and Canuel 1988).
5.4.2
Diagenetic Indicators
Although phytoplankton producers initially dominate the organic composition of sinking particles, consumers can substantially alter this composition through degradation
and alteration reactions. Not only can organic compounds be consumed, but waste
products that are a structural portion of the original compound can be left behind.
Or, bacteria colonizing a particle can synthesize new bacterial biomass either from
the organic matter within the particle or by consuming DOC. A number of individual
compounds indicate the freshness or diagenetic state of organic matter. For example,
certain labile phytoplankton constituents, such as polyunsaturated fatty acids, are
readily degraded in the environment and/or herbivore guts, and thus are depleted in
more degraded particles (de Baar et al.1983; Wakeham and Canuel 1988). Preferential
loss of labile algal fatty acids resulting in the enrichment of more stable components
in the products of heterotrophic metabolism has been observed both in field studies
and laboratory feeding experiments (Prahl et al. 1985; Wakeham and Canuel 1988;
Harvey et al. 1987). Similarly, high relative percentages of nitrogen and carbon in the
form of chromatographically-measured amino acids (Whelan 1977; Lee and Cronin
1984; Cowie and Hedges 1992) and carbohydrates (Cowie et al.1992; Cowie and Hedges
1992) are also indicative of relatively undegraded organic remains. Conversely, high
relative concentrations of characteristic diagenetic products, such as the non-protein
amino acids ornithine and f3-alanine can indicate the presence of bacterially degraded
material (Lee and Cronin 1982,1984; Ittekkot et al. 1984a,b). Use of multiple compounds
that indicate the stage of diagenesis provides a sensitive and consistent means of comparing the quality of organic matter in aquatic environments. For example, using a
combination of only sixteen lipid, amino acid, pigment and carbohydrate compounds
or groups of compounds, Wakeham et al. (1997) divided organic matter collected in
the water column and sediment of the equatorial Pacific into four clearly-defined diagenetic classes that could be traced from sea surface to sediment (Fig. 5.7).
Freshness of organic matter can also be influenced by an intimately associated
mineral or organic matrix. Resistant organic matrices may protect otherwise labile
lipids. For example, waxy coatings common to land plants apparently protect higher
plant alkanes, fatty acids, and fatty alcohols from degradation. Thus, terrestrial
biomarkers are abundant in abyssal sediments, even though they are minor components of the particles produced in the overlying waters (Wakeham et al. 1984; Volkman
et al. 1983; Gagosian et al. 1983). Marine-derived compounds seem to lack such a protective matrix and thus are preferentially degraded in the water column.
Several studies have reported that the adsorption of organic compounds serves to protect them from microbial degradation (Christensen and Blackburn 1980; Marshman
and Marshall 1981; Gordon and Millero 1985). Mayer (1994) has recently postulated that
adsorption of organic matter into micropores of inorganic sedimentary material might
physically remove it from the action of hydrolyzing enzymes, which cannot function
within the tiny pores. This mechanism may be responsible for the almost universal correlation between organic carbon content and mineral grain size observed in marine sedi-