128
S.G. Wakeham
5.2
Lipids as Tracers of Organic Carbon
Lipids are widely used indicators for inferring organic carbon source and alteration
processes. Lipids are generally defined operationally as those compounds that are
water-insoluble but extracted with non-polar solvents, and analytical methodologies
are often tuned for specific compound classes (Wakeham and Volkman 1991). Along
with amino acids and carbohydrates, lipids constitute the major biochemicals in living organisms. Although usually less abundant than amino acids but more abundant
than carbohydrates, lipids typically account for 10-60% of organic carbon (DC) in
marine organisms (Parsons et al. 1984; Sargent and Henderson 1986). In living organisms, lipids play major roles in energy storage and mobilization, membrane structure,
and control of metabolic processes.
The utility of lipids as biomarkers lies in the fact that organisms biosynthesize lipids of diverse molecular structures that contain a great variety of organic functional
groups (e.g. Cranwelll982; de Leeuw and Largeau 1993). Organic geochemists are continually searching for novel biomarkers, and in a number of cases biomarkers are quite
unique to select organisms. For example, the long-chain C3TC39 alkenones that are of
such great interest to paleoceanographers (Brassell 1993) are unique to a very limited
set of haptophytes (Volkman et al. 1980; Conte et al. 1994). 4-Methylsterols are commonly (but apparently not exclusively) attributed to dinoflagellates (Boon et al. 1979;
Robinson et al. 1984). Crustacean zooplankton, primarily calanoid copepods,
biosynthesize and store wax (alkyl) esters in the C28-C38 carbon number range (Sargent
and Henderson 1986) while most other marine organisms use triacylglycerols as energy storage lipids (Sargent 1976). Long-chain hydrocarbons, alcohols, fatty acids (>C 20 )
and wax esters (C«-C 60 ) are constituents of epicuticular waxes of terrigenous vascular plants (Kolattukudy 1976) and are thus robust indicators of inputs from higher
plants. Bacterial contributions to organic matter are indicated by, for example,
branched-chain (e.g. iso- and anteiso-C1S) fatty acids (Kaneda 1991), acyclic isoprenoids
(Risatti et al. 1984) and hopanoids (Ourisson et al. 1987).
Lipids are relatively labile toward degradation in the ocean, potentially more reactive than amino acids and carbohydrates (Wakeham et al. 1997a). Rapid degradation
of lipid components, either by autolysis or by hydrolytic attack by enzymes from heterotrophic consumers, usually follows the death of the producer organism. Degradation results in qualitative changes in composition between surface water particulate
matter and sediment that can be significant as the more labile compounds are lost and
more refractory compounds are conserved, further complicating assessments of upper-ocean environmental conditions.
An example of this qualitative decoupling of surface water lipid composition from
that of sediments is illustrated in Fig. 5-1, contrasting the molecular distributions of
hydrocarbons in a sediment trap sample from a 500 m depth in the Arabian Sea and
with that of underlying sediments at a 3500 m depth. Hydrocarbons in the trap are
dominated by compounds of phytoplanktonic origin, in keeping with the high primary
production in the Arabian Sea. Diatoms are abundant during the monsoon upwelling
season and are the probable sources of n-CJ7> 3,6,9,12,15,18-heneicosahexaene (HEH),
and a series of C2s-isoprenoid alkenes (bf25) (Rowland and Robson 1990; Volkman et al.
1994 The sterenes (steroidal hydrocarbons) in the trap sample are thought to be prod-
S.G. Wakeham
5.2
Lipids as Tracers of Organic Carbon
Lipids are widely used indicators for inferring organic carbon source and alteration
processes. Lipids are generally defined operationally as those compounds that are
water-insoluble but extracted with non-polar solvents, and analytical methodologies
are often tuned for specific compound classes (Wakeham and Volkman 1991). Along
with amino acids and carbohydrates, lipids constitute the major biochemicals in living organisms. Although usually less abundant than amino acids but more abundant
than carbohydrates, lipids typically account for 10-60% of organic carbon (DC) in
marine organisms (Parsons et al. 1984; Sargent and Henderson 1986). In living organisms, lipids play major roles in energy storage and mobilization, membrane structure,
and control of metabolic processes.
The utility of lipids as biomarkers lies in the fact that organisms biosynthesize lipids of diverse molecular structures that contain a great variety of organic functional
groups (e.g. Cranwelll982; de Leeuw and Largeau 1993). Organic geochemists are continually searching for novel biomarkers, and in a number of cases biomarkers are quite
unique to select organisms. For example, the long-chain C3TC39 alkenones that are of
such great interest to paleoceanographers (Brassell 1993) are unique to a very limited
set of haptophytes (Volkman et al. 1980; Conte et al. 1994). 4-Methylsterols are commonly (but apparently not exclusively) attributed to dinoflagellates (Boon et al. 1979;
Robinson et al. 1984). Crustacean zooplankton, primarily calanoid copepods,
biosynthesize and store wax (alkyl) esters in the C28-C38 carbon number range (Sargent
and Henderson 1986) while most other marine organisms use triacylglycerols as energy storage lipids (Sargent 1976). Long-chain hydrocarbons, alcohols, fatty acids (>C 20 )
and wax esters (C«-C 60 ) are constituents of epicuticular waxes of terrigenous vascular plants (Kolattukudy 1976) and are thus robust indicators of inputs from higher
plants. Bacterial contributions to organic matter are indicated by, for example,
branched-chain (e.g. iso- and anteiso-C1S) fatty acids (Kaneda 1991), acyclic isoprenoids
(Risatti et al. 1984) and hopanoids (Ourisson et al. 1987).
Lipids are relatively labile toward degradation in the ocean, potentially more reactive than amino acids and carbohydrates (Wakeham et al. 1997a). Rapid degradation
of lipid components, either by autolysis or by hydrolytic attack by enzymes from heterotrophic consumers, usually follows the death of the producer organism. Degradation results in qualitative changes in composition between surface water particulate
matter and sediment that can be significant as the more labile compounds are lost and
more refractory compounds are conserved, further complicating assessments of upper-ocean environmental conditions.
An example of this qualitative decoupling of surface water lipid composition from
that of sediments is illustrated in Fig. 5-1, contrasting the molecular distributions of
hydrocarbons in a sediment trap sample from a 500 m depth in the Arabian Sea and
with that of underlying sediments at a 3500 m depth. Hydrocarbons in the trap are
dominated by compounds of phytoplanktonic origin, in keeping with the high primary
production in the Arabian Sea. Diatoms are abundant during the monsoon upwelling
season and are the probable sources of n-CJ7> 3,6,9,12,15,18-heneicosahexaene (HEH),
and a series of C2s-isoprenoid alkenes (bf25) (Rowland and Robson 1990; Volkman et al.
1994 The sterenes (steroidal hydrocarbons) in the trap sample are thought to be prod-