1
Determination of Total Lipid,
Lipid Classes, and Fatty Acids
in Aquatic Samples
Christopher C. Parrish
1.1. Introduction
The hydrophobic nature of lipids provides a convenient means of separating them
from other compounds in an aqueous sample matrix. Extraction in nonpolar
solvents is universally employed and is the basis of the operational definition of
lipids. This approach is used routinely in algal biosynthetic studies in which the
fate of a radiolabeled precursor is followed into the lipid pool. By adding 14C
bicarbonate to a sample from the field (Wainman and Lean, 1992) or a culture
(Rai, 1995) and then later extracting the sample with a water-immiscible organic
solvent, the "lipid fraction of carbon fixation" (LFCF) can be detennined (Wainman and Lean, 1992). By perfonning a chromatographic separation before counting, this procedure can be further refined to detennine the subclasses in which the
14C ends up (Smith and D'Souza, 1993). Subfractionation is important when a
differentiation between allocation to lipid storage and membrane synthesis is
required. The radiolabeling approach is convenient, sensitive, and not prone to
contamination. However, many ecological studies are not amenable to this approach, and so chemical analysis of the constituents of lipid extracts has to be
perfonned.
A lipid extract may contain as many as 16 different subclasses of both biogenic
and anthropogenic origin (Parrish, 1988). Figure 1.1 shows some of the more
important biogenic classes. Triacylglycerols and phospholipids are biochemically
related, as they both possess a glycerol backbone to which two or three fatty acids
are esterified, and they also share common precursors: diacylglycerols derived
from phosphatidic acid. Triacylglycerols are a very important energy storage
substance, whereas phospholipids are essential components of membranes.
Sterols share with phospholipids a structural function in membranes, but in terms
of polarity they are grouped with triacylglycerols in the neutral lipids. Phospholipids are grouped with the polar lipids including glycolipids. Glycolipids contain
one or more molecules of a sugar and are found in bacteria, plants, and animals.
Glycoglycerolipids of the fonn shown in Figure 1.1 are found predominantly in
the plant kingdom in association with chloroplasts.
4
Determination of Total Lipid,
Lipid Classes, and Fatty Acids
in Aquatic Samples
Christopher C. Parrish
1.1. Introduction
The hydrophobic nature of lipids provides a convenient means of separating them
from other compounds in an aqueous sample matrix. Extraction in nonpolar
solvents is universally employed and is the basis of the operational definition of
lipids. This approach is used routinely in algal biosynthetic studies in which the
fate of a radiolabeled precursor is followed into the lipid pool. By adding 14C
bicarbonate to a sample from the field (Wainman and Lean, 1992) or a culture
(Rai, 1995) and then later extracting the sample with a water-immiscible organic
solvent, the "lipid fraction of carbon fixation" (LFCF) can be detennined (Wainman and Lean, 1992). By perfonning a chromatographic separation before counting, this procedure can be further refined to detennine the subclasses in which the
14C ends up (Smith and D'Souza, 1993). Subfractionation is important when a
differentiation between allocation to lipid storage and membrane synthesis is
required. The radiolabeling approach is convenient, sensitive, and not prone to
contamination. However, many ecological studies are not amenable to this approach, and so chemical analysis of the constituents of lipid extracts has to be
perfonned.
A lipid extract may contain as many as 16 different subclasses of both biogenic
and anthropogenic origin (Parrish, 1988). Figure 1.1 shows some of the more
important biogenic classes. Triacylglycerols and phospholipids are biochemically
related, as they both possess a glycerol backbone to which two or three fatty acids
are esterified, and they also share common precursors: diacylglycerols derived
from phosphatidic acid. Triacylglycerols are a very important energy storage
substance, whereas phospholipids are essential components of membranes.
Sterols share with phospholipids a structural function in membranes, but in terms
of polarity they are grouped with triacylglycerols in the neutral lipids. Phospholipids are grouped with the polar lipids including glycolipids. Glycolipids contain
one or more molecules of a sugar and are found in bacteria, plants, and animals.
Glycoglycerolipids of the fonn shown in Figure 1.1 are found predominantly in
the plant kingdom in association with chloroplasts.
4