Function and metabolism
Functions and metabolism of lipids
in marine organisms: an overview
J.R. Sargent
Umversity of Stirling, Institute of Aquaculture, Stirling FK9 4LA, UK
Marine animals can contain large amounts of neutral lipids, including
triacylglycerols, wax esters and alkyldiacylglycerols whose fundamental
role is the provision of metabolic energy, above all for growth and
reproduction, this being seen particularly clearly in the highly seasonal
reproduction of zooplankton and fish. However, all of these neutral
lipids have a lower specific gravity than sea water, so that roles in regulating buoyancy has often been postulated, especially for wax esters. The
fatty acids triacylglycerol oils from Northern Hemisphere frsh aregenerally dominated by 16:0, 18:1 (n-9), 20:1 (n-9) and 22:1 (n-11).These
fatty acids are major sources of metabolic energy in fish and are oxidised
by conventional mitochondrial (J-oxidation pathways with peroxisomal
β-oxidation also probably being involved for 22:1 (n-11).
Fish triacylglycerol oils also contain substantial amounts of 20:5 (n-3)
and 22:6 (n-3). Current evidence is that 20:5 (n-3) is oxidized in fish
by mitochondrial β-oxidation' whereas peroxisomal β-oxidation is
necessary for the catabolism of 22:6 (n-3). Marine fish acquire all of the
fatty acyl groups of their constituent body lipids from their diets and
have only a limited capacity of biosynthesis fatty acids de novo. The
20:1 (n-9) and 22:1 (n-11 ) that are so characteristic of certain northern
hemisphere fish oils originate from 20:1 (n-9) and 22:1 (n-11) fatty
alcohols in zooplanktonic wax esters, these esters commonly accounting
for more than 50% of the dry weight of the zooplankton. The polar lipids
of marine organisms, whether the glycolipids that predominate in the
thylakoid membranes of unicellular photosynthetic organisms that
constitute phytoplankton, or the phosphoglycerides that predominate in
the cell membrane bilayers of animals, are characteristically highly
unsaturated. The glycolipids of photosynthetic organisms contain a range
of polyunsaturated fatty acids (PUFAs) including 16:2 (n-7), 16:3 (n-4),
16:4 (n-1 ), 18:4 (n-3), 18:5 (n-3) and 20:5 (n-3). Where 22:6 (n-3) is
present in unicellular, photosynthetic organisms such as prymnesiophytes
ancl dinoflagellates, it is present in phosphaticlylcholine, often as cli22:6 (n-3) molecular species. Phosphoglycerides of cell membranes of
marine animals are generally highly unsaturated with 20:5 (n-3) and
22:6 (n-3) being the dominant PUFAs. The commonly held view that this
is an adaptation to low environmental temperatures is not supported
by experimental evidence, e.g. the phosphoglycerides of the warm
181
Functions and metabolism of lipids
in marine organisms: an overview
J.R. Sargent
Umversity of Stirling, Institute of Aquaculture, Stirling FK9 4LA, UK
Marine animals can contain large amounts of neutral lipids, including
triacylglycerols, wax esters and alkyldiacylglycerols whose fundamental
role is the provision of metabolic energy, above all for growth and
reproduction, this being seen particularly clearly in the highly seasonal
reproduction of zooplankton and fish. However, all of these neutral
lipids have a lower specific gravity than sea water, so that roles in regulating buoyancy has often been postulated, especially for wax esters. The
fatty acids triacylglycerol oils from Northern Hemisphere frsh aregenerally dominated by 16:0, 18:1 (n-9), 20:1 (n-9) and 22:1 (n-11).These
fatty acids are major sources of metabolic energy in fish and are oxidised
by conventional mitochondrial (J-oxidation pathways with peroxisomal
β-oxidation also probably being involved for 22:1 (n-11).
Fish triacylglycerol oils also contain substantial amounts of 20:5 (n-3)
and 22:6 (n-3). Current evidence is that 20:5 (n-3) is oxidized in fish
by mitochondrial β-oxidation' whereas peroxisomal β-oxidation is
necessary for the catabolism of 22:6 (n-3). Marine fish acquire all of the
fatty acyl groups of their constituent body lipids from their diets and
have only a limited capacity of biosynthesis fatty acids de novo. The
20:1 (n-9) and 22:1 (n-11 ) that are so characteristic of certain northern
hemisphere fish oils originate from 20:1 (n-9) and 22:1 (n-11) fatty
alcohols in zooplanktonic wax esters, these esters commonly accounting
for more than 50% of the dry weight of the zooplankton. The polar lipids
of marine organisms, whether the glycolipids that predominate in the
thylakoid membranes of unicellular photosynthetic organisms that
constitute phytoplankton, or the phosphoglycerides that predominate in
the cell membrane bilayers of animals, are characteristically highly
unsaturated. The glycolipids of photosynthetic organisms contain a range
of polyunsaturated fatty acids (PUFAs) including 16:2 (n-7), 16:3 (n-4),
16:4 (n-1 ), 18:4 (n-3), 18:5 (n-3) and 20:5 (n-3). Where 22:6 (n-3) is
present in unicellular, photosynthetic organisms such as prymnesiophytes
ancl dinoflagellates, it is present in phosphaticlylcholine, often as cli22:6 (n-3) molecular species. Phosphoglycerides of cell membranes of
marine animals are generally highly unsaturated with 20:5 (n-3) and
22:6 (n-3) being the dominant PUFAs. The commonly held view that this
is an adaptation to low environmental temperatures is not supported
by experimental evidence, e.g. the phosphoglycerides of the warm
181