11
Comparison of Lipids in Marine and
Freshwater Organisms
Robert G. Ackman
11.1. Introduction
Lipids, carbohydrates, and proteins are the basic components of aquatic organisms, and all have distinct roles. For example, the carbohydrates can be structural
components in phytoplankton and macrophytes and energy reserves in bivalve
mollusks. Carbohydrates are barely mentioned in fish biochemistry except for
glucose, which has a function in muscle energy metabolism (Kiessling et aI.,
1995). The proteins do not seem important in primitive organisms, but some
invertebrates use free amino acids for ionic balance and, in moving up the evolutionary scale, the role of proteins in muscle of mobile life forms becomes very
important. The role of lipids in photosynthetic carbon fixation is not obvious
although the photosynthetic apparatus depends on certain fatty acids and lipid
classes (GUIT and Harwood, 1991). In most organisms, fatty acids are commonly
three-quarters of the mass of phospholipids, which are critical in membranes.
Aquatic bacteria survive in a highly stressful environment because they are encased in lipids. Keweloh and Heipieper (1996) point out that stable saturated fatty
acids are present in these lipids, but the high-melting trans monoethylenic fatty
acids may be formed in parallel to, or from, the common cis isomers, to adapt this
type of organism to a hostile environment. More intriguing is the recent but
repeated discovery of eicosapentaenoic acid (20:50)3, popularly designated EPA)
in marine bacteria (Nichols et aI., 1996; Yazawa, 1996; Henderson et aI., 1995a;
Akimoto et aI., 1990). Hitherto, this fatty acid, sensitive to oxidation, would have
been associated with invertebrates that accumulate it from phytoplankton (Ackman and Kean-Howie, 1995). In moving up the evolutionary scale, lipids also play
a major role in the neurotransmission system critical to mobile animals and in
sensory perception organs such as the retina or tapetum lucidum of the fish eye,
where docosahexaenoic acid (22:60)3, popularly designated DHA) is important
(Nicol et aI., 1972). However, marine invertebrates do not seem to have this
specific requirement for high levels of DHA, and in the case of Artemia sp. eye
phospholipids, there is no 22:60)3 (Navarro et aI., 1992).
263
Comparison of Lipids in Marine and
Freshwater Organisms
Robert G. Ackman
11.1. Introduction
Lipids, carbohydrates, and proteins are the basic components of aquatic organisms, and all have distinct roles. For example, the carbohydrates can be structural
components in phytoplankton and macrophytes and energy reserves in bivalve
mollusks. Carbohydrates are barely mentioned in fish biochemistry except for
glucose, which has a function in muscle energy metabolism (Kiessling et aI.,
1995). The proteins do not seem important in primitive organisms, but some
invertebrates use free amino acids for ionic balance and, in moving up the evolutionary scale, the role of proteins in muscle of mobile life forms becomes very
important. The role of lipids in photosynthetic carbon fixation is not obvious
although the photosynthetic apparatus depends on certain fatty acids and lipid
classes (GUIT and Harwood, 1991). In most organisms, fatty acids are commonly
three-quarters of the mass of phospholipids, which are critical in membranes.
Aquatic bacteria survive in a highly stressful environment because they are encased in lipids. Keweloh and Heipieper (1996) point out that stable saturated fatty
acids are present in these lipids, but the high-melting trans monoethylenic fatty
acids may be formed in parallel to, or from, the common cis isomers, to adapt this
type of organism to a hostile environment. More intriguing is the recent but
repeated discovery of eicosapentaenoic acid (20:50)3, popularly designated EPA)
in marine bacteria (Nichols et aI., 1996; Yazawa, 1996; Henderson et aI., 1995a;
Akimoto et aI., 1990). Hitherto, this fatty acid, sensitive to oxidation, would have
been associated with invertebrates that accumulate it from phytoplankton (Ackman and Kean-Howie, 1995). In moving up the evolutionary scale, lipids also play
a major role in the neurotransmission system critical to mobile animals and in
sensory perception organs such as the retina or tapetum lucidum of the fish eye,
where docosahexaenoic acid (22:60)3, popularly designated DHA) is important
(Nicol et aI., 1972). However, marine invertebrates do not seem to have this
specific requirement for high levels of DHA, and in the case of Artemia sp. eye
phospholipids, there is no 22:60)3 (Navarro et aI., 1992).
263