8. Lipids and Essential Fatty Acids in Aquatic Food Webs
177
tive supply and composition of EFA and because highly polyunsaturated EFAs
(e.g., DHA) are potentially toxic in high concentrations. As a result of these
underlying mechanisms, the optimum is most likely wide for most organisms, in
particular for species with high metabolic flexibility or low specific requirements.
The optimum range may tum out to be more narrow with respect to reproduction
and in the very early larval stages. These speculations are further elaborated
below.
8.2.6. Fatty Acid Transport and Metabolism in Food Webs
The literature provides a rapidly accumulating database for lipid and fatty acid
metabolism in aquatic organisms and the patterns of fatty acid transport in the
lower parts of aquatic food webs. I present some results from marine algae, the
rotifer B. plicatilis, which is used as live feed for marine larvae worldwide, and
farmed Atlantic salmon (S. salar). The results are mostly previously unpublished.
8.2.6.1. Algae
Many species of plankton algae store energy as carbohydrates, but some species
use lipids or more precisely, TAGs, for storage (Reitan et aI., 1994b; Olsen, 1989).
This obviously has a great impact on the absolute 003 fatty acid content of algae.
Lipid-storing species are probably found within all taxonomic groups of algae,
and most algal species used in marine larviculture seem to be lipid-storing species.
Algae are treated in Wainman et al. (this volume), and only some general features
of marine species are covered in this section.
8.2.6.1.1. Lipids of Algae
The limiting factor for algal growth is crucial for the absolute lipid content of
lipid-storing species, whereas this is of less importance for species that store
carbohydrates. Nutrient limitation tends to depress the rate of cell division more
extensively than it does of carbon accumulation through photosynthesis, and
nutrient limitation should therefore result in algal cells that are rich in carbon
storage products, be it lipids or carbohydrates (Reitan et aI., 1994b; Olsen, 1989;
Shifrin and Chisholm, 1981). Energy (i.e., light) and carbon limitation should
instead result in a lower level of storage products (i.e., lipids) in the algal cells,
because energy or carbon compounds are limiting cell division.
Lipids in algae normally range from 5 to 70% of dry weight, with 15-30% and
8-13% as the most common ranges for species that tend to store lipids and
carbohydrates, respectively. The highest values are representative for severely
nutrient-limited lipid-storing species. The fraction of fatty acids is normally in the
range of 30-50% of total lipids in algae (Reitan et aI., 1994b). The above generalization is only valid for rough comparisons with other organisms or products.
8.2.6.1.2. Essential Fatty Acids of Algae
De novo synthesis of high amounts of long-chain 003 PUFA, like EPA and
DHA, occurs only in algae, in particular marine benthic and planktonic species
177
tive supply and composition of EFA and because highly polyunsaturated EFAs
(e.g., DHA) are potentially toxic in high concentrations. As a result of these
underlying mechanisms, the optimum is most likely wide for most organisms, in
particular for species with high metabolic flexibility or low specific requirements.
The optimum range may tum out to be more narrow with respect to reproduction
and in the very early larval stages. These speculations are further elaborated
below.
8.2.6. Fatty Acid Transport and Metabolism in Food Webs
The literature provides a rapidly accumulating database for lipid and fatty acid
metabolism in aquatic organisms and the patterns of fatty acid transport in the
lower parts of aquatic food webs. I present some results from marine algae, the
rotifer B. plicatilis, which is used as live feed for marine larvae worldwide, and
farmed Atlantic salmon (S. salar). The results are mostly previously unpublished.
8.2.6.1. Algae
Many species of plankton algae store energy as carbohydrates, but some species
use lipids or more precisely, TAGs, for storage (Reitan et aI., 1994b; Olsen, 1989).
This obviously has a great impact on the absolute 003 fatty acid content of algae.
Lipid-storing species are probably found within all taxonomic groups of algae,
and most algal species used in marine larviculture seem to be lipid-storing species.
Algae are treated in Wainman et al. (this volume), and only some general features
of marine species are covered in this section.
8.2.6.1.1. Lipids of Algae
The limiting factor for algal growth is crucial for the absolute lipid content of
lipid-storing species, whereas this is of less importance for species that store
carbohydrates. Nutrient limitation tends to depress the rate of cell division more
extensively than it does of carbon accumulation through photosynthesis, and
nutrient limitation should therefore result in algal cells that are rich in carbon
storage products, be it lipids or carbohydrates (Reitan et aI., 1994b; Olsen, 1989;
Shifrin and Chisholm, 1981). Energy (i.e., light) and carbon limitation should
instead result in a lower level of storage products (i.e., lipids) in the algal cells,
because energy or carbon compounds are limiting cell division.
Lipids in algae normally range from 5 to 70% of dry weight, with 15-30% and
8-13% as the most common ranges for species that tend to store lipids and
carbohydrates, respectively. The highest values are representative for severely
nutrient-limited lipid-storing species. The fraction of fatty acids is normally in the
range of 30-50% of total lipids in algae (Reitan et aI., 1994b). The above generalization is only valid for rough comparisons with other organisms or products.
8.2.6.1.2. Essential Fatty Acids of Algae
De novo synthesis of high amounts of long-chain 003 PUFA, like EPA and
DHA, occurs only in algae, in particular marine benthic and planktonic species