184
Y. Olsen
selmis sp. is higher than the percentage DHA in I. galbana. In fact, absolute DHA
contents of I. galbana were two to three times higher than EPA contents of
Tetraselmis sp. This clearly shows that the relative fatty acid distribution of the
food algae has a greater impact on the rate of change in fatty acid distribution than
the absolute fatty acid content (Olsen et aI., 1993b). The catabolic rate of DHA
degradation in the rotifers may regrettably also have been slightly higher than the
catabolic rate of EPA (see above), but this is believed to be less important.
Rotifers fed I. galbana for 4 h (lower curve, Fig. 8. lOA) showed a logarithmic
reduction in the percentage DHA content when given Baker's yeast or Tetraselmis
sp. (Fig. 8.1OC). By contrast, rotifers given Tetraselmis sp. for 4 h (upper curve,
Fig. 8.IOA) showed a logarithmic reduction in percentage EPA when fed with
Baker's yeast or I. galbana for another 4 h (Fig. 8.108). The two types of feed
used during the second stage differed with respect to EFA contents but gave the
same time course for EPA or DHA reduction. This illustrates how dietary EPA and
DHA contents, rather than the type or taxonomic class of the food, are decisive in
determining the fatty acid profile of rotifers. This implies that any metabolic
conversion of DHA and EPA, which are believed to be among the most labile fatty
acids of the rotifer tissues (Olsen et aI., 1993a), is insignificant compared with the
net changes in fatty acid distribution taking place when the food is changed.
The above dynamic response of the fatty acid composition of rotifer tissues
following changes in fatty acid composition of the diet is probably typical for
TAG-zooplankton, including freshwater species. The change in different fatty
acids, expressed in terms of percentage of total fatty acids, can be interpreted as a
continuous dilution of the fatty acids contained in zooplankton tissues due to the
constant consumption of new dietary fatty acids.
If no selective catabolism or anabolism of fatty acids takes place in the rotifer
tissues, we can anticipate that their fatty acid composition will become very close
to that of their feed, provided that the rotifers are cultivated for a long time on a
fixed diet, exceeding the lifetime of the individual rotifer (i.e., more than five
doublings of biomass or IS d). The fatty acid compositions of dietary lipids and
rotifers, grown for more than five generations on a fixed diet, are indeed very
closely related, but the fatty acid profiles are not identical (Fig. 8.11). The DHA
content is lower in rotifers than in the food, whereas the content of docosapentaenoic acid (DPA) (22:5w3) is slightly higher. The differences found in shorter
w3 fatty acids and w6 fatty acids are relatively minor. The distribution of monounsaturated fatty acids suggests reduced levels of the long-chain fatty acids 20: I and
22: I and increased level of the short-chain 16: I and 18: I fatty acids in rotifers
compared with their food. Saturated fatty acids were generally found in slightly
higher proportions in the rotifers than in the diet. The data in Figure 8.11 therefore
suggest that B. plicatilis selectively catabolizes DHA into DPA (and EPA?). It also
catabolizes the long-chained monounsaturated fatty acids 20: I and 22: I, considered to be important fuel in the marine food web (Sargent and Henderson,
1986), at a slightly higher rate than most other fatty acids. No indications of chain
elongation have been found for this strain of B. plicatilis, which is not surprising
considering the EFA-rich diets used to cultivate the rotifers.
Y. Olsen
selmis sp. is higher than the percentage DHA in I. galbana. In fact, absolute DHA
contents of I. galbana were two to three times higher than EPA contents of
Tetraselmis sp. This clearly shows that the relative fatty acid distribution of the
food algae has a greater impact on the rate of change in fatty acid distribution than
the absolute fatty acid content (Olsen et aI., 1993b). The catabolic rate of DHA
degradation in the rotifers may regrettably also have been slightly higher than the
catabolic rate of EPA (see above), but this is believed to be less important.
Rotifers fed I. galbana for 4 h (lower curve, Fig. 8. lOA) showed a logarithmic
reduction in the percentage DHA content when given Baker's yeast or Tetraselmis
sp. (Fig. 8.1OC). By contrast, rotifers given Tetraselmis sp. for 4 h (upper curve,
Fig. 8.IOA) showed a logarithmic reduction in percentage EPA when fed with
Baker's yeast or I. galbana for another 4 h (Fig. 8.108). The two types of feed
used during the second stage differed with respect to EFA contents but gave the
same time course for EPA or DHA reduction. This illustrates how dietary EPA and
DHA contents, rather than the type or taxonomic class of the food, are decisive in
determining the fatty acid profile of rotifers. This implies that any metabolic
conversion of DHA and EPA, which are believed to be among the most labile fatty
acids of the rotifer tissues (Olsen et aI., 1993a), is insignificant compared with the
net changes in fatty acid distribution taking place when the food is changed.
The above dynamic response of the fatty acid composition of rotifer tissues
following changes in fatty acid composition of the diet is probably typical for
TAG-zooplankton, including freshwater species. The change in different fatty
acids, expressed in terms of percentage of total fatty acids, can be interpreted as a
continuous dilution of the fatty acids contained in zooplankton tissues due to the
constant consumption of new dietary fatty acids.
If no selective catabolism or anabolism of fatty acids takes place in the rotifer
tissues, we can anticipate that their fatty acid composition will become very close
to that of their feed, provided that the rotifers are cultivated for a long time on a
fixed diet, exceeding the lifetime of the individual rotifer (i.e., more than five
doublings of biomass or IS d). The fatty acid compositions of dietary lipids and
rotifers, grown for more than five generations on a fixed diet, are indeed very
closely related, but the fatty acid profiles are not identical (Fig. 8.11). The DHA
content is lower in rotifers than in the food, whereas the content of docosapentaenoic acid (DPA) (22:5w3) is slightly higher. The differences found in shorter
w3 fatty acids and w6 fatty acids are relatively minor. The distribution of monounsaturated fatty acids suggests reduced levels of the long-chain fatty acids 20: I and
22: I and increased level of the short-chain 16: I and 18: I fatty acids in rotifers
compared with their food. Saturated fatty acids were generally found in slightly
higher proportions in the rotifers than in the diet. The data in Figure 8.11 therefore
suggest that B. plicatilis selectively catabolizes DHA into DPA (and EPA?). It also
catabolizes the long-chained monounsaturated fatty acids 20: I and 22: I, considered to be important fuel in the marine food web (Sargent and Henderson,
1986), at a slightly higher rate than most other fatty acids. No indications of chain
elongation have been found for this strain of B. plicatilis, which is not surprising
considering the EFA-rich diets used to cultivate the rotifers.