152
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
possible when using the 14C method, after having measured the Cr values in
the labeled food in the same bacteria, for example (see Sect. 3.4.1). The
3H methyl groups have very labile bonds within the thymidine molecules, and
thus are subject to hydrolithical dissipation. Therefore, the radioactivity of
TDR incorporated in the bacterial cells being related to the carbon of their
biomass (e.g. Cr) cannot be stable, even when being Jelcrmin0d. In any case,
most researchers who used labeled natural bacterioplankton for grazing
experiments actually never estimated such specific radioactivity. It should
also be mentioned that the TDR label is largely incorporated into bacteria's
nucleic acids (DNA). Being a rather stable polymer, DNA is digested more
slowly by the consumers than other components of the ingested bacterial
biomass. Therefore the data thus received can be interpreted only as the
"clearance" or filtration rates, but not the assimilation or even consumption
rates in carbon units. While using 14C protein hydrolyzate or glucose as label,
the latter goal is easily attainable via the estimation of Cr in bacteria.
Similarly, it is quite possible to label natural bacterioplankton or bacteria in
sediments with 14C keeping the samples in the dark or with the DCMU-the
inhibitor of photosynthesis (Taylor and Sullivan 1984). At the same time,
another use of 3H-Iabel, for experiments on feeding selectivity or food preference, can be quite efficient because it permits double labeling: the algae with
14C02 and bacterioplankton with 3H-methyl-thymidine (Gophen et al. 1974;
Roman and Rublee 1981).
In another point it is necessary to be careful when evaluating the
adequacy of radioisotopic techniques for the study of microzooplankton nutrition. I refer to the use of Nucleopore filters or 7-10-llm mesh screens for the
separation of consumers from labeled food, the particles of which are often
commensurable in size with that of the consumers. It especially concerns
bacterioplankton, which is aggregated or attached to the detritus particles to
dead algae, to skins of crustaceans, or to the homes of appendicularians in
native environments. When using such filters, it is first necessary to estimate
their possible retention of labeled bacterioplankton.
In conjunction with the latter, it should also be recalled that in most cases
microscopic methods for the study of the sources of nutrition and for
quantification of feeding activity among microplankters like microzooplanktonic larvae, ciliates, zoofiagellates, and phagotrophic dinofiagellates very
often appear to be more efficient than just the use of the radioisotopic
technique and especially that of TDR. The best way to progress in this direction is therefore the use of both the microscopic (see Sect. 6.3) and the
14C-based radioisotopic methods in close interconnection.
Finally, concerning the evaluation of radioisotopic methodology used for
direct estimations of the transfer of labeled carbon via the food web in
enclosed in situ planktonic communities, it will have a place in future investigations; but to recommend it at the present stage as established and ready for
use is premature. The transfer of 14C-Iabeled molecules and material through
the food web is a very complex phenomenon. The incorporation rates of 14C_
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
possible when using the 14C method, after having measured the Cr values in
the labeled food in the same bacteria, for example (see Sect. 3.4.1). The
3H methyl groups have very labile bonds within the thymidine molecules, and
thus are subject to hydrolithical dissipation. Therefore, the radioactivity of
TDR incorporated in the bacterial cells being related to the carbon of their
biomass (e.g. Cr) cannot be stable, even when being Jelcrmin0d. In any case,
most researchers who used labeled natural bacterioplankton for grazing
experiments actually never estimated such specific radioactivity. It should
also be mentioned that the TDR label is largely incorporated into bacteria's
nucleic acids (DNA). Being a rather stable polymer, DNA is digested more
slowly by the consumers than other components of the ingested bacterial
biomass. Therefore the data thus received can be interpreted only as the
"clearance" or filtration rates, but not the assimilation or even consumption
rates in carbon units. While using 14C protein hydrolyzate or glucose as label,
the latter goal is easily attainable via the estimation of Cr in bacteria.
Similarly, it is quite possible to label natural bacterioplankton or bacteria in
sediments with 14C keeping the samples in the dark or with the DCMU-the
inhibitor of photosynthesis (Taylor and Sullivan 1984). At the same time,
another use of 3H-Iabel, for experiments on feeding selectivity or food preference, can be quite efficient because it permits double labeling: the algae with
14C02 and bacterioplankton with 3H-methyl-thymidine (Gophen et al. 1974;
Roman and Rublee 1981).
In another point it is necessary to be careful when evaluating the
adequacy of radioisotopic techniques for the study of microzooplankton nutrition. I refer to the use of Nucleopore filters or 7-10-llm mesh screens for the
separation of consumers from labeled food, the particles of which are often
commensurable in size with that of the consumers. It especially concerns
bacterioplankton, which is aggregated or attached to the detritus particles to
dead algae, to skins of crustaceans, or to the homes of appendicularians in
native environments. When using such filters, it is first necessary to estimate
their possible retention of labeled bacterioplankton.
In conjunction with the latter, it should also be recalled that in most cases
microscopic methods for the study of the sources of nutrition and for
quantification of feeding activity among microplankters like microzooplanktonic larvae, ciliates, zoofiagellates, and phagotrophic dinofiagellates very
often appear to be more efficient than just the use of the radioisotopic
technique and especially that of TDR. The best way to progress in this direction is therefore the use of both the microscopic (see Sect. 6.3) and the
14C-based radioisotopic methods in close interconnection.
Finally, concerning the evaluation of radioisotopic methodology used for
direct estimations of the transfer of labeled carbon via the food web in
enclosed in situ planktonic communities, it will have a place in future investigations; but to recommend it at the present stage as established and ready for
use is premature. The transfer of 14C-Iabeled molecules and material through
the food web is a very complex phenomenon. The incorporation rates of 14C_