102
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
Attempts to find alternative methods of quantitative study of nutrition in
aquatic animals resulted in the appearance in 1952-1955 of the radioisotopic
study method for planktonic crustaceans (Marcolini 1953; Rodina and Troshin
1954; Marshall and Orr 1955; Rodina 1957). These researchers preferred as
label the radioisotope of phosphorus 32p, primarily because of its high radiation energy. Correspondingly, preparations labeled with this isotope had a high
counting efficiency, which was important at that time, when less efficient OM
counters were in use. This isotope was also used later by many researchers
(Rigler 1961; Mc Manon and Rigler 1965); but, in addition to the higher counting efficiency, the 32p label has an important negative feature - the versatility
of its chemical bonds with the organic molecules produced photosynthetically
by algae incubated in the presence of radiolabeled phosphate 35P04-2. Being
consumed by the algal cells, it is converted cellularly into labile phospho organic substances, especially into ATP. Depending on their physiological state,
the cells will consume or release their radiophosphorus label, thus changing
the specific radioactivity (Rigler 1961). An accurate quantitative estimation of
ingested or assimilated food by 32p label is practically impossible. For such an
estimation, the stability of the specific radioactivity of food matter, e.g., its
radioactivity per unit of dry weight or carbon, is necessary. Therefore all the
data on feeding of aquatic animals obtained by 32p label can be only semiquantitative, estimating absolute amounts of ingested and assimilated food.
The data obtained using 32p as label may at best be only an estimation of the
filtration rates.
From the point of view of the stability of specific radioactivity in food
material, radiocarbon 14C represents an ideal label. Being assimilated by food
organisms autotrophically from 14C02 or heterotrophically from labeled
organic matter, the radioactive atoms of 14C are incorporated into the carbon
chains of the labeled food thus produced. Therefore its specific radioactivity
remains stable when subjected to ingestion and digestion by consumers during
short-term experiments. In living 14C-Iabeled food organisms, the specific
radioactivity of the body carbon can also change if, for example, they are transferred from a labeled to an unlabeled medium. These changes are, however,
predictable, while to predict changes in specific radioactivity with 32p label
is practically impossible. The single disadvantage of 14C as a label is its low
radiation energy, and consequently high self-absorption, which complicates the
exact determination of radioactivity. This was especially a problem during its
earlier use before the 1970s, when it was measured with low-efficiency OM
counters. Nevertheless, this problem of self-absorption was methodologically
solved even by that time, and thus it did not delay the development of
the radiocarbon method. At the present, with the use of modern liquid
scintillation techniques, it presents no significant difficulties.
The radiocarbon method for the study of nutrition in aquatic animals was
initiated in 1955-1960. It found practical application in the study of nutrition
in most common tropho-species of aquatic animals inhabiting the Rybinsk
water reservoir (Sorokin 1958b; Sorokin and Meshkov 1958; 1959). Employ-
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
Attempts to find alternative methods of quantitative study of nutrition in
aquatic animals resulted in the appearance in 1952-1955 of the radioisotopic
study method for planktonic crustaceans (Marcolini 1953; Rodina and Troshin
1954; Marshall and Orr 1955; Rodina 1957). These researchers preferred as
label the radioisotope of phosphorus 32p, primarily because of its high radiation energy. Correspondingly, preparations labeled with this isotope had a high
counting efficiency, which was important at that time, when less efficient OM
counters were in use. This isotope was also used later by many researchers
(Rigler 1961; Mc Manon and Rigler 1965); but, in addition to the higher counting efficiency, the 32p label has an important negative feature - the versatility
of its chemical bonds with the organic molecules produced photosynthetically
by algae incubated in the presence of radiolabeled phosphate 35P04-2. Being
consumed by the algal cells, it is converted cellularly into labile phospho organic substances, especially into ATP. Depending on their physiological state,
the cells will consume or release their radiophosphorus label, thus changing
the specific radioactivity (Rigler 1961). An accurate quantitative estimation of
ingested or assimilated food by 32p label is practically impossible. For such an
estimation, the stability of the specific radioactivity of food matter, e.g., its
radioactivity per unit of dry weight or carbon, is necessary. Therefore all the
data on feeding of aquatic animals obtained by 32p label can be only semiquantitative, estimating absolute amounts of ingested and assimilated food.
The data obtained using 32p as label may at best be only an estimation of the
filtration rates.
From the point of view of the stability of specific radioactivity in food
material, radiocarbon 14C represents an ideal label. Being assimilated by food
organisms autotrophically from 14C02 or heterotrophically from labeled
organic matter, the radioactive atoms of 14C are incorporated into the carbon
chains of the labeled food thus produced. Therefore its specific radioactivity
remains stable when subjected to ingestion and digestion by consumers during
short-term experiments. In living 14C-Iabeled food organisms, the specific
radioactivity of the body carbon can also change if, for example, they are transferred from a labeled to an unlabeled medium. These changes are, however,
predictable, while to predict changes in specific radioactivity with 32p label
is practically impossible. The single disadvantage of 14C as a label is its low
radiation energy, and consequently high self-absorption, which complicates the
exact determination of radioactivity. This was especially a problem during its
earlier use before the 1970s, when it was measured with low-efficiency OM
counters. Nevertheless, this problem of self-absorption was methodologically
solved even by that time, and thus it did not delay the development of
the radiocarbon method. At the present, with the use of modern liquid
scintillation techniques, it presents no significant difficulties.
The radiocarbon method for the study of nutrition in aquatic animals was
initiated in 1955-1960. It found practical application in the study of nutrition
in most common tropho-species of aquatic animals inhabiting the Rybinsk
water reservoir (Sorokin 1958b; Sorokin and Meshkov 1958; 1959). Employ-