104
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
processes of organic matter utilization and transformation by protozoans
within the "microbial loop" (Taylor and Sallinvan 1984; Nygaard and Hessen
1990) and applied them to evaluate in situ feeding and production in pelagic
communities (Tchmyr 1967; Shushkina and Sorokin 1969; Smith et al. 1973;
Daro 1978; Roman and Rublee 1981; Bjornsen et al. 1986; Martin et al. 1986;
Napp and Long 1989). To label bacteria as potential fuuo ~uurce, many authors
began to use tritiated 3H-methyl thymidine (TDR), which bacteria incorporate into their nucleic acids. This approach appeared to be quite reliable
for the dual label, when studying food preference in animals in relation to
bacteria and algae (Gophen et al. 1974). An advantage of the use of TDR as
label is its selectivity in labeling bacteria within natural microplankton assemblages (Caron et al. 1993). The disadvantages are less stability of specific
radioactivity in relation to the body carbon of labeled food, and significantly
lower (ca. five times less) energy radiation of 3H. This entails problems in
radioassay of animal tissues experimentally, while natural bacterioplankton
could be labeled with 14C as well, which does not raise these problems (see
Sect. 3.3).
Below, details of 14C techniques as they were refined during the above
experiments and resumed in Sorokin (1968; see Fig. 3.1) are described.
3.2 General Principles
As was pointed out above, the great advantage of using the 14C method for
trophic studies is the possibility of obtaining practically all the basic quantitative characteristics of feeding for a given hydrobiont species. Most of these
characteristics (or parameters), such as food spectrum, food preference, filtration rate, dependence of feeding rate on food concentration, and even assimilability of food (ratio of assimilation to ingested food), could be measured as
relative values of the ingestion, assimilation, or clearance rates (in the case of
quantification the filtration activity). This greatly reduces the time required,
and avoids the necessity of correcting for respiratory losses of label, losses with
liquid excretion, or incomplete labeling, which is a frequent source of criticism
and confusion (Conover and Francis 1973; Lampert 1977). Measurements of
absolute rates of feeding, accounting for the above losses, are needed only as
measuring values for food rations (rates of food ingestion) and absolute rates
of assimilation. This latter can be estimated in balance experiments, taking into
account all possible losses and using equally labeled food material or food
organisms.
The main principle in the use of 14C to quantify amounts of ingested and
digested food is as in any other tracer method; it is based upon measuring the
inverse specific radioactivity (Cr ) of organic matter in food material labeled
with 14C. After exposure to labeled food with known inverse specific radioactivity, animal consumers are removed from the labeled food and the radioac-
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
processes of organic matter utilization and transformation by protozoans
within the "microbial loop" (Taylor and Sallinvan 1984; Nygaard and Hessen
1990) and applied them to evaluate in situ feeding and production in pelagic
communities (Tchmyr 1967; Shushkina and Sorokin 1969; Smith et al. 1973;
Daro 1978; Roman and Rublee 1981; Bjornsen et al. 1986; Martin et al. 1986;
Napp and Long 1989). To label bacteria as potential fuuo ~uurce, many authors
began to use tritiated 3H-methyl thymidine (TDR), which bacteria incorporate into their nucleic acids. This approach appeared to be quite reliable
for the dual label, when studying food preference in animals in relation to
bacteria and algae (Gophen et al. 1974). An advantage of the use of TDR as
label is its selectivity in labeling bacteria within natural microplankton assemblages (Caron et al. 1993). The disadvantages are less stability of specific
radioactivity in relation to the body carbon of labeled food, and significantly
lower (ca. five times less) energy radiation of 3H. This entails problems in
radioassay of animal tissues experimentally, while natural bacterioplankton
could be labeled with 14C as well, which does not raise these problems (see
Sect. 3.3).
Below, details of 14C techniques as they were refined during the above
experiments and resumed in Sorokin (1968; see Fig. 3.1) are described.
3.2 General Principles
As was pointed out above, the great advantage of using the 14C method for
trophic studies is the possibility of obtaining practically all the basic quantitative characteristics of feeding for a given hydrobiont species. Most of these
characteristics (or parameters), such as food spectrum, food preference, filtration rate, dependence of feeding rate on food concentration, and even assimilability of food (ratio of assimilation to ingested food), could be measured as
relative values of the ingestion, assimilation, or clearance rates (in the case of
quantification the filtration activity). This greatly reduces the time required,
and avoids the necessity of correcting for respiratory losses of label, losses with
liquid excretion, or incomplete labeling, which is a frequent source of criticism
and confusion (Conover and Francis 1973; Lampert 1977). Measurements of
absolute rates of feeding, accounting for the above losses, are needed only as
measuring values for food rations (rates of food ingestion) and absolute rates
of assimilation. This latter can be estimated in balance experiments, taking into
account all possible losses and using equally labeled food material or food
organisms.
The main principle in the use of 14C to quantify amounts of ingested and
digested food is as in any other tracer method; it is based upon measuring the
inverse specific radioactivity (Cr ) of organic matter in food material labeled
with 14C. After exposure to labeled food with known inverse specific radioactivity, animal consumers are removed from the labeled food and the radioac-