Concluding Remarks
151
3.7 Concluding Remarks
The reader may be disappointed by the evidently personal selection of
radioisotopic methods described in this chapter. Indeed, the list of methods
given here actually ignores many alternative ones based on the use of other
than 14C radioisotopes, like 3H, 32p, and 33p' The list also omits the rapidly
expanding direction of radioisotopic studies on the phagotrophic nutrition of
protozoa and mixotrophic algae with the use of 3H and 14C-Iabeled dissolved
organic matter to mark bacteria - their main food (Hollibaugh et al. 1980;
Taylor and Sullivan 1984; Lessard and Swift 1985; Nygaard and Hessen 1990).
Finally, no mention is made of attempts to use 14C label for the sequencing
transfer of organic matter within food webs in enclosures in situ after the addition either of 14C02 carbonate, which enters the food web via phototrophic
assimilation (Tchmyr 1967; Shushkina and Sorokin 1969; Daro 1978; Roman
and Rublee 1981), via labeled DOM (Sorokin and Tcherdyntzeva 1971;
Roman and Rublee 1981), or via particulated organic matter, like algae, bacteria, or zooplankton labeled with 14C or 3H (Haney 1971; Smith et al. 1973;
Bjornsen et al. 1986; Caron et al. 1993).
The significant achievements in studies of trophodynamics in aquatic
ecosystems, especially at the initial, most important, microbial steps of the food
web, which have been attained with the aid of the these approaches based on
the application of various radioisotopic techniques is an indisputable fact.
However, for future progress in this field it is important now to make a critical evaluation of the great variety of methods in order to have the possibility
to select the simplest, the most direct, and really quantitative among them. A
need for comprehensive information about the trophical status of a key
trophospecies (Turner and Roff 1993) is widely acknowledged. Experimental
studies of nutrition of aquatic animals are often made by scientists or engineers who have no physiological or zoological specialization.
The above considerations lead me to strongly recommend totally abandoning the application of 32p or 33p as labels for trophological studies. These
isotopes are very suitable for the study of nutrient cycling, but not the
trophodynamics of carbon. I also recommend care in the use of 3H methylthymidine (TDR) as tracer for labeled natural microbial assemblages, even for
the study of food preference by double labeling with 14C (algae) and 3H (bacteria) in the case where the results of uptake estimations are given in dpm per
animal without measuring the specific radioactivity in the labeled food particles either in algae or in bacteria, which have been thus labeled in situ in
natural water.
TDR should have definite preference as a label because it is selectively
incorporated by bacteria. (Birkbeck et al. 1982; Marvalin and Lazarek 1988);
but the specific radioactivity of microbial biomass in relation to 3H label
cannot be stable to be safely used for recalculation per the absolute amount
of consumed or assimilated labeled microbial carbon after the radioactivity
values are thus measured in the consumers' bodies. However, the latter is
151
3.7 Concluding Remarks
The reader may be disappointed by the evidently personal selection of
radioisotopic methods described in this chapter. Indeed, the list of methods
given here actually ignores many alternative ones based on the use of other
than 14C radioisotopes, like 3H, 32p, and 33p' The list also omits the rapidly
expanding direction of radioisotopic studies on the phagotrophic nutrition of
protozoa and mixotrophic algae with the use of 3H and 14C-Iabeled dissolved
organic matter to mark bacteria - their main food (Hollibaugh et al. 1980;
Taylor and Sullivan 1984; Lessard and Swift 1985; Nygaard and Hessen 1990).
Finally, no mention is made of attempts to use 14C label for the sequencing
transfer of organic matter within food webs in enclosures in situ after the addition either of 14C02 carbonate, which enters the food web via phototrophic
assimilation (Tchmyr 1967; Shushkina and Sorokin 1969; Daro 1978; Roman
and Rublee 1981), via labeled DOM (Sorokin and Tcherdyntzeva 1971;
Roman and Rublee 1981), or via particulated organic matter, like algae, bacteria, or zooplankton labeled with 14C or 3H (Haney 1971; Smith et al. 1973;
Bjornsen et al. 1986; Caron et al. 1993).
The significant achievements in studies of trophodynamics in aquatic
ecosystems, especially at the initial, most important, microbial steps of the food
web, which have been attained with the aid of the these approaches based on
the application of various radioisotopic techniques is an indisputable fact.
However, for future progress in this field it is important now to make a critical evaluation of the great variety of methods in order to have the possibility
to select the simplest, the most direct, and really quantitative among them. A
need for comprehensive information about the trophical status of a key
trophospecies (Turner and Roff 1993) is widely acknowledged. Experimental
studies of nutrition of aquatic animals are often made by scientists or engineers who have no physiological or zoological specialization.
The above considerations lead me to strongly recommend totally abandoning the application of 32p or 33p as labels for trophological studies. These
isotopes are very suitable for the study of nutrient cycling, but not the
trophodynamics of carbon. I also recommend care in the use of 3H methylthymidine (TDR) as tracer for labeled natural microbial assemblages, even for
the study of food preference by double labeling with 14C (algae) and 3H (bacteria) in the case where the results of uptake estimations are given in dpm per
animal without measuring the specific radioactivity in the labeled food particles either in algae or in bacteria, which have been thus labeled in situ in
natural water.
TDR should have definite preference as a label because it is selectively
incorporated by bacteria. (Birkbeck et al. 1982; Marvalin and Lazarek 1988);
but the specific radioactivity of microbial biomass in relation to 3H label
cannot be stable to be safely used for recalculation per the absolute amount
of consumed or assimilated labeled microbial carbon after the radioactivity
values are thus measured in the consumers' bodies. However, the latter is