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Use of Radioisotopic Methodology in Aquatic Microbial Ecology
bacteria). In order to achieve this, the experiment should be: (1) short-term to
determine the original contents of active cells in the sample, e.g., less than one
generation in duration, and (2) accomplished by quasi uniform conditions for
all series of samples. By measuring the absolute rates, the ambient conditions
of incubations must be preserved in each individual sample. The amount
of labeled substrate added to them is of particular importance. Its ambiental
content in the sample must not be significantly changed after the injection of
a portion of isotopic working solution, to measure just the in situ absolute rates
of its uptake or decomposition by the intrinsic microbial population. Another
stipulation in this case is the known exact specific radioactivity of the labeled
substrate thus used. In most cases, researchers either ignore it or do not
determine it personally; they simply believe the labels on the concentrated
commercially distributed batches. These batches, as a rule, contain labeled substances with an extremely high specific radioactivity, so that it cannot be controlled by the researcher himself. Often the labels do not reflect reality, and in
many cases even cannot reflect it in principle because minor (usually microgram or nanogram) quantities of substances contained in the batches easily
change their chemical composition due to chemical transformation or radiolysis. Often in such preparations only a single atom in a large molecule is
labeled, as in TDR-tritiated methyl thymidine, for example. An unquestioning
acceptance of what is written on the batches is one of the main sources of
errors in the use of radioisotopes in microbial ecology. The means to deal with
this problem will be discussed below.
When returning to estimations of the relative rates of microbial processes and possible applications of these data, it is helpful to remember the
main reasons for such an analysis and their corresponding radioisotopic
techniques:
1. The estimation of the percentage of metabolically active cells within a given
microbial population, or the percentage of cells in this population which
can take up or metabolyze the given substance, for example the TOR or
labeled amino acids. The best methodological solution for this is the use of
microautoradiography of natural microbial populations. The main reason
for such an investigation is derived from the data, that a significant part of
ambiental microbial populations is represented by senescent or dormant
cells. From this point of view, it is important to quantitatively evaluate the
ratio between their dormant inactive and active parts.
2. The localization on vertical profiles or the spatial distribution of metabolically active and metabolically specific microbial populations, their relative
density and distribution in the water basins. The accumulation of specific
metabolically active microflora (For example, ordinary heterotrophic
bacteria or specific methane-oxidizing bacteria) itself still cannot serve as
evidence (even relative) of the in situ rates of corresponding processes,
because such an accumulation often occurs during the physical processes
of transfer or sedimentation of microbial populations and not just because
Use of Radioisotopic Methodology in Aquatic Microbial Ecology
bacteria). In order to achieve this, the experiment should be: (1) short-term to
determine the original contents of active cells in the sample, e.g., less than one
generation in duration, and (2) accomplished by quasi uniform conditions for
all series of samples. By measuring the absolute rates, the ambient conditions
of incubations must be preserved in each individual sample. The amount
of labeled substrate added to them is of particular importance. Its ambiental
content in the sample must not be significantly changed after the injection of
a portion of isotopic working solution, to measure just the in situ absolute rates
of its uptake or decomposition by the intrinsic microbial population. Another
stipulation in this case is the known exact specific radioactivity of the labeled
substrate thus used. In most cases, researchers either ignore it or do not
determine it personally; they simply believe the labels on the concentrated
commercially distributed batches. These batches, as a rule, contain labeled substances with an extremely high specific radioactivity, so that it cannot be controlled by the researcher himself. Often the labels do not reflect reality, and in
many cases even cannot reflect it in principle because minor (usually microgram or nanogram) quantities of substances contained in the batches easily
change their chemical composition due to chemical transformation or radiolysis. Often in such preparations only a single atom in a large molecule is
labeled, as in TDR-tritiated methyl thymidine, for example. An unquestioning
acceptance of what is written on the batches is one of the main sources of
errors in the use of radioisotopes in microbial ecology. The means to deal with
this problem will be discussed below.
When returning to estimations of the relative rates of microbial processes and possible applications of these data, it is helpful to remember the
main reasons for such an analysis and their corresponding radioisotopic
techniques:
1. The estimation of the percentage of metabolically active cells within a given
microbial population, or the percentage of cells in this population which
can take up or metabolyze the given substance, for example the TOR or
labeled amino acids. The best methodological solution for this is the use of
microautoradiography of natural microbial populations. The main reason
for such an investigation is derived from the data, that a significant part of
ambiental microbial populations is represented by senescent or dormant
cells. From this point of view, it is important to quantitatively evaluate the
ratio between their dormant inactive and active parts.
2. The localization on vertical profiles or the spatial distribution of metabolically active and metabolically specific microbial populations, their relative
density and distribution in the water basins. The accumulation of specific
metabolically active microflora (For example, ordinary heterotrophic
bacteria or specific methane-oxidizing bacteria) itself still cannot serve as
evidence (even relative) of the in situ rates of corresponding processes,
because such an accumulation often occurs during the physical processes
of transfer or sedimentation of microbial populations and not just because