Estimation of Relative Microbial Activity in Aquatic Habitats
161
plate counts or than the BOD technique. The high sensitivity of radioisotopic
methods enables the relative activity of specific groups of bacteria in series
of samples in short-term experiments to be measured, providing a precise
picture of the native distribution of their populations. The main criterion of
relative abundance of a given metabolically specific group of bacteria, for
example ordmary heterotrophs or methanotrophs, is, in this case, a relative
uptake rate of a corresponding radiolabeled substrate, such as a mixture of
amino acids or methane by microbial populations which are present in
these samples. The comparative rate of uptake in this case is determined under
standard temperature and oxygen availability conditions and calculated in
equal units (cpm per 100ml of the sample, for example). Because of a shortterm exposure time, the uptake of a labeled substrate thus measured reflects
the relative abundance of a given group which is in existence at the time of
sampling.
Radioisotopic methodology has been most often applied for the evaluation of the heterotrophic potential in natural waters, e.g., for determination of
the relative activity of heterotrophic bacteria in water basins with the use of
individual amino acids (most often glutamate), of their mixture, or of protein
hydrolyzate as a labeled substrate. It was also used for the investigation of the
spatial distribution in aquatic habitats of other specific groups of microbial
communities, such as bacteria-oxidizing hydrocarbons, thiobacilli, or methaneoxidizing bacteria. The techniques of experiments in relation to each of these
groups have their specific features.
Measuring the uptake rates of radiolabeled dissolved organic compounds
like glucose or amino acids by the aquatic microflora in natural waters became
extremely popular in the 1960s and early 1970s, in part because of the simplicity of the technique itself, since the pioneering works by Parsons and
Strickland (1962) and Wright and Hobbie (1965, 1966) led to numerous publications reviewed by Hobbie and Rublee (1977) and Van Es and Meyer Reil
(1982); but most of the authors actually did not clearly delineate the reasons
for these measurements. It was understood that the uptake rates thus measured and expressed as cpm per volume of the water sample could not be
applied for the evaluation of real rates of absolute bacterial decomposition or
production in situ, first of all as a sequence of the unknown and inestimable
degree of the isotopic dilution of the radiolabeled substances, nanomolar
quantities of which were being injected into the samples of natural waters.
Thus, isotopic dilution in fact should have been very high in this case because
the researchers preferred to use commercially supplied labeled substrates
with a very high specific radioactivity of 0.3 up to 20Cimmol- 1 for these experiments. One reason for this was that an addition of nanomolar amounts
of labeled organic substrates excluded or decreased their uptake by algae
(Wright and Hobbie 1966); but now that we know about the abundance in
natural waters of the picophytoplanktonic cells of the bacterial size and especially of picocyanobacteria, which have a high affinity for the uptake of dissolved organic matter, this argument cannot be accepted.
161
plate counts or than the BOD technique. The high sensitivity of radioisotopic
methods enables the relative activity of specific groups of bacteria in series
of samples in short-term experiments to be measured, providing a precise
picture of the native distribution of their populations. The main criterion of
relative abundance of a given metabolically specific group of bacteria, for
example ordmary heterotrophs or methanotrophs, is, in this case, a relative
uptake rate of a corresponding radiolabeled substrate, such as a mixture of
amino acids or methane by microbial populations which are present in
these samples. The comparative rate of uptake in this case is determined under
standard temperature and oxygen availability conditions and calculated in
equal units (cpm per 100ml of the sample, for example). Because of a shortterm exposure time, the uptake of a labeled substrate thus measured reflects
the relative abundance of a given group which is in existence at the time of
sampling.
Radioisotopic methodology has been most often applied for the evaluation of the heterotrophic potential in natural waters, e.g., for determination of
the relative activity of heterotrophic bacteria in water basins with the use of
individual amino acids (most often glutamate), of their mixture, or of protein
hydrolyzate as a labeled substrate. It was also used for the investigation of the
spatial distribution in aquatic habitats of other specific groups of microbial
communities, such as bacteria-oxidizing hydrocarbons, thiobacilli, or methaneoxidizing bacteria. The techniques of experiments in relation to each of these
groups have their specific features.
Measuring the uptake rates of radiolabeled dissolved organic compounds
like glucose or amino acids by the aquatic microflora in natural waters became
extremely popular in the 1960s and early 1970s, in part because of the simplicity of the technique itself, since the pioneering works by Parsons and
Strickland (1962) and Wright and Hobbie (1965, 1966) led to numerous publications reviewed by Hobbie and Rublee (1977) and Van Es and Meyer Reil
(1982); but most of the authors actually did not clearly delineate the reasons
for these measurements. It was understood that the uptake rates thus measured and expressed as cpm per volume of the water sample could not be
applied for the evaluation of real rates of absolute bacterial decomposition or
production in situ, first of all as a sequence of the unknown and inestimable
degree of the isotopic dilution of the radiolabeled substances, nanomolar
quantities of which were being injected into the samples of natural waters.
Thus, isotopic dilution in fact should have been very high in this case because
the researchers preferred to use commercially supplied labeled substrates
with a very high specific radioactivity of 0.3 up to 20Cimmol- 1 for these experiments. One reason for this was that an addition of nanomolar amounts
of labeled organic substrates excluded or decreased their uptake by algae
(Wright and Hobbie 1966); but now that we know about the abundance in
natural waters of the picophytoplanktonic cells of the bacterial size and especially of picocyanobacteria, which have a high affinity for the uptake of dissolved organic matter, this argument cannot be accepted.