172
Use of Radioisotopic Methodology in Aquatic Microbial Ecology
biosynthesis into the proteinaceous particulated food accessible for aquatic
fauna, thus being preconditioned for rapid decomposition within the food web.
Finally, it is necessary to keep in mind that microbial respiration itself comprises only 40-70% of the overall plankton respiration in the water column
within the euphotic zone, and some 70 to 90% in dark deeper layers and in
the bottom sediments. Thus, the microbial decumpusitiun rate clusely reflects
the level of integrated respiration of the whole biota in a given biotope.
For the measuring the overall in situ respiration (e.g., decomposition)
we have quite reliable methods based upon the determination of oxygen
consumption in dark bottles or in sediment cores taken with the overlaying
water (see Sect. 6.4). To estimate the shares of bacterioplankton or benthic
microfiora in this process, approaches have been proposed based upon
prefiltration to separate bacterioplankton from other plankton organisms and
upon the use of eukaryotic or prokaryotic inhibitors. Nevertheless, the use of
the radioisotope method may give useful additional information also in this
field. Again, there are two main approaches in attempts to use radioisotopic
labels: the determinations of the relative (or potential) rates of respiration in
natural microbial assemblages and measuring its absolute rates.
The estimation of relative rates of respiratory decomposition of radiolabeled substances added to samples of water or sediments became very popular
by the late 1970s after Kadota et al. (1966) and Hobbie and Crawford (1969)
discovered a simple procedure for measuring radioactivity in 14C02 respired
by natural bacterioplankton by the uptake of labeled organic substrate added
to the samples of natural waters. This method was later even more refined by
Herbland and Bois (1976) and Kuparinen and Uusi-Rauva (1979). One goal
of its use was to calculate the total uptake rates of these substrates, accounting for losses of assimilated labeled substrates during microbial respiration,
and the second to evaluate in situ rates of decomposition of labeled organic
matter by natural microfiora (Williams and Askew 1968; Romaqnenko and
Kuznetsov 1976; Williams et al. 1976; Cristensen and Blackburn 1982; Namsaraev 1982; Sepers et al. 1982); but because the measurements of uptake of
the nanomolar amounts of labeled substrates added to natural water samples
may at best be evidence of its relative rates only, it is not a reaSOn to correct
them for the respiratory losses. Concerning the second goal, it is possible to
measure the turnover time and decomposition rate of a given individual
labeled substrate or a mixture of them, but under conditions that they are
added in micromolar but not nanomolar concentrations, and that their specific
activity is known. In any case, most often their nanomolar concentrations are
used. In this case, the decomposition rates of the individual labeled substrates
thus measured remain indefinite because of isotopic dilution (Sepers et al.
1982).
As for the measurement of the turnover time and the absolute rate of respiration decomposition of micromolar concentrations of labeled organic substrates with known specific radioactivity, this becomes the method of choice,
but mainly for the case of using specific organic pollutants as such substrates.
Use of Radioisotopic Methodology in Aquatic Microbial Ecology
biosynthesis into the proteinaceous particulated food accessible for aquatic
fauna, thus being preconditioned for rapid decomposition within the food web.
Finally, it is necessary to keep in mind that microbial respiration itself comprises only 40-70% of the overall plankton respiration in the water column
within the euphotic zone, and some 70 to 90% in dark deeper layers and in
the bottom sediments. Thus, the microbial decumpusitiun rate clusely reflects
the level of integrated respiration of the whole biota in a given biotope.
For the measuring the overall in situ respiration (e.g., decomposition)
we have quite reliable methods based upon the determination of oxygen
consumption in dark bottles or in sediment cores taken with the overlaying
water (see Sect. 6.4). To estimate the shares of bacterioplankton or benthic
microfiora in this process, approaches have been proposed based upon
prefiltration to separate bacterioplankton from other plankton organisms and
upon the use of eukaryotic or prokaryotic inhibitors. Nevertheless, the use of
the radioisotope method may give useful additional information also in this
field. Again, there are two main approaches in attempts to use radioisotopic
labels: the determinations of the relative (or potential) rates of respiration in
natural microbial assemblages and measuring its absolute rates.
The estimation of relative rates of respiratory decomposition of radiolabeled substances added to samples of water or sediments became very popular
by the late 1970s after Kadota et al. (1966) and Hobbie and Crawford (1969)
discovered a simple procedure for measuring radioactivity in 14C02 respired
by natural bacterioplankton by the uptake of labeled organic substrate added
to the samples of natural waters. This method was later even more refined by
Herbland and Bois (1976) and Kuparinen and Uusi-Rauva (1979). One goal
of its use was to calculate the total uptake rates of these substrates, accounting for losses of assimilated labeled substrates during microbial respiration,
and the second to evaluate in situ rates of decomposition of labeled organic
matter by natural microfiora (Williams and Askew 1968; Romaqnenko and
Kuznetsov 1976; Williams et al. 1976; Cristensen and Blackburn 1982; Namsaraev 1982; Sepers et al. 1982); but because the measurements of uptake of
the nanomolar amounts of labeled substrates added to natural water samples
may at best be evidence of its relative rates only, it is not a reaSOn to correct
them for the respiratory losses. Concerning the second goal, it is possible to
measure the turnover time and decomposition rate of a given individual
labeled substrate or a mixture of them, but under conditions that they are
added in micromolar but not nanomolar concentrations, and that their specific
activity is known. In any case, most often their nanomolar concentrations are
used. In this case, the decomposition rates of the individual labeled substrates
thus measured remain indefinite because of isotopic dilution (Sepers et al.
1982).
As for the measurement of the turnover time and the absolute rate of respiration decomposition of micromolar concentrations of labeled organic substrates with known specific radioactivity, this becomes the method of choice,
but mainly for the case of using specific organic pollutants as such substrates.