186
Use of Radioisotopic Methodology in Aquatic Microbial Ecology
The possibility to separate heterotrophic and chemoautotrophic dark
CO2 uptake by natural microbial populations opened the way to resolve
the problem of expected fluctuations of bacterial production to dark CO2
uptake ratio (K) in the vicinity of redox zones, which occurs because in this
layer the dark 14C02 uptake is accomplished by both heterotrophic chemosynthesis and methylophilic bacteria. It also enables the measurement of
the true chemosynthesis rate in natural waters with corresponding correction
of the heterotrophic CO2 assimilation (Sorokin 1972b, 1983b; Sorokin et al.
1992a, 1994, 1995a). Finally, the use of the 14C04-uptake technique is practically the only way to measure anaerobic photosynthesis in purple and green
bacteria, which form exclusively dense populations in meromictic lakes,
brackish coastal marine lakes, and shallow embayments, where the light over
0.5% PAR reaches the redox or anoxic zone either in the water column or
in the surface layer reduced bottom sediments. In such a biotope, these bacteria create a significant biomass which is readily involved in the pelagic or
benthic food webs (Pfenning 1967; Sorokin 1970b; Sorokin and Donato 1975;
Gorlenko 1978). The quantification of photosynthetic production by this group
of bacteria becomes especially important because it is enhanced in areas of
pollution and eutrophication, where the latter stimulate anoxia in water basins.
4.4.2.2 Measuring Production of Heterotrophic Bacteria
4.4.2.2.1 General Remarks
As mentioned above, the 14C dark uptake method for measuring the production of heterotrophic bacteria is based upon their ability to involve definite
amounts of exogenous CO2 in the biosynthesis of their cell material. This
process proceeds even in animal tissues. The mechanism of CO2 assimilation
by the cells of heterotrophic organisms is known as the anaplerotic reaction
of phosphoenolpyruvate carboxylation. This reaction compensates for the
deficiency of the tricarbonic acid cycle (TCA) elements, which is created by
their permanent uptake for the biosynthesis of biomass, mainly for the production of amino acids, lipids, and nucleic acids (Kornberg 1966). Therefore,
the rate of this involvement of exogenous CO2 is directly coupled with the rate
of cell growth. The more rapid the growth, and the more elements of TCA
uptaken, the more CO2 is needed to compensate for this loss (Overbeck 1979,
1984). This feature of the 14C02 uptake mechanism provides a stable relationship between its rate and cell growth. It also explains the decrease in the ratio
of CO2 uptake to growth in the presence of the proteinaceous food substrate:
the more amino acids are available for a bacterial cell from water, the fewer
TCA elements will be needed for biosynthesis, and the less CO2 will be
uptaken to compensate for their deficiency.
The theoretically calculated demand of heterotrophic bacteria growing on
glucose in external CO2 was calculated to be about 11 % (Wiame and Bour-
Use of Radioisotopic Methodology in Aquatic Microbial Ecology
The possibility to separate heterotrophic and chemoautotrophic dark
CO2 uptake by natural microbial populations opened the way to resolve
the problem of expected fluctuations of bacterial production to dark CO2
uptake ratio (K) in the vicinity of redox zones, which occurs because in this
layer the dark 14C02 uptake is accomplished by both heterotrophic chemosynthesis and methylophilic bacteria. It also enables the measurement of
the true chemosynthesis rate in natural waters with corresponding correction
of the heterotrophic CO2 assimilation (Sorokin 1972b, 1983b; Sorokin et al.
1992a, 1994, 1995a). Finally, the use of the 14C04-uptake technique is practically the only way to measure anaerobic photosynthesis in purple and green
bacteria, which form exclusively dense populations in meromictic lakes,
brackish coastal marine lakes, and shallow embayments, where the light over
0.5% PAR reaches the redox or anoxic zone either in the water column or
in the surface layer reduced bottom sediments. In such a biotope, these bacteria create a significant biomass which is readily involved in the pelagic or
benthic food webs (Pfenning 1967; Sorokin 1970b; Sorokin and Donato 1975;
Gorlenko 1978). The quantification of photosynthetic production by this group
of bacteria becomes especially important because it is enhanced in areas of
pollution and eutrophication, where the latter stimulate anoxia in water basins.
4.4.2.2 Measuring Production of Heterotrophic Bacteria
4.4.2.2.1 General Remarks
As mentioned above, the 14C dark uptake method for measuring the production of heterotrophic bacteria is based upon their ability to involve definite
amounts of exogenous CO2 in the biosynthesis of their cell material. This
process proceeds even in animal tissues. The mechanism of CO2 assimilation
by the cells of heterotrophic organisms is known as the anaplerotic reaction
of phosphoenolpyruvate carboxylation. This reaction compensates for the
deficiency of the tricarbonic acid cycle (TCA) elements, which is created by
their permanent uptake for the biosynthesis of biomass, mainly for the production of amino acids, lipids, and nucleic acids (Kornberg 1966). Therefore,
the rate of this involvement of exogenous CO2 is directly coupled with the rate
of cell growth. The more rapid the growth, and the more elements of TCA
uptaken, the more CO2 is needed to compensate for this loss (Overbeck 1979,
1984). This feature of the 14C02 uptake mechanism provides a stable relationship between its rate and cell growth. It also explains the decrease in the ratio
of CO2 uptake to growth in the presence of the proteinaceous food substrate:
the more amino acids are available for a bacterial cell from water, the fewer
TCA elements will be needed for biosynthesis, and the less CO2 will be
uptaken to compensate for their deficiency.
The theoretically calculated demand of heterotrophic bacteria growing on
glucose in external CO2 was calculated to be about 11 % (Wiame and Bour-