Use of the Radiocarbon Method to Determine Primary Production
93
changing jars, should be continued until dusk. The curve of the diurnal course
of photosynthesis is constructed using the results of radioassay of the thalli
thus incubated, with the time intervals on the abscissa and the radioactivity of
the plant material of respective specimens on the ordinate.
e) Controls for the dark uptake of 14C-bicarbonate by benthic hydrophytes
"Blank" dark incubations of various hydrophytes in water containing 14C_
carbonate have showed that 14COZ dark uptake is very different in different
kinds and their various associations (Kremer 1979; Arnold and Littler
1985). In the noncalcareous representatives of macrophytes belonging to
Rhodophyta and Chlorophyta, it comprises usually not more than 3% of light
14COZ uptake, while in some representatives of Phaeophyta, this percentage
can be significantly more. It can be especially high - up to 30 to 50% - in associations of periphyton and microphytobenthos, where abundant communities
of heterotrophic microflora of chemosynthetic (mostly sulfur-oxidizing) bacteria and calcareous coralline algae are present, which have a high affinity for
consumption of external 14COZ' The mechanism of non photosynthetic uptake
in individual noncalcareous plants can vary. It proceeds in the tissues of the
thallus itself, or COz is assimilated in the dark by the heterotrophic bacteria
overgrowing the surfaces of hydrophytes. The simple diffusion of labeled COz
into the tissues and lacunes of macrophytes thalli (Thomas 1963) is also possible. The calcareous algae consume external 14COZ to produce lime elements
by calcification. Thus dark control experiments appear to be necessary; but
when using the 14C-method, such dark control incubations as routine parallels
to all light incubations are not as obligatory as with the dark-light oxygen
bottle method. Dark incubations may be performed once for the whole set of
benthic plants or the associations selected for research of primary production.
Dark incubation in this case can be carried out separately from light incubation, which is most convenient to keep in the laboratory or in some building on the shore, where the experiments can be arranged in a darkened place,
and where it is easy to simulate the in situ temperature. The jars are filled
with prefiltered water and standard portions of working solution of 14C_
bicarbonate are added. After injection of the isotope and thorough mixing, the
jars are transferred one by one into an aquarium enclosed in a dark box, and
the plant specimens are placed into them; the jars are then closed with lids
and covered with black tape, and the dark box is closed. The jars are incubated
in the dark for 3-4h. To stop incubation, the plants, still in the dark, are
extracted from the jars and rinsed with water. Then they are placed into black
plastic bags and finally radioassayed. The radioactivity thus measured in cpm
units is calculated per 1 g of dry plant material per hour (Rd) , cpmg- 1 h- 1 . These
values are used for correction of the radioactivity values obtained during light
incubations.
Another also rather simple way to estimate the Rd values is by nocturnal
incubation. A series of jars are filled with water just before dusk; the isotopic
solution is added into them and mixed. When darkness comes, the preprepared
93
changing jars, should be continued until dusk. The curve of the diurnal course
of photosynthesis is constructed using the results of radioassay of the thalli
thus incubated, with the time intervals on the abscissa and the radioactivity of
the plant material of respective specimens on the ordinate.
e) Controls for the dark uptake of 14C-bicarbonate by benthic hydrophytes
"Blank" dark incubations of various hydrophytes in water containing 14C_
carbonate have showed that 14COZ dark uptake is very different in different
kinds and their various associations (Kremer 1979; Arnold and Littler
1985). In the noncalcareous representatives of macrophytes belonging to
Rhodophyta and Chlorophyta, it comprises usually not more than 3% of light
14COZ uptake, while in some representatives of Phaeophyta, this percentage
can be significantly more. It can be especially high - up to 30 to 50% - in associations of periphyton and microphytobenthos, where abundant communities
of heterotrophic microflora of chemosynthetic (mostly sulfur-oxidizing) bacteria and calcareous coralline algae are present, which have a high affinity for
consumption of external 14COZ' The mechanism of non photosynthetic uptake
in individual noncalcareous plants can vary. It proceeds in the tissues of the
thallus itself, or COz is assimilated in the dark by the heterotrophic bacteria
overgrowing the surfaces of hydrophytes. The simple diffusion of labeled COz
into the tissues and lacunes of macrophytes thalli (Thomas 1963) is also possible. The calcareous algae consume external 14COZ to produce lime elements
by calcification. Thus dark control experiments appear to be necessary; but
when using the 14C-method, such dark control incubations as routine parallels
to all light incubations are not as obligatory as with the dark-light oxygen
bottle method. Dark incubations may be performed once for the whole set of
benthic plants or the associations selected for research of primary production.
Dark incubation in this case can be carried out separately from light incubation, which is most convenient to keep in the laboratory or in some building on the shore, where the experiments can be arranged in a darkened place,
and where it is easy to simulate the in situ temperature. The jars are filled
with prefiltered water and standard portions of working solution of 14C_
bicarbonate are added. After injection of the isotope and thorough mixing, the
jars are transferred one by one into an aquarium enclosed in a dark box, and
the plant specimens are placed into them; the jars are then closed with lids
and covered with black tape, and the dark box is closed. The jars are incubated
in the dark for 3-4h. To stop incubation, the plants, still in the dark, are
extracted from the jars and rinsed with water. Then they are placed into black
plastic bags and finally radioassayed. The radioactivity thus measured in cpm
units is calculated per 1 g of dry plant material per hour (Rd) , cpmg- 1 h- 1 . These
values are used for correction of the radioactivity values obtained during light
incubations.
Another also rather simple way to estimate the Rd values is by nocturnal
incubation. A series of jars are filled with water just before dusk; the isotopic
solution is added into them and mixed. When darkness comes, the preprepared