Studying Sulfur Cycling with the Aid of 35S
237
5.5.2.2 Experimental Procedure
Samples for estimation of HzS oxidation rates in situ are taken with plastic
water bottles of not less than 31 to have enough water for flushing the bottles.
The depths for sampling have to be selected depending on the vertical profile
of dissolved oxygen: rare samples U-2) are taken in the oxic zone duwn to Oz
content (0.3mgl- 1 ), while the rest, 5-7, are taken in the suboxic layer which
starts from 0.3mgl- 1 Oz content depth down to HzS content of 1-2mgl and
zero oxygen. The sampling procedure is as during Winkler oxygen titration,
e.g., the bottles are filled through a silicon tube going down to their bottom
by passing two to three volumes of water through them. The bottles are closed
without air bubbles. Two extra samples are taken from the redox zone, one to
replenish water lost during the opening of the bottles for injecting the radioisotopic solution and the other for zero time incubation. The optimal volume of
bottles is 0.25-0.301. Instead of bottles, the glass balloons of the same capacity can be used as vessels for these experiments with suboxic water samples
(Fig. 5.7). Before sampling, they are flushed with nitrogen. Balloons are very
convenient for field work as they are kept in boxes in special stands and preserve the redox potential better than bottles (Sorokin 1970b).
When sampling is finished, all the bottles are charged with 20-50)11 of
working isotope solution added to the experimental bottles with the aid of a
capillary end attached to an automatic pipet. After the isotope is injected, the
neck of the vessel is instantly refilled with the anoxic water sample and closed
without air bubbles. The charged samples are incubated under temperature
and illumination conditions which simulate those in situ. In meromictic basins,
where the redox layer is situated at a depth less than the Secchi disk transparency 4, the development of photosynthetic sulfur bacteria is possible. In
this case, the experimental bottles should be incubated in situ at the depths of
the sampling. To select the most appropriate time for incubation, when a still
significant part of the initially added labeled sulfide (20-30%) is present in the
samples, it is very helpful to make a time course curve of the sulfide oxidation, measuring the time course of the radioactivity decrease in the ZnS precipitates (see below) from a series of parallel subsamples taken at the depth
of the supposed maximum of sulfide oxidation and equally charged with the
labeled sulfide (Fig. 5.8). The time of incubation should be selected within the
linear part of this curve, and is between 10 and 20 h.
After the series of samples taken on the vertical profile has been prepared
for incubation, the zero time bottle is charged with the same volume of isotopic solution, its contents is mixed, and the 0.5-ml portion of combined carrier
solution is immediately added to it. The composition of this solution was given
above when the control procedure of batch isotopic solutions was described
(see Sect. 5.4.2.1). During work in marine waters, sulfate can be omitted from
this solution. After injection of the carrier solution, the bottle is closed without
air bubbles, and the process of sulfide oxidation in it practically stops, at least
within the couple of hours needed for processing the samples it is unde-
237
5.5.2.2 Experimental Procedure
Samples for estimation of HzS oxidation rates in situ are taken with plastic
water bottles of not less than 31 to have enough water for flushing the bottles.
The depths for sampling have to be selected depending on the vertical profile
of dissolved oxygen: rare samples U-2) are taken in the oxic zone duwn to Oz
content (0.3mgl- 1 ), while the rest, 5-7, are taken in the suboxic layer which
starts from 0.3mgl- 1 Oz content depth down to HzS content of 1-2mgl and
zero oxygen. The sampling procedure is as during Winkler oxygen titration,
e.g., the bottles are filled through a silicon tube going down to their bottom
by passing two to three volumes of water through them. The bottles are closed
without air bubbles. Two extra samples are taken from the redox zone, one to
replenish water lost during the opening of the bottles for injecting the radioisotopic solution and the other for zero time incubation. The optimal volume of
bottles is 0.25-0.301. Instead of bottles, the glass balloons of the same capacity can be used as vessels for these experiments with suboxic water samples
(Fig. 5.7). Before sampling, they are flushed with nitrogen. Balloons are very
convenient for field work as they are kept in boxes in special stands and preserve the redox potential better than bottles (Sorokin 1970b).
When sampling is finished, all the bottles are charged with 20-50)11 of
working isotope solution added to the experimental bottles with the aid of a
capillary end attached to an automatic pipet. After the isotope is injected, the
neck of the vessel is instantly refilled with the anoxic water sample and closed
without air bubbles. The charged samples are incubated under temperature
and illumination conditions which simulate those in situ. In meromictic basins,
where the redox layer is situated at a depth less than the Secchi disk transparency 4, the development of photosynthetic sulfur bacteria is possible. In
this case, the experimental bottles should be incubated in situ at the depths of
the sampling. To select the most appropriate time for incubation, when a still
significant part of the initially added labeled sulfide (20-30%) is present in the
samples, it is very helpful to make a time course curve of the sulfide oxidation, measuring the time course of the radioactivity decrease in the ZnS precipitates (see below) from a series of parallel subsamples taken at the depth
of the supposed maximum of sulfide oxidation and equally charged with the
labeled sulfide (Fig. 5.8). The time of incubation should be selected within the
linear part of this curve, and is between 10 and 20 h.
After the series of samples taken on the vertical profile has been prepared
for incubation, the zero time bottle is charged with the same volume of isotopic solution, its contents is mixed, and the 0.5-ml portion of combined carrier
solution is immediately added to it. The composition of this solution was given
above when the control procedure of batch isotopic solutions was described
(see Sect. 5.4.2.1). During work in marine waters, sulfate can be omitted from
this solution. After injection of the carrier solution, the bottle is closed without
air bubbles, and the process of sulfide oxidation in it practically stops, at least
within the couple of hours needed for processing the samples it is unde-