240
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
of the comparative importance of chemical and biological ways of sulfide oxidation schemes appears to be among the most important within this sphere of
research. The main features of biological sulfur oxidation are: (1) the formation of molecular sulfur by the purple sulfur bacteria in layers of bacterial photosynthesis, and (2) the absence or insignificant formation of thiosulfate as an
intermediate during H 2S uxidatiun. Tu verify the impact uf native thiobacterial populations and of sulfur bacteria upon this process, the experiments
described above on sulfide oxidation are performed in the presence of
antiseptics like chloroform with analysis of its basic products (S2-, So, S2032-,
SOl-). Such experiments can be done during two time-series experiments with
samples taken from the redox zone, one with the addition of antiseptic. In the
latter, the sulfide is oxidized only chemically, while in the other, intact, series
it proceeds with the participation of ambient thiomicrofiora. The examples of
such an experiment made with water samples from the redox zones of
meromictic basins are given in Figs. 5.5 and 5.6. In the Black Sea the thiobacilli
enter the scene when a significant part of the sulfide added is also oxidized
chemically to sulfate and thiosulfate. Thus they participate in this basin only
in the oxidation of thiosulfate formed during the chemical oxidation of sulfide.
In the Gek Gel lake their population is denser and they participate in sulfide
oxidation also at its initial stage.
The anoxic oxidation of sulfur compounds which proceeds in water bodies
with the participation of nitrate as an electron acceptor (thio-denitrification)
proves be a purely biological process. H 2S does not react directly with N03-,
and thus the chemical oxidation cannot compete with the biological, are
happens with free oxygen (Sorokin 1970b, 1972b). Therefore, the oxidation of
sulfide in this case proceeds with the participation of thiobacilli which starts
directly from the oxidation of H 20 to sulfate involving combined oxygen of
nitrate. To measure the in situ rate of such an biological anaerobic HzS oxidation the samples are taken at the lower boundary of the redox zone where
the water contains practically no free oxygen. The incubations are carried out
with three subsamples: one the zero time, the second with NaNO, added
(30 mg 1-1) and the third as the second plus antiseptic (chloroform or thymol).
The protocol of their processing should include measuring the radioactivity of
S2-, So, SZ03z-, sol-.
5.5.3 Oxidation Rates of Thiosulfate and Elemental Sulfur
As was also pointed out above, thiosulfate and SO comprise the major energy
source for bacterial popUlations inhabiting the redox layers in water columns
and in the water-bottom interface layer. Both these substrates have a high
energy value and are used by litho trophic and photosynthesis bacteria as main
electron donors. Being rather resistant to direct chemical oxidation with free
oxygen, they are oxidized in their native environments mainly by thiobacilli
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
of the comparative importance of chemical and biological ways of sulfide oxidation schemes appears to be among the most important within this sphere of
research. The main features of biological sulfur oxidation are: (1) the formation of molecular sulfur by the purple sulfur bacteria in layers of bacterial photosynthesis, and (2) the absence or insignificant formation of thiosulfate as an
intermediate during H 2S uxidatiun. Tu verify the impact uf native thiobacterial populations and of sulfur bacteria upon this process, the experiments
described above on sulfide oxidation are performed in the presence of
antiseptics like chloroform with analysis of its basic products (S2-, So, S2032-,
SOl-). Such experiments can be done during two time-series experiments with
samples taken from the redox zone, one with the addition of antiseptic. In the
latter, the sulfide is oxidized only chemically, while in the other, intact, series
it proceeds with the participation of ambient thiomicrofiora. The examples of
such an experiment made with water samples from the redox zones of
meromictic basins are given in Figs. 5.5 and 5.6. In the Black Sea the thiobacilli
enter the scene when a significant part of the sulfide added is also oxidized
chemically to sulfate and thiosulfate. Thus they participate in this basin only
in the oxidation of thiosulfate formed during the chemical oxidation of sulfide.
In the Gek Gel lake their population is denser and they participate in sulfide
oxidation also at its initial stage.
The anoxic oxidation of sulfur compounds which proceeds in water bodies
with the participation of nitrate as an electron acceptor (thio-denitrification)
proves be a purely biological process. H 2S does not react directly with N03-,
and thus the chemical oxidation cannot compete with the biological, are
happens with free oxygen (Sorokin 1970b, 1972b). Therefore, the oxidation of
sulfide in this case proceeds with the participation of thiobacilli which starts
directly from the oxidation of H 20 to sulfate involving combined oxygen of
nitrate. To measure the in situ rate of such an biological anaerobic HzS oxidation the samples are taken at the lower boundary of the redox zone where
the water contains practically no free oxygen. The incubations are carried out
with three subsamples: one the zero time, the second with NaNO, added
(30 mg 1-1) and the third as the second plus antiseptic (chloroform or thymol).
The protocol of their processing should include measuring the radioactivity of
S2-, So, SZ03z-, sol-.
5.5.3 Oxidation Rates of Thiosulfate and Elemental Sulfur
As was also pointed out above, thiosulfate and SO comprise the major energy
source for bacterial popUlations inhabiting the redox layers in water columns
and in the water-bottom interface layer. Both these substrates have a high
energy value and are used by litho trophic and photosynthesis bacteria as main
electron donors. Being rather resistant to direct chemical oxidation with free
oxygen, they are oxidized in their native environments mainly by thiobacilli