Environmental Engineering Reference
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were revealed in the chemocline [8, 56]. Kodina and her colleagues [58] have
detected carbon isotopic signal in the chemocline. Samples with isotopically
light carbon were collected at 5 m (station 3804), 15 m (station 3794), and
20 m (station 3794) above the upper boundary of the sulfide zone (Fig. 6c).
The isotopic trend was explained by the fractionation of carbon isotopes by the
chemolithoautotrophic bacteria [31, 32, 58, 59].
We have investigated the possible effect of autotrophic microorganisms on
the isotopic composition of POC in the chemocline during the joint Russian-
Swiss expedition in May 1998 [50]. The distribution of microorganisms, rates of
CO 2 dark fixation, sulfate reduction, and methanogenesis as well as the isotopic
composition of POC, oxygen and hydrogen sulfide content along the transect
to the deep-sea zone have been studied (Fig. 3). Water samples were collected
in the chemocline as well as every 5 m above and below. Figure 6c shows the
data onδ
13 C-POC; the results of hydrochemical and microbiological analyses at
stations 3 and 5 (Fig. 3) are listed in Table 11. Significant changes in the isotopic
composition of POC are evident in the upper layer (15-20 m) of the sulfide zone.
This can be explained by the presence of autotrophic microorganisms, because
the total number of microorganisms and dark CO 2 fixation rates increase in
this zone. The analysis of δ
13 C distribution in deeper layers showed that the
isotopically light organic matter consisting of the autotrophic microbial biomass
was rapidly consumed by anaerobic bacteria. The main consumers of this fresh
organic matter were heterotrophic sulfate reducers as demonstrated by the peak
of sulfate reduction rates under the layer with maximum rate of dark CO 2
fixation. This phenomenon was first discovered by Sorokin [91] and confirmed
later in our studies (Table 11) [50].
The noticeable lightening of the isotopic composition of POC in the chemos-
ynthesis zone was recently reported by a group of Turkish scientists at two
stations in the south-western Black Sea [102]. In addition to δ
13 C determina-
tions of POC, the rate of dark CO 2 fixation has been measured, which revealed
active chemosynthesis in the upper sulfide zone [101]. The maximum depletion
of POC in δ
13 C isotope in the chemosynthesis zone at one of the stations was
5‰ compared to POC samples from the oxycline. Similar to our observations,
isotopically light organic matter was rapidly consumed in the zone below the
chemosynthesis layer [102]. In meromictic Lake Mogil'noye on Kil'din Is-
land (Barents Sea) we have discovered similar isotope effect due to activity of
autotrophic microorganisms in the upper sulfide zone [49].
In sum, δ
13 C values of POC in the sulfide zone of the Black Sea (below 200
m) range between -24.0 and -26.0‰; the averageδ
13 C of POC of 22 samples is
-25.3‰ (Fig. 5). These data are of great interest for understanding the organic
carbon genesis in Black Sea sediments.
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