Environmental Engineering Reference
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to sulfate using CO 2 fixation via pyruvate carboxylation [50]. Two strains of
sulfite-oxidizing bacteria were described as a new species of a new genus,
named Sulfitobacter pontiacus [49]. MPN counts demonstrated that density
of the thiosulfate-oxidizing bacteria in the Black Sea interface zone may be
as high as 10 1 -10 4 cells ml 1 [53]. Obligately chemolithoautotrophic bacteria
related to the genus Thiomicrospira sp . were also isolated from the Black Sea
chemocline. Recent molecular data confirmed that Thiomicrospira denitrificans
related microorganisms belonging to ε-Proteobacteria are an abundant group
of microorganisms inhabiting oxic/anoxic interface [58]. This group can be a
candidate for biologically-mediated sulfide oxidation.
Which group(s) of microorganisms is mainly responsible for hydrogen sul-
fide oxidation in the Black Sea is debated and a highly controversial issue among
microbiologists. Sorokin was among the first who suggested that chemolitho-
autotrophic sulfide-oxidizing Thiobacillus are the main oxidation agents. Using
MPN technique he estimated that the density of these microorganisms in the
redox zone can reach 10 5 cells ml 1 [51]. Studies on chlorophorm-poisoned
samples showed that the initial step of hydrogen sulfide oxidation is mediated
chemically. Bacteria were responsible for the subsequent elemental sulfur and
thiosulfate oxidation to sulfate. Jannasch and coworkers [15] isolated from the
Black Sea chemocline obligately chemolithoautotrophic sulfur-oxidizing bac-
teria related to two genera, Thiobacillus and Thiomicrospira. These bacteria
were able to oxidize hydrogen sulfide, elemental sulfur and tetrathionate up to
sulfate and did not use nitrate or Mn and Fe oxides as alternative to oxygen
electron acceptors. In experimental studies these authors showed that these or-
ganisms may potentially compete with chemically mediated sulfide oxidation
if present in cell densities higher than 10 4 cells ml 1 [15].
The occurrence of photosynthetic bacteria in the Black Sea water column
at the depth of 500 m was first reported by Kriss [20]. Based on morpho-
logical features these bacteria were later identified along the southern coasts
at Kiloys and Trabson at 160-200 m depths and referred to the phototrophic
Thiocapsa roseopersicina [6]. The possibility of anoxygenic photosynthesis at
these depths was not considered during that time. The presence of Thiocapsa
type phototropic bacteria in the western and southern Black Sea was explained
by oxygen intrusions in the anoxic part with modified Mediterranean waters or
by transport to the open sea from surface shelf sediments exposed to favorable
light conditions.
The presence of bacteriochlorophyll e and carotenoids in the Black Sea
oxic/anoxic interface was interpreted as direct evidence that anoxygenic pho-
tosynthesis occurs and may be an important factor for sulfide oxidation [42,
43]. The presence of these pigments is a diagnostic feature of phototrophic
bacteria Chlorobium that are able to survive at low light intensities in stratified
water columns. Phototrophic bacteria related to Chlorobium phaeobacteroides
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