Environmental Engineering Reference
In-Depth Information
represent excellent model systems for the study of low-light adaptation and
of mechanisms of adaptation towards extreme energy-limited environments
in general. As outlined below, the green sulfur bacteria recovered from the
Black Sea chemocline represent a particularly well suited model system for the
investigation of these basic scientific questions.
2. ANOXYGENIC PHOTOTROPHIC BACTERIA IN
THE BLACK SEA
The first report on the occurrence of phototrophic sulfur bacteria in the Black
Sea dates back to 1953, when Kriss and Rukina [41] described the enrichment
of purple sulfur bacteria from the dark water layers between 500 and 2000 m
depths. These findings were corroborated by microscopic observations of cells
resembling
Thiocapsa roseopersicina
which occurred in water samples from
160 and 200 m depth [16]. Similar reports were made by others [79], and it was
suggested that these bacteria may be major primary producers in the Black Sea
[66].
However, the presence of anoxygenic phototrophic bacteria in the water
column could not be confirmed in two parallel studies in which pure cultures of
Chromatium warmingii
and
Thiocapsa roseopersicina
and enrichment cultures
of
Chlorobium phaeovibrioides
were obtained exclusively from 660 m and
2240 m-deep sediment layers, but not from 129 different pelagic water samples
[25, 30]. Because of the low salt requirements for growth of these strains, and
because of the absence of light and the presence of high amounts of organic
carbon in the deep sediments it was proposed that the cells of phototrophic sulfur
bacteria in sediments originate in coastal lagoons of the Black Sea and survive
by means of a fermentative metabolism [25]. Active proliferation still seems
unlikely due to extremely low turnover rates at deep sea hydrostatic pressures
[29, 31]. The presence of large spherical cells below the chemocline observed
by Dickman and Artuz [16] was confirmed more recently [4, 69]. These cells
constituted 5-10% of the total living biomass, but resemble
Achromatium
and
thus most likely are not phototrophic.
More specific evidence for the presence of phototrophic sulfur bacteria be-
came available more than a decade ago. Bacterial cells exhibiting autofluo-
rescence were observed at the oxic/anoxic interface [5]. Since cells of green
sulfur bacteria become red fluorescent upon treatment with fixative due to the
formation of free bacteriopheophytins [73], the bacterial cells accumulated in
the chemocline most likely contained bacteriochlorophylls and consequently
were identified as green sulfur bacteria. These cells were reported to account
for 10% of the total bacterial cell number [5]. Traces of different bacterio-
chlorophyll
e
homologues were directly detected by the U.S.-Turkish expe-
dition on the RV
Knorr
in May 1988 [60]. These pigments are characteristic
for the obligate anaerobic photolithotrophic green sulfur bacteria
Chlorobium
