Environmental Engineering Reference
In-Depth Information
l
−
1
were determined at 74 m at a central station in the western basin, much
lower concentrations of bacteriochlorophyll
e
(214 ng l
−
1
) were found closer to
the Turkish coast, where the chemocline was located at a greater depth of 100
m depth [60]. This pronounced decrease in the chemocline bacteriochlorophyll
e
concentrations towards the coastal sampling stations was fully confirmed by
the recent comparison of the population densities of green sulfur bacteria at
100 m and 150 m depths [45].
Direct evidence for a specific adaptation of green sulfur bacteria dwelling
in the chemocline of the Black Sea comes from the study of five strains of
brown-colored green sulfur bacteria which were isolated from chemocline water
samples during the U.S.-Turkish expedition with the RV
Knorr
in May 1988
[54]. All strains contained bacteriochlorophyll
e
as the main photosynthetic
pigment and revealed an extreme low-light adaptation compared to 12 other
green and purple sulfur bacterial strains. One isolate was chosen for a detailed
study of this low-light adaptation. Under severely limiting light intensities
BChl
e
≤
1 µmol Quanta·m
−
2
·s
−
1
, the Black Sea isolates grew significantly faster than
related green sulfur bacteria. In contrast, growth rates at light saturation were
lower than in other phototrophic sulfur bacteria.
Most notably, the Black Sea bacterium is capable of growing at 0.25 µmol
Quanta·m
−
2
·s
−
1
of daylight fluorescent tubes (0.04 % of surface irradiance),
which is too low to support growth of most other anoxygenic phototrophic bac-
teria. Only two strains of the brown-colored
Chlorobium phaeovibrioides
could
also grow at this low quantum flux, albeit at much slower rate than the Black
Sea strain. Since not only the growth rate, but also the sulfide oxidation rates of
whole cells were significantly higher under light limitation [54], the photosyn-
thetic reaction itself must be more efficient in the Black Sea strain, which has
been attributed to an increase in light-harvesting pigments by a factor of
2as
compared to other green sulfur bacteria. This might also be an explanation for
the high concentrations of bacteriochlorophyll compared to rather low amounts
of biomass detected in this water depth [5, 60]. Green sulfur bacteria employ
specialized intracellular structures, so-called chlorosomes, for photosynthetic
light-harvesting [56]. As demonstrated by ultrathin sectioning, the Black Sea
strain MN1 produced two-fold larger chlorosomes than another
Chlorobium
phaeobacteroides
strain (DSMZ 266
T
) [18]. The number of chlorosomes per
cell was found to be constant and independent of light intensity. Changes in
chlorosome volume, hence cellular pigment content, are due to changes in
chlorosome length, but not width [18]. A second unusual feature of the Black
Sea green sulfur bacterium is its extraordinarily low maintenance energy re-
quirement which is commensurate with the extremely low doubling time of
2.8 years calculated for green sulfur bacterial cells under
in situ
conditions
[54]. Recent assimilation experiments with
14
C-labeled bicarbonate revealed
that the green sulfur bacteria from the Black Sea are capable of anoxygenic
∼
