Environmental Engineering Reference
In-Depth Information
were enriched from the Black Sea chemocline at depths of 77-83 m by Over-
mann [36]. Black Sea
Chlorobium sp.
strain BS-1 is the only known green
sulfur bacteria, which is able to grow at extremely low light intensities of 0.25
µEinst m
−
2
sec
−
2
. At these light intensities, each bacteriochlorophyll molecule
adsorbs only 1 photon every 8 hours. The Black Sea strain required for growth
the presence of sulfide and a strongly reduced environment (Eh=-560 mV).
In May 1998 Gorlenko et al. [9] attempted to detect the activity and abun-
dance of phototrophic bacteria in the Black Sea chemocline. They were unable
to either isolate anoxygenic phototrophic bacteria on specific media or to mea-
sure the presence of bacteriochlorophyll
e
in the chemocline. Gorlenko and
colleagues, however, succeeded with isolation of two strains of green sulfur
bacteria from the surface 0-1 cm of deep-sea Black Sea sediment. Pigment
composition and morphological characteristics of these strains were used to as-
sign these isolates to earlier described green sulfur bacteria
Chlorobium sp.
[9,
39]. Furthermore pure cultures of green
Chlorobium sp.,
purple
Chromatium
sp.
and
Thiocapsa sp.
were isolated from the Black Sea northwestern shelf
surface sediments. These data suggest that anoxygenic photosynthesis in Black
Sea waters may be a transient feature with spatial and temporal variability.
Light penetration depth varies between the continental slope and the open sea
as well as seasonally and may be the main factor regulating the abundance and
activity of phototrophs in the chemocline. The hypothesis is partially supported
by recent data of Overmann and Manske (this volume), who calculated that
anoxygenic photosynthesis accounted for 4 to 13% in summer and for 0.002 to
0.01% in winter to the total sulfide flux in the Black Sea chemocline.
Bacteria of the Reductive Part of the Sulfur Cycle.
Sulfate reducing bac-
teria are present in the entire anoxic water column as confirmed by direct MPN
counts and measurements of sulfate reduction rates using
35
SO
4
2
−
[1, 31, 52].
The highest rates of up to 1569 nmol l
−
1
d
−
1
were observed in the upper anoxic
water below the zone of active chemosynthesis down to 250-300 m (Fig. 4).
Vertical profiles of sulfate reduction rates show a significant variability, possi-
bly due to activity changes associated with seasonal and spatial variable organic
detritus fluxes. MPN counts of sulfate-reducing bacteria give very low numbers
in the range between 30 and 600 cells L
−
1
, which was explained by the low
efficiency of the MPN method for estimating the abundance of sulfate reducers
[3]. Quantification of the
dsrA
gene of sulfate-reducing bacteria by quantita-
tive polymerase chain reaction in the Black Sea water column in winter 2001
showed that their numbers increased at the interface and were rather uniform
throughout the anoxic water column with cell densities of 10
2
-10
3
cells ml
−
1
.
Sulfate-reducers accounted for less than 1.5% of total bacteria in the anoxic
zone (Neretin et al., unpublished data). δ-Proteobacteria (most of them are
sulfate-reducers) were detected both, at the oxic/anoxic interface and in the
