Environmental Engineering Reference
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photosynthesis even at light intensities as low as 0.015 - 0.08µmol Quanta m 2
s 1 depending on the light adaptation state of the culture [45].
However, growth of the green sulfur bacteria under the extremely low light
intensities in situ may also be supported by the assimilation of organic car-
bon compounds. Under suboptimal light conditions, organic carbon substrates
strongly influence the phototrophic growth of different green sulfur bacteria
[2, 3]. Green sulfur bacteria are capable of incorporating acetate into cellular
components in the CO 2 -dependent reaction of pyruvate synthase [64]. This reac-
tion requires reduced ferredoxin and hence depends on light for the regeneration
of reducing power. While the photosynthetic rate is decreased in the presence
of organic substrates, the lowest light intensity supporting growth of a brown-
colored Chlorobium phaeobacteroides isolated from Lake Kinneret is decreased
from 1.0 µmol Quanta·m 2 ·s 1 in the absence to 0.3 µmol Quanta·m 2 ·s 1 in
the presence of acetate [2]. The hypolimnion of Lake Kinneret contains acetate
at concentrations of 3.3µM [2]. At present, the concentrations and types of low
molecular weight organic compounds in the chemocline of the Black Sea have
not been investigated.
From the in situ data mentioned in the preceding section, it can be calcu-
lated that each bacteriochlorophyll e molecule present in the chemocline layer
absorbs one photon every 8 hours. The ecological situation of phototrophic
organisms in the Black Sea chemocline is comparable to that of growing plants
at a distance of 50 m from a little candle in an otherwise pitch black glass
house. The evidence accumulated to date indicates that the particular strain of
green sulfur bacteria dwelling in the Black Sea chemocline is indeed capable
of exploiting this minute light intensity. The green sulfur bacterium dwelling in
the Black Sea chemocline so far represents the most extreme case of low-light
adaptation. It therefore represents a valuable model system for the study of
the molecular basis of low light adaptation. Comparison of its 16S rRNA gene
sequence with all known sequences in the databases revealed that the Black
Sea bacterium so far is unique and has not been discovered in any other system
(Fig. 1).
5. BIOGEOCHEMICAL SIGNIFICANCE OF
ANOXYGENIC PHOTOSYNTHESIS
Karl and Knauer [34] determined a total rate of oxic primary productivity
in the photic zone of 575 mg C m 2 day 1 . Because the total amount of
bacteriochlorophyll e determined in the chemocline of the Black Sea surpasses
the amount of chlorophyll a in the overlying oxygenated water layers, it has
been argued that anoxygenic photosynthesis has become a significant process in
the Black Sea carbon cycle [60, 61]. In many oxic/anoxic aquatic ecosystems,
anoxygenic phototrophic bacteria reoxidize a major fraction of the sulfide
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