Environmental Engineering Reference
In-Depth Information
However, the actual values of the light intensities available to the green
sulfur bacteria present in the Black Sea chemocline remained obscure until
2001, when the first direct measurement of light intensities in the Black Sea
chemocline, then positioned between 90 and 110 m depth, was made with
an integrating quantum meter [45]. According to these recent measurements,
in situ light intensities during winter reach 0.001 µmol Quanta m 2 s 1 in
the chemocline, corresponding to 0.0007% of surface light intensity. These
measurements hence confirm the earlier estimates.
4. SPECIFIC LOW-LIGHT ADAPTATION OF GREEN
SULFUR BACTERIA
Previous to the discovery of green sulfur bacteria in the Black Sea chemo-
cline, the deepest populations of green sulfur bacteria were detected at depths
of 25 - 30 m in some freshwater lakes [20, 23, 48, 74]. So far, all attempts to dis-
cover photosynthetic activity in natural samples from the Black Sea chemocline
have failed [5, 33, 68] with only one possible exception where light-dependent
14 CO 2 -incorporation was observed once and at intensities above in situ values
[12]. It has therefore been questioned whether the green sulfur bacteria in this
environment are at all photosynthetically active [69]. As an independent line of
evidence, modeling of sulfide fluxes revealed that direct and indirect oxidation
by molecular oxygen accounts for most, if not all of the sulfide removal in and
beneath the Black Sea chemocline [38, 39]. In accordance with these results, a
stimulation of sulfide oxidation by light could not be detected in natural water
samples [60]. As a consequence, anoxygenic photosynthesis most likely does
not represent a biogeochemically important process in this system (see below).
Furthermore, a large portion of microorganisms present in water layers below
50 m depth may be moribund or dead based on their low specific ATP content
[5].
However, oxygenic coralline red algae have been observed at a depth of
268 m in the Caribbean where in situ light intensities reach only as little as
0.007-0.025µmol Quanta·m 2 ·s 1 (0.0005% of the surface light intensity) [42].
These values are comparable to those determined for the Black Sea chemocline
(see above). Since they strictly depend on sulfide-containing environments
and are obligately phototrophic, at least some green sulfur bacterial lineages
would be expected to have experienced a high selection pressure towards low-
light adaptation. The fact that fluorescence measurements of pigment extracts
demonstrated the presence of intact bacteriochlorophyll e pigments, together
with the very low abundance of bacteriopheophytins [12, 60] contradicts the
assumption that green sulfur bacterial cells in the Black Sea chemocline are in a
moribund state. Further indirect evidence for a light dependence of anoxygenic
photosynthetic bacteria comes from the variation of their population densities
with depth of the chemocline. Whereas pigment concentrations of 940 ng
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