Environmental Engineering Reference
In-Depth Information
[46]. The occurrence of fossil 16S rRNA gene sequences of the extremely low-
light adapted green sulfur bacterium may now be used as a more specific
biomarker to infer photic zone anoxia and hence the vertical extent of the oxic
zone more specifically.
Currently, the vertical position of the anoxic interface in the Black Sea
is positioned between
60 and 200 m depending on sampling location, and
also varies over time [9]. The chemocline is bent towards the shelf edges
similar to the situation during the deposition of Unit I and II [70, 71]. If
the maximum penetration of light in the marine environment is considered
[42] green sulfur bacteria could theoretically colonize water layers down to
250 m depth. Since the vertical attenuation is usually higher in oxic/anoxic
environments, populations of green sulfur bacteria would be expected to occur
in shallower depths. In the Black Sea, a depth of 121 m has been considered the
maximum depth for photoautotrophic growth [59]. The compensation depth
for the extremely low-light adapted green sulfur bacteria in the Black Sea
may actually be even lower, since bacteriochlorophyll
e
has recently been
detected even at a sampling location where the chemocline was positioned at
150 m [45]. In the Cariaco Basin, where lipids and pigments of anoxygenic
phototrophic bacteria are entirely absent, the oxic/anoxic interface is positioned
at 275-300 m depth [43, 76]. In conclusion, the presence of specific 16S rRNA
gene sequence biomarkers of the low-light adapted green sulfur bacterium in a
potentially oxic/anoxic marine environment most likely indicates photic zone
anoxia down to a maximum of 150 m depth.
However, a more detailed analysis of the fossil DNA sequences of green sul-
fur bacteria present in the Black Sea sediments suggests that molecular markers
have to be interpreted with caution. Firstly, in contrast to other oxic/anoxic envi-
ronments with accumulations of phototrophic sulfur bacteria [53], the vertical
concentration profile of bacterial pigments showed an unusually sharp drop
below the chemocline, where BChl
e
concentrations declined below the de-
tection limit at a depth of 120 m [60, 61]. Secondly, chlorophyll
a
, but no
pigments of green sulfur bacteria were detected in the sedimenting matter
(at 87.5 hours deployment of sediment traps) [61] although the total amount
of bacteriochlorophyll
e
determined in the chemocline of the Black Sea sur-
passes the total chlorophyll a in the overlying oxygenated water layers [60].
Both observations indicate that only a very small fraction of the green sul-
fur bacteria present in the Black Sea chemocline actually reach the deep-sea
sediments. Similarly, biomarkers of archaea involved in anaerobic methane
oxidation could not be detected in sinking particulate matter and even not in
underlying sediments [78], indicating that important bacterial or archaeal bio-
markers may be entirely absent or underrepresented in Black Sea sediments.
On the opposite, 16S rRNA gene sequences have been detected in sediments
from the Eastern Mediterranean as old as 240,000 years [13]. All the Mediter-
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