Environmental Engineering Reference
In-Depth Information
explain most of the sulfide oxidation in the Black Sea. This indirect evidence
suggests that anoxygenic photosynthesis is of little importance for the sulfur
cycle in the Black Sea. Presently it is unknown, whether the intrusion of oxygen
has direct effects on the physiological activity of green sulfur bacteria in the
chemocline of the Black Sea.
Support for this conclusion comes from specific sulfide oxidation rates which
were determined in laboratory cultures of the green sulfur bacterium from the
chemocline of the Black Sea [54]. An extrapolation from data obtained at
higher light intensities to
in situ
light levels yielded sulfide oxidation rates of
anoxygenic photosynthesis which could account for not more than 4 to 13% of
total sulfide oxidation in the chemocline during summer [54]. Using the most
recent data for the rate of anoxygenic photosynthesis (211 ngC m
−
2
day
−
1
,or
17.6 nmol m
−
2
day
−
1
, see above) a sulfide oxidation rate of 8.8 - 35.2 nmolH
2
S
m
−
2
day
−
1
can be inferred depending on the oxidation product formed (sulfate or
elemental sulfur, respectively). This range of predicted oxidation rates accounts
for 0.002 - 0.01% of total sulfide oxidation in the Black Sea chemocline during
winter.
Nevertheless, the significance of anoxygenic photosynthesis in the carbon
and sulfur cycles may actually be higher due to a vertical displacement of
the chemocline caused by internal waves [35]. Such short-term exposure of
green sulfur bacteria to higher quantum fluxes can significantly enhance their
contribution to photosynthetic carbon assimilation [2].
6.
MOLECULAR FOSSILS OF GREEN SULFUR
BACTERIA AND THEIR IMPLICATIONS FOR
THE RECONSTRUCTION OF THE
PALEOENVIRONMENT
Caused by the postglacial sea-level rise, the Black Sea was first inundated
with Mediterranean water about 9000 yr BP, leading to the deposition of a
sequence of sediment horizons [9], which indicates a conversion of the fresh-
water lake to an anoxic marine basin [65]. Most of the paleoceanographic data
obtained to date cover the last 7000 - 8000 years (radiocarbon dating) of Black
Sea history [69]. Only few excursions yielded sediment cores long enough to
document earlier periods. Two of them were the Glomar challenger deep sea
drilling operation [63] and the recent excursion of the french research vessel
Marion Dufresne
in May 2004.
In the Black Sea, oxic freshwater conditions started 22,000 years BP and
lasted for a period of over 13,000 years [14]. After deposition of this sediment
Unit III, a microlaminated sapropel layer formed at about 8200 yr BP [59]
(or 7500, or 5000 yr BP depending on the dating method, see [77]), probably
indicating the onset of euxinic conditions in the center of the basin. Unit II is an
organic rich (12 - 15% organic matter, [77]) sapropel and was deposited during
