Environmental Engineering Reference
In-Depth Information
produced in the chemocline or in deeper water layers [74]. Since the electrons
from sulfide oxidation are almost completely transferred onto CO
2
, anoxygenic
photosynthesis couples the carbon and sulfur cycles much more efficiently than
chemolithotrophic sulfide oxidation. Phototrophic sulfur bacteria hence may
substantially alter the pelagic carbon and sulfur cycles [51], for example via the
accumulation of a high microbial biomass which may enter the aerobic pelagic
food web.
One attempt to determine the ecological significance of green sulfur bacteria
in the Black Sea was made by Repeta et al. [60]. They tried to translate the
measured bacteriochlorophyll values into biomass with POC data (particulate
organic matter) and estimated a total photosynthetic bacterial biomass of 0.5
g·m
−
2
in the chemocline compared to a total phytoplankton biomass in the
upper water layers of 0.9 g·m
−
2
. They also detected the highest rates of H
2
S
oxidation
in situ
at the base of the bacteriochlorophyll maximum which shows a
correlation to anoxygenic photosynthetic bacteria. However, a significant light
induced increase in bacterial photosynthesis rates
in situ
could not be shown.
The light intensity determined
in situ
was 0.001 µmol Quanta m
−
2
s
−
1
in
winter (Section 3). Based on the very slow growth, the lowest light intensity
which could be employed in growth experiments with laboratory cultures was
0.25 µmol Quanta m
−
2
s
−
1
[54] and hence far higher than the
in situ
light
intensity values. Therefore,
14
CO
2
assimilation rates were recently assessed at
much decreased intensities of as low as 0.006 µmol Quanta·m
−
2
·s
−
1
, and then
used to calculate photosynthesis rates and sulfide oxidation rates (see below)
in the chemocline of the Black Sea [45]. These calculations yielded a rate of
anaerobic primary production of 211 ngC m
−
2
day
−
1
under
in situ
conditions.
Considering the integrated biomass of green sulfur bacteria in the chemocline
of 798 µg
BChle
m
−
2
(Fig. 3), equal to 2.89 mgC m
−
2
, this primary production
corresponds to an average growth rate of 7.31·10
−
5
d
−
1
and a doubling time of
26 years. Although experimental conditions in the laboratory cultures may be
suboptimal, and anoxygenic photosynthesis will be higher during summertime,
green sulfur bacteria in the chemocline of the Black Sea clearly form the
least dense and slowest growing population known to date as revealed by a
cross-system comparison of stratified aquatic ecosystems (Figs. 2, 3). Specific
adaptations, like the assimilation of organic carbon substrates generated by
accompanying bacteria could theoretically result in a higher growth rate as
calculated from CO
2
-incorporation alone. Auxiliary metabolic reactions and the
role of interactions with accompanying bacteria should therefore be the focus
of future investigations of bacterial physiology in the Black Sea chemocline.
From the very low growth rates calculated for green sulfur bacteria in the
chemocline, it also has to be concluded that losses of these bacteria must be
extraordinarily small in order to explain the persistence of these bacteria even
in winter. Indeed, ciliates of the order Scuticociliatida and isokont flagellates
