Environmental Engineering Reference
In-Depth Information
they do not compete for substrates. For example, methylamines are common
substrates for methanogens, whereas they are not used by sulfate-reducers. (2)
Another mechanism facilitating co-existence between two groups could be spa-
tial separation. Detritus particles can provide niches for active methanogenesis
and separate them from a direct substrate competition and/or interaction with
sulfate-reducers.
6. PROTOZOA IN THE BLACK SEA REDOX-ZONE
The analysis of microbial communities in the Black Sea chemocline would
not be complete without considering the role of protozoa. Protists of the redox
zone represent a diverse and vertically structured community. In spring 1988
suboxic waters are populated by the ciliate
Pleuronema marinum
(up to 30 cells
L
−
1
) as well as by the ciliate's families
Tracheliidae, Holophryidae,
and
Am-
phileptidae
(up to 130 cells L
−
1
) [64]. The same authors reported that most of
these ciliates were fed on
Thiovulum
cells. Other communities dominated by a
ciliate
Askenasia sp.
populated the lower part of the redox zone and upper layers
of the hydrogen sulfide zone.
Scuticociliatida
and heterotrophic flagellates were
associated with the upper anoxic layers down to 300 m. Most of these scutic-
ocilliates beared bacteria as ecto- and/or endosymbionts. Sulfide-oxidizing or
sulfate-reducing bacteria were proposed as possible candidates. Several studies
in stratified fresh and marine environments such as e.g., in Mariager Fjord [7]
and Cariaco Basin [Taylor et al., this volume] clearly demonstrated that protist
communities play an important role in utilization and controlling chemosyn-
thetic production. Sorokin [52] proposed that in the Black Sea transfer of
chemosynthetic bacterial production to higher trophic levels occurs via ciliates
of the redox zone that are grazed by migrating zooplankton at the base of the
oxycline. Systematics and occurrence of the Black Sea protozoa is well doc-
umented (reviewed in [52]). However, we know very little about physiology
and in situ activity of this group of microorganisms in the Black Sea water
column. Without this information, our understanding of the role of bacterial
communities in the food web and overall biogeochemical carbon cycle in the
sea will be unsatisfactory.
7. CONCLUSIONS AND DIRECTIONS FOR FUTURE
RESEARCH
The Black Sea redox-zone harbors a diverse microbial community, which
composition and activities were the subject of this chapter. Total number of
microorganisms increases by an order of magnitude within the redox zone and
bacterial chemosynthesis estimated using dark CO
2
fixation rates is the main
source of newly synthesized organic matter as supported by rate and carbon
isotopic composition measurements. This process accounts for 20-50% of total
primary production in the oxic zone. The surprisingly high value suggests that
