Environmental Engineering Reference
In-Depth Information
bition experiments, 16S rDNA libraries, and enrichment cultures all support the
assertion that the redoxcline is inhabited by highly stratified guilds of diverse
chemoautotrophs that organize in response to resource availability. They appear
to largely rely on energy from S in reduced and intermediate oxidation states,
which are potentially oxidized by a variety of chemical species (O
2
,NO
3
−
,
Mn
4
+
,Fe
3
+
).
A growing culture collection of chemoautotrophic -proteobacteria, capable
of oxidizing H
2
,H
2
S, S
0
,S
2
O
3
at the expense of O
2
,S
0
and NO
3
−
is emerging
from deep-sea hydrothermal field exploration [1,17,29,53,54]. These newly de-
scribed isolates closely cluster with phylotypes present in Cariaco and Black
Sea 16S rDNA libraries. However in the case of the Cariaco, O
2
and NO
3
−
are undetectable where peak chemoautotrophy is observed (Fig. 7 in [56]) and
S
0
concentrations are presently unknown. Alternate electron acceptors, such as
metals and sulfur intermediates, must be invoked to explain observed activity
distributions. While anaerobic, manganese-reducing chemo-organotrophs are
well-known (e.g.,
Shewanella
sp.), strict chemoautotrophs that use metals as
final electron acceptors are yet to be described. Moreover, the relative impor-
tance of specific reductants and oxidants, including NH
4
+
,H
2
S, S
0
,S
2
O
3
,SO
3
,
H
2
,CH
4
,Mn
4
+
and Fe
3
+
, in supporting microbial community growth remains
unresolved for anoxic basins.
4. CONCLUSIONS
As summarized in Fig. 10, information amassed from the Cariaco Basin,
Black Sea and other anoxic water columns all show that redoxclines support
active microbial food webs with multiple trophic levels but appear to be less
complex than food webs in oxic waters [4,9,56,57,67]. Heterotrophic produc-
tion directly fueled by passively sinking biogenic debris can only support a
small fraction of this food web. Rather this food web appears to be supported
primarily by chemoautotrophic prokaryotes, potentially dividing at rates of
about 0.4 - 0.6 d
−
1
. However, determining actual fluxes of inorganic ions
driving this production, defining efficiency of their biological utilization and
describing community composition of these prokaryotes are among the largest
challenges facing researchers of these systems.
The hypothesis that large prokaryotic inventories observed in redoxclines of
anoxic water columns result from diminished mortality rates and slow growth
rates is contrary to our observations. Large and variable communities of anaer-
obic flagellates and ciliates have been observed repeatedly in the Cariaco Basin.
While protozoan grazing and growth rate measurements are unavailable, flagel-
lates and many taxa of ciliates are bacterivorous and can only proliferate if
coexisting with actively growing prey. These organisms represent the second
trophic level. Preliminary phylogenetic information suggests that anaerobic cil-
