Environmental Engineering Reference
In-Depth Information
iate communities are diverse, and may include hosts to prokaryotic symbionts
and larger taxa that prey upon flagellates and other ciliates (third trophic level)
[4,9,46,52,67]. Typical of many food webs, our biomass estimates within the
redoxcline diminish by an order of magnitude between ascending trophic levels,
from prokaryotes to flagellates to ciliates.
The fates of top predators and ungrazed production in the chemoautotrophic
layer are intriguing unknowns. Are larger protozoans culled by migratory sus-
pension feeders? Do predators die off during periods of low prey availability
or viral epidemics, permitting the redoxcline community to simply recycle
nutrients fueled by fluxes of reductants and oxidants from below and above
as proposed by Ho et al. [15]? Does a portion of the redoxcline's production
sink into the anoxic layer and eventually to the seabed? We have observed
that in more than 20% of Cariaco's sediment trap collections, POC flux to 455
m exceeded fluxes to 265 m [56]. However, there is currently no direct evi-
dence of the redoxcline's contribution to sedimentation (Thunell, pers. comm.).
The largest predators (ciliates) in this web produce uncompacted feces of in-
completely digested cells with no appreciable sinking velocity. Nevertheless,
various microbes are known to stimulate particle aggregation by exuding poly-
meric material (reviewed in [48]). Perhaps this aggregation process contributes
to the sedimenting flux (Fig. 10).
Viral infection is dependent on susceptible host population densities and
proliferation is dependent on host growth rates, irrespective of whether hosts
are aerobic or anaerobic [11,57]. Hence bacteriophages most likely dominate
viral communities in the redoxcline, because prokaryotes are the most abundant
hosts. We speculate that viral attack on abundant protozoan populations in
the redoxcline community is also possible (Fig. 10). Therefore, viruses may
represent competitors to grazer populations at two or more trophic levels. Viral
lysis effectively returns high-quality organic matter to non-living pools, thereby
stimulating heterotrophic activity and elemental cycling [34]. Waters supporting
high population densities and low diversity are more prone to viral epidemics
than species-rich communities [11]. Therefore, the relative importance of viral
lysis to prokaryotic mortality, elemental cycling and energy flow within the
redoxcline are likely to depend on productivity and host diversity.
Full understanding of how redoxclines sustain the high standing stocks and
productivity presented above awaits further study and new approaches. Im-
proved descriptions of water circulation and horizontal structure, inferences
provided by stable isotopes, phylogenetic and physiological studies and inves-
tigations of symbioses will all be useful in solving this fascinating problem.
Detailed phylogenetic and biogeochemical studies of prokaryotes and protists
residing in the Cariaco's redoxcline are currently underway.
