Geochemical energy sources for microbial primary production in the ...

C. Schmidt et al. / Marine Chemistry 108 (2008) 18–31
29
short-term studies however may not fully reflect the
temporal temperature variation pattern within a swarm.
Over these narrow ranges, the potential energy sources
available to the epibionts for the chemolithoautotrophic
CO 2 fixation is unambiguous. The conditions within the
shrimp habitat at Rainbow appear energetically optimal
for chemolithoautotrophic growth relying on iron II, and
potentially on hydrogen oxidation. At TAG, the oxidation
of sulfide provides the main energy budgets. These
bioenergetic considerations support previous studies that
have suggested iron oxidation as an important metabolic
pathway, based on the analysis of iron oxide deposits in
the branchial cavity of shrimp sampled at Rainbow
(Gloter et al., 2004; Zbinden et al. 2004). The fast abiotic
oxidation of iron in alkaline to neutral media at ambient
temperature (∼25 °C) is expected to limit the microbially
mediated conversion to low O 2 conditions. According to
the temperature dependence of the kinetic rate constant
(Millero et al. 1987), this effect should not be limiting in
the tepid Rimicaris habitat.
Although the presence of H 2 in the shrimp environment
at Rainbow remains speculative, it is interesting to
note that the available energy budget provided by
hydrogen oxidation could be in the same range as the
one liberated by the oxidation of iron. Reaction kinetics
is a determinant criteria that should be investigated in
order to further asses the importance of this electron
donor. The commonly described electron donors in
chemosynthetic ecosystems, i.e., sulfide and methane,
do not represent the predominant energy sources at
Rainbow. Still, their oxidation may constitute a possible
chemosynthetic pathway, as some energy can be derived,
although the yield is much lower than for iron and
hydrogen oxidation.
5. Conclusion
Estimations on the energy budget as a function of the
mixing ratio suggested that chemolithoautotrophic primary
producers may find optimal conditions for growth
in association with highly active shrimps that aggregate
in swarms in the hydrothermal fluid-seawater mixing
interface. This study confirmed that the chemical energy
sources that could be utilized by primary producers
associated with R. exoculata differ substantially between
Rainbow and TAG, even though they colonize
similar habitats. Although the shrimps were observed in
comparable narrow temperature ranges, they may harbor
a highly diversified microbial epiflora at different sites,
depending on the hydrothermal fluid chemistry. Consideration
based on thermodynamic constraints need to
be further supported by microbial studies since the most
energetic processes are not necessarily the one which
dominantly fuel the shrimp epibionts. Rimicaris shrimps
maintain themselves in a quite narrow environmental
range within the mixing zone. The electron donors that
are commonly described to fuel chemosynthetic growth,
like sulfide and methane, do not appear as the main
energy sources at Rainbow. Here, the oxidation of iron
yields the maximum energy in the mixing zone. A firstorder
calculation revealed that hydrogen could act as
well as a major electron donor for CO 2 -fixation in the
shrimp habitat, as long as its residence time is long
enough to allow microbial uptake. Field studies are still
required to provide a direct evidence of this hypothesis
and to estimate to what extend hydrogen will be preserved
in the mixing zone.
Acknowledgments
This work was financially supported by IFREMER,
University Pierre and Marie Curie-Paris 6, and the
European Community (PhD grant to C.S. / MOMARNET
RTN contract 2004-5050026). The authors would like
to particularly acknowledge the chief scientists of the
research cruises, Pierre-Marie Sarradin for ATOS and
Anne Godfroy for EXOMAR, the captains and crews of
the RV Atalante and the Victor 6000 operation group, as
well as instrumentation engineers and technicians for their
essential support at sea.
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