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

28 C. Schmidt et al. / Marine Chemistry 108 (2008) 18–31
iron II in the mixing zone may be conservative, but
sulfide appears depleted by about 80% of its endmember
value. These results suggest that the shrimp
environment is fuelled by a secondary fluid source from
which chemical compounds have been partly removed
by precipitation or microbial consumption as shown in
other hydrothermal environments (Le Bris et al., 2003;
Von Damm and Lilley, 2004).
The oxygen variation with pH at Rainbow substantially
departs from conservative mixing. These data
provided clues to identify the redox couple that could
reach equilibrium with O 2 /H 2 O, either in situ or during
the sample recovery process. As previously documented
(Le Bris et al., 2006), sulfide appears to be oxidized
during sample recovery. This result is consistent with
the fast sulfide oxidation rate in presence of Fe II (Zhang
and Millero, 1994). A similar assumption can be done
for hydrogen, which is a major constituent of Rainbow
fluids. Its presence in the source fluid is suggested by
oxygen content in the samples, which is lower than it
could be explained by sulfide oxidation. A maximum
amount of 40% hydrogen in the source fluid with
respect to the reference end-member (Charlou et al.,
2002) was estimated from these data, assuming that iron
oxidation is kinetically inhibited.
The presence of hydrogen in diffuse fluids has been
reported by Butterfield et al. (2004) and Von Damm and
Lilley (2004). It is not surprising that hydrogen could be
enriched in the secondary fluid source at Rainbow, as it
is highly enriched in the hydrothermal end-member
fluid. However, H 2 may be completely oxidized in the
oxic part of the mixing zone, as long as the reaction is
fast enough to be completed in this dynamic environment.
Alternatively, hydrogen may be maintained to a
substantial level if the renewal rate is high enough to
balance its fast oxidation. Two scenarios were proposed
on this basis. In order to draw conclusions on the
accuracy of either of them it would be required to
perform hydrogen in situ measurements. It is indeed
unlikely that H 2 could remain stable for hours in
oxygenated fluid samples. Therefore, hydrogen determination
from samples would always be afflicted with
imprecision. Comparison of in situ oxygen measurements
with discrete sample contents would provide
further clues to assess the presence of hydrogen in the
medium. Unfortunately these data are lacking to date.
4.2. Potential chemosynthetic energy pathways
In both scenarios, iron oxidation appears as a major
energy source in the mild part of the mixing zone at
Rainbow. This peculiar result is due to the exceptional
enrichment of iron in the local vent fluid. The enrichment
of hydrogen in the source fluid could have as well important
biogeochemical implications. In the first scenario,
assuming that hydrogen is fully oxidized in the
mixing zone, the O 2 availability will be limited to the low
temperature range (b25 °C). In the second scenario
hydrogen remains available in the whole mixing zone.
The energetical yield for hydrogen oxidizers would be in
the same range as for iron oxidizers below 18 °C and
slightly more above. This last scenario provides an upper
estimate of the energy available as the hydrogen and
oxygen contents where maximized in this assumption. In
comparison, sulfide and methane only appear as secondary
energy sources for the shrimp epibionts at Rainbow.
In contrast, at the TAG site, total sulfide oxidation to
sulfate appears as the dominant potential energy source
below 30 °C. In this thermal range, the energy that
would be available for chemosynthetic microbes using
iron II is much lower but still substantial. Methane is
only a minor component of TAG fluids and should not
constitute a significant electron donor for primary
production. The energy that could be supplied from
the oxidation of these reduced compounds in the shrimp
habitat at the two Mid-Atlantic Ridge vent sites thus
depict very different patterns.
4.3. Energetic and physico-chemical characteristics of
the shrimp habitat
Although the shrimp are highly mobile, they aggregate
in swarms of several thousand of individuals within
very sharp thermal limits. Similar temperature ranges
have been determined for two swarms at two locations at
the Rainbow site in 2001, and again in a swarm at one of
these location in 2005 (Table 3). Slightly lower data
were obtained for a swarm at TAG.
This result is generally consistent with the thermal
ranges reported in the literature for the R. exoculata
habitat, but it does not confirm the highest values
presented in some studies (Table 3). Results acquired at
Rainbow and at TAG show a certain homogeneity. This
suggests that the thermal range defined in this study may
be representative for Rimicaris swarms. Even though the
presence of shrimp at higher temperatures cannot be
ruled out, Ravaux et al. (2003) demonstrated that the
shrimp expresses a biochemical response to thermal
stress above 25 °C. We believe that the present study
provides a more accurate definition of the thermal
conditions in the Rimicaris habitat than early studies.
Temperature ranges were carefully assessed from large
data sets and followed by close-up video-control in order
to ensured the precise location of the probe tip. These