THESE Anne POSTEC Diversité de populations microbiennes ...

Résultats - Chapitre 3
that the related microorganisms are particularly well adapted to the culture conditions.
Sequences of autotrophic bacteria related to Deferribacter and Thermodesulfatator could be
detected only in the continuous culture in bioreactor at T28, and were subsequently
successfully subcultivated. This confirmed the advantage of the enrichment by continuous
culture in bioreactor rather than in vials since a wider diversity was revealed.
Although bacterial sequences were related to moderately thermophilic microorganisms,
growth of Archaea belonging to the hyperthermophilic genus Thermococcus occurred in a
first time despite the temperature of 60°C. We deduced that 99% of the cells thriving in the
continuous culture at T2 were related to Thermococcus. Their metabolism through sulphur
reduction might have generated a propitious environment for the successive growth of
bacterial species: the heterotrophic strains Caminicella spp., Thermosipho spp. and
Marinitoga spp., and the autotrophic strains Deferribacter spp. and Thermodesulfatator spp..
Sequences related to autotrophic microorganisms could be detected in the T28 library, but
not in the T7 one. Interestingly, Nercessian et al. (2003) showed similarly that the proportion
of Thermococcales-related phylotypes decreased in longer deployments of in situ collectors
in hydrothermal environments, whereas previously undetected phylotypes and those related
to facultative and/or strict chemolithoautotrophs increased or emerged.
Among heterotrophic bacteria evidenced in this study, a sequence related to Caminicella
sporogenes was detected both in the cultures in vials and in the bioreactor. C. sporogenes is
a thermophilic bacterium belonging to the Clostridiales and isolated from the East Pacific
Rise (EPR) hydrothermal vent (Alain, et al. 2002b). This study showed the occurrence of
members of this species on the Mid-Atlantic Ridge. Among the order Thermotogales, two
strains were isolated from the enrichment culture in bioreactor and corresponded to the
phylotypes evidenced by molecular analyses. Strain sp. AT1272 belonged to the genus
Thermosipho. Its 16S rDNA sequence shared 96% similarity with T. melanesiensis and 99%
with Thermosipho strain MV1063, both originating from the Mid-Atlantic Ridge. Strain
AT1271 T belonged to the genus Marinitoga. Its 16S rDNA sequence shared 96% of similarity
with the closest relative M. camini. This strain named ‘M. hydrogenitolerans’ was recently
characterized (Postec, et al. 2005). Its growth was not affected by the high hydrogen
concentration in the culture headspace, and this constitutes its main distinctive feature
among the Marinitoga spp. This tolerance could result from adaptation of strain AT1271 T to
the high hydrogen concentrations (16 mmol kg -1 ) measured in all vent fluids at the Rainbow
field from which strain AT1271 T was recovered (Charlou et al. 2002).
Among autotrophic microorganisms co-cultivated with heterotrophs in the bioreactor, a new
strain belonged to the genus Thermodesulfatator, order Thermodesulfobacteriales. This
genus is represented so far by the unique species T. indicus (Moussard et al. 2004), a
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