Olivier Gros

Bacterial host specificity of Lucinacea endosymbionts: Interspecific variation in 16s rRNA sequences

Three tropical lucinid clams (Codakia orbiculata, Codakia pectinella and Lucina nassula) from a shallow coastal environment have been studied regarding to their thioautotrophic bacterial endosymbionts. The 16S rRNA genes (rDNA) from these three endosymbionts were amplified using PCR. Phylogenetic analysis by distance matrix and parsimony methods always placed the newly examined symbionts within the monophyletic group composed of symbionts of the bivalve superfamily Lucinacea. A same single 16S rRNA sequence was found in C. orbiculata and C. pectinella and was identical to that found in C. orbicularis and Linga pensylvanica, two other lucinids living in the same type of environment. These data indicate that a same symbiont species may be associated with different host species. Lucina nassula host a symbiont with a distinct 16S rDNA sequence, but very closely related to the former.

First description of a new uncultured epsilon sulfur bacterium colonizing marine mangrove sediment in the Caribbean: Thiovulum sp. strain karukerense

Here, the first description is reported of an epsilon sulfur-oxidizing bacterium from sulfide-rich sediments of marine mangrove in the Caribbean. By transition electron microscopy it was shown that this new strain contains intracytoplasmic large internal sulfur granules, which was confirmed by energy-dispersive X-ray spectroscopy analyses performed using an environmental scanning electron microscope. The sulfur distribution obtained for this sulfur-oxidizing bacterial strain allowed us to conclude that elemental sulfur is formed as an intermediate oxidation product and stored intracellularly. By conventional scanning electron microscopy it was shown that the bacterial cells are ovoid and extremely motile by lophotrichous flagella. Phylogenetic analyses based on partial sequence of the 16S rRNA gene confirmed that the bacterial strain belongs to the Thiovulum cluster and could be a representative of a new species in this poorly studied genus of sulfur-oxidizing free-living bacteria. Thus, reduced sediment of marine mangrove represents a sulfide-rich environment sustaining development of both gamma and epsilon sulfur-oxidizing Proteobacteria.

Thioautotrophic ectosymbiosis in Pseudovorticella sp., a peritrich ciliate species colonizing wood falls in marine mangrove

Ciliates represent a diversified group of protists known to establish symbioses with prokaryotic micro-organisms. They are mainly phagotrophs and symbiotic relationships with bacteria can give them an important advantage in chemosynthetic environments. The aim of this study is to describe the thiotrophic association that occurs between the peritrich ciliate Pseudovorticella sp. and potential sulfur-oxidizing bacteria. Investigations at microscopic scale (LM, SEM, TEM) showed ectosymbiotic bacteria covering the surface of the body of Pseudovorticella sp. According to 16S rDNA phylogenetic analysis, these ectosymbiotic bacteria belong to γ-proteobacteria and are phylogenetically close to the symbiont of the recently described Zoothamnium ignavum, which inhabits shallow-water wood falls. FISH experiments, using symbiont specific probes, clearly indicate that these ectosymbiotic bacteria are also ingested into food vacuoles. Electron lucent granules observed in TEM in the cytoplasm of the ectosymbiotic bacteria have been identified as sulfur granules by Raman microspectrometry analyses. Raman microspectrometry analyses confirmed the thiotrophic nature of this relationship already suggested by the results obtained by TEM and phylogeny. A complete sulfur map was then performed to investigate the sulfur distribution in the zooid. Results show that the relationship between this protist and its bacterial partner is a thiotrophic ectosymbiosis.

First description of a new uncultured purple sulfur bacterium colonizing marine mangrove sediment in the Caribbean: Halochromatium-like PSB from Guadeloupe

Here, we report the first description of a marine purple sulfur bacterium (PSB) from sulfide-rich sediments of a marine mangrove in the Caribbean. TEM shows that this new isolate contains intracytoplasmic vesicular membrane systems (containing bacteriochlorophyll a) and larger internal sulfur granules, confirmed by EDXS analyses performed using ESEM. The sulfur distribution and mapping obtained for this PSB strain has allowed us to conclude that elemental sulfur is formed as an intermediate oxidation product and stored intracellularly. SEM shows that the bacterial cells are ovoid and extremely motile via lophotrichous flagella. Phylogenetic characterization, based on the analysis of 16S rDNA and functional gene pufM sequences, demonstrate that this strain belongs to the Chromatiaceae and may be a representative of a new species of the genus Halochromatium. Thus, reduced sediments of marine mangrove represent a sulfide-rich environment that sustains the development of Chromatiaceae, in addition to sulfur-oxidizing bacteria and cyanobacteria, as previously reported.

Research letterBacterial host specificity of Lucinacea endosymbionts: Interspecific variation in 16S rRNA sequences

Three tropical lucinid clams (Codakia orbiculata, Codakia pectinella and Lucina nassula) from a shallow coastal environment have been studied regarding to their thioautotrophic bacterial endosymbionts. The 16S rRNA genes (rDNA) from these three endosymbionts were amplified using PCR. Phylogenetic analysis by distance matrix and parsimony methods always placed the newly examined symbionts within the monophyletic group composed of symbionts of the bivalve superfamily Lucinacea. A same single 16S rRNA sequence was found in C. orbiculata and C. pectinella and was identical to that found in C. orbicularis and Linga pensylvanica, two other lucinids living in the same type of environment. These data indicate that a same symbiont species may be associated with different host species. Lucina nassula hosts a symbiont with a distinct 16S rDNA sequence, but very closely related to the former.