BackgroundSince the discovery of thioautotrophic bacterial symbiosis in the giant tubeworm Riftia pachyptila, there has been great impetus to investigate such partnerships in other invertebrates. In this study, we present the occurrence of a sulphur-oxidizing symbiosis in a metazoan belonging to the phylum Cnidaria in which this event has never been described previously.Methodology/Principal FindingsScanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) observations and Energy-dispersive X-ray spectroscopy (EDXs) analysis, were employed to unveil the presence of prokaryotes population bearing elemental sulphur granules, growing on the body surface of the metazoan. Phylogenetic assessments were also undertaken to identify this invertebrate and microorganisms in thiotrophic symbiosis. Our results showed the occurrence of a thiotrophic symbiosis in a cnidarian identified as Cladonema sp.Conclusions/SignificanceThis is the first report describing the occurrence of a sulphur-oxidizing symbiosis in a cnidarian. Furthermore, of the two adult morphologies, the polyp and medusa, this mutualistic association was found restricted to the polyp form of Cladonema sp.

A large (47.75±3.56 µm in diameter) Thiovulum bacterial strain forming white veils is described from marine mangrove ecosystem. High sulfide concentrations (up to 8 mM of H2S) were measured on the sunken organic matters (wood/bone debris) in laboratory conditions. This sulfur-oxidizing bacterium colonized such organic matter forming white veil. According to conventional scanning electron microscope (SEM) observations, bacterial cells are ovoid and slightly motile by numerous small flagella present through the cell surface. Intracytoplasmic large internal sulfur granules were shown suggesting a sulphidic-based metabolism. Observations were confirmed by sulfur elemental sulfur distribution detected by energy-dispersive X-ray spectroscopy (EDXS) analysis using environmental scanning electron microscope (ESEM) on non-dehydrated samples. Phylogenetic analysis of partial sequence of 16S rDNA obtained from purified fractions of this -proteobacteraeota strain indicates that this bacterium belongs to the Thiovulaceae cluster and could be one of the largest Thiovulum ever described. We propose to name this species “Candidatus Thiovulum sp. strain imperiosus”.

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.