Thiotrophic Bacteria Research Articles

Lipid Biomarker Patterns Reflect Nutritional Strategies of Seep-Dwelling Bivalves From the South China Sea

Invertebrates living at methane seeps such as mussels and clams gain nutrition through symbiosis with chemosynthetic, chiefly methanotrophic and thiotrophic bacteria. Lipid biomarkers, including their compound-specific carbon stable isotope compositions, extracted from the host tissues are predestined for deciphering the various sources of diets and the associations among varying environments, endosymbionts, and hosts. Here, we investigated lipid inventories of soft tissues of two bathymodiolin mussel species hosting aerobic methanotrophic bacteria (Gigantidas platifrons from Site F and Gigantidas haimaensis from Haima seeps), one bathymodiolin mussel with thiotrophic bacteria (Bathymodiolus aduloides from Haima seeps), and one vesicomyid clam (Archivesica marissinica from Haima seeps) from the South China Sea. The gills of mussels hosting methanotrophic symbionts were found to contain high amounts of lipids of aerobic methanotrophic bacteria, such as the 4,4-dimethyl lanosterol, and other 4-methyl sterols, and the type I methanotroph-specific monounsaturated fatty acids (MUFAs) C16:1ω9 and C16:1ω8. Production of methyl-sterols is favored over fatty acids at low oxygen concentrations, as demonstrated in culture experiments with Methylococcus capsulatus. Since lesser fatty acids and abundant sterols are found in G. haimaensis compared to G. platifrons, G. haimaensis apparently lived at very low oxygen levels. Extremely high levels of MUFAs C16:1ω7 and C18:1ω7 were found in gill tissue of both B. aduloides and the vesicomyid clam A. marissinica. Given the absence of ω8 fatty acids, both B. aduloides and the vesicomyid clam contain thiotrophic bacteria only. The occurrence of 13C-enriched 24-methylenecholesterol in B. aduloides indicates that the animal complemented its diet by filter-feeding (ca. 3% of the total sterol inventory) on photosynthetically derived carbon, whereas the majority of sterols are pointing to a diet relying on endosymbionts. Different types of 4-methyl sterols were observed between the thiotroph-containing mussel and methanotroph-containing mussels, suggesting different biosynthetic steps are present from lanosterol to cholesterol between animal hosts and aerobic methanotrophs. Among the four bivalve species, specific lipid biomarker patterns diagnostic for either the symbionts or the hosts yielded similar δ13C values in each species, indicating that the host obtained its nutrition either directly from the symbionts or derived at least most of its carbon in this way. The information derived from lipid biomarkers of bivalves and their corresponding symbionts in modern environments is vital to interpret data from the rock record, where most other methods to study microbial community composition are not applicable.

Anaerobic Sulfur Oxidation Underlies Adaptation of a Chemosynthetic Symbiont to Oxic-Anoxic Interfaces.

ABSTRACTChemosynthetic symbioses occur worldwide in marine habitats, but comprehensive physiological studies of chemoautotrophic bacteria thriving on animals are scarce. Stilbonematinae are coated by thiotrophic Gammaproteobacteria. As these nematodes migrate through the redox zone, their ectosymbionts experience varying oxygen concentrations. However, nothing is known about how these variations affect their physiology. Here, by applying omics, Raman microspectroscopy, and stable isotope labeling, we investigated the effect of oxygen on “Candidatus Thiosymbion oneisti.” Unexpectedly, sulfur oxidation genes were upregulated in anoxic relative to oxic conditions, but carbon fixation genes and incorporation of 13C-labeled bicarbonate were not. Instead, several genes involved in carbon fixation were upregulated under oxic conditions, together with genes involved in organic carbon assimilation, polyhydroxyalkanoate (PHA) biosynthesis, nitrogen fixation, and urea utilization. Furthermore, in the presence of oxygen, stress-related genes were upregulated together with vitamin biosynthesis genes likely necessary to withstand oxidative stress, and the symbiont appeared to proliferate less. Based on its physiological response to oxygen, we propose that “Ca. T. oneisti” may exploit anaerobic sulfur oxidation coupled to denitrification to proliferate in anoxic sand. However, the ectosymbiont would still profit from the oxygen available in superficial sand, as the energy-efficient aerobic respiration would facilitate carbon and nitrogen assimilation.IMPORTANCE Chemoautotrophic endosymbionts are famous for exploiting sulfur oxidization to feed marine organisms with fixed carbon. However, the physiology of thiotrophic bacteria thriving on the surface of animals (ectosymbionts) is less understood. One longstanding hypothesis posits that attachment to animals that migrate between reduced and oxic environments would boost sulfur oxidation, as the ectosymbionts would alternatively access sulfide and oxygen, the most favorable electron acceptor. Here, we investigated the effect of oxygen on the physiology of “Candidatus Thiosymbion oneisti,” a gammaproteobacterium which lives attached to marine nematodes inhabiting shallow-water sand. Surprisingly, sulfur oxidation genes were upregulated under anoxic relative to oxic conditions. Furthermore, under anoxia, the ectosymbiont appeared to be less stressed and to proliferate more. We propose that animal-mediated access to oxygen, rather than enhancing sulfur oxidation, would facilitate assimilation of carbon and nitrogen by the ectosymbiont.

Lipid biomarker patterns reflect different formation environments of mussel- and tubeworm-dominated seep carbonates from the Gulf of Mexico (Atwater Valley and Green Canyon)

Mussels and tubeworms thriving at many methane seeps typically live in symbiosis with chemosynthetic, chiefly methanotrophic or thiotrophic bacteria. It has been shown that the activities of chemosymbiotic animals can result in large differences in the sedimentary environments of their habitats. Here, we put forward the concept that such environmental variability can be archived in the lipid biomarker inventories of authigenic carbonates forming in different, locally confined environments at seeps, mussel beds and tubeworm bushes in this case. To test this hypothesis, lipid biomarker patterns of carbonates from mussel and tubeworm environments from two seep sites (Atwater Valley 340 and Green Canyon 852) of the Gulf of Mexico were analyzed. Previous work revealed stronger carbon isotope fractionation between the methane source and biomarkers of anaerobic methane oxidizing archaea-2 (ANME-2)/sulfate-reducing Desulfosarcina/Desulfococcus (DSS) consortia than for ANME-1/DSS consortia, both performing anaerobic oxidation of methane (AOM). Similar δ13Cmethane values were found at the mussel and tubeworm sites from the same seeps and the local microbial consortia also appear to be largely similar based on the observed AOM biomarker inventories. Yet, a large average offset of 32‰ between the δ13C values of molecular fossils of sulfate-reducing bacteria (SRB) involved in AOM was observed, with lower values typifying tubeworm carbonates than mussel carbonates. This pattern is interpreted to reflect local effects on isotope fractionation caused by the chemosymbiotic metazoans at mussel- and tubeworm-dominated sites. At tubeworm-dominated sites, the excess sulfate produced by thiotrophic symbionts of tubeworms and pumped down into the sediment results in persistent production of AOM-derived bicarbonate and the enrichment of 12C in sub-surface sediments. Interestingly, tubeworm carbonates also contain high amounts of non-isoprenoidal dialkyl glycerol diethers (DAGEs) with extreme 13C depletions, representing compounds that derived from non-DSS cluster SRB. Most likely, 13C-depleted AOM-derived organic intermediates were used as carbon sources by the DAGE-producing non-DSS cluster SRB, possibly performing organoclastic sulfate reduction. Our study identifies significant variation in biomarker patterns between mussel and tubeworm carbonates at two seep sites in the Gulf of Mexico. Such variation allows to characterize different habitats at seeps, which are shaped by the interaction of chemosymbiotic seep metazoans and their symbionts with the local environment. Metazoan community composition apparently controls geobiological interaction in seep ecosystems to a large degree, which may allow tracing of the effects of chemosymbiosis into the rock record.

Life and death in facultative chemosymbioses: control of bacterial population dynamics in the Thyasiridae

Stable associations between marine invertebrates and their chemosynthetic bacterial symbionts are predicated on both the adequate transfer of resources and the restriction of bacterial cells to a finite population within host tissues. In symbioses between thyasirid bivalves and thiotrophic bacteria, symbionts are extracellular, acquired from a free-living pool, and periodically endocytosed and digested by host bacteriocytes. Thyasirid symbionts require reduced sulphur to fuel their autotrophic metabolism; the host makes this energy source accessible to symbionts through its burrowing and irrigation behaviours. Here, we demonstrate that unlike the bacterial symbionts of many chemosymbiotic bivalves, those of Thyasira flexuosa and T. cf. gouldi divide while associated with host gill epithelial cells, possibly constituting a second method of symbiont replacement in thyasirid bivalves alongside environmental acquisition. Furthermore, exposure of T. flexuosa and T. cf. gouldi to elevated concentrations of thiosulphate, a reduced sulphur species used by many sulphur-oxidizing bacterial symbionts, results in the rapid onset of bacterial division and expansion of symbiont populations within host gills and can result in host mortality. These results highlight the flexible nature of symbioses in thyasirid bivalves and the possibility for symbiosis breakdown in some thyasirid lineages. Furthermore, results suggest a role for behaviours such as sulphide mining and irrigation in maintaining and controlling a stable population of bacterial symbionts in this family.

Specificity in diversity: single origin of a widespread ciliate-bacteria symbiosis.

Symbioses between eukaryotes and sulfur-oxidizing (thiotrophic) bacteria have convergently evolved multiple times. Although well described in at least eight classes of metazoan animals, almost nothing is known about the evolution of thiotrophic symbioses in microbial eukaryotes (protists). In this study, we characterized the symbioses between mouthless marine ciliates of the genus Kentrophoros, and their thiotrophic bacteria, using comparative sequence analysis and fluorescence in situ hybridization. Ciliate small-subunit rRNA sequences were obtained from 17 morphospecies collected in the Mediterranean and Caribbean, and symbiont sequences from 13 of these morphospecies. We discovered a new Kentrophoros morphotype where the symbiont-bearing surface is folded into pouch-like compartments, illustrating the variability of the basic body plan. Phylogenetic analyses revealed that all investigated Kentrophoros belonged to a single clade, despite the remarkable morphological diversity of these hosts. The symbionts were also monophyletic and belonged to a new clade within the Gammaproteobacteria, with no known cultured representatives. Each host morphospecies had a distinct symbiont phylotype, and statistical analyses revealed significant support for host–symbiont codiversification. Given that these symbioses were collected from two widely separated oceans, our results indicate that symbiotic associations in unicellular hosts can be highly specific and stable over long periods of evolutionary time.

The Relative Abundance and Transcriptional Activity of Marine Sponge-Associated Microorganisms Emphasizing Groups Involved in Sulfur Cycle

During the last decades, our knowledge about the activity of sponge-associated microorganisms and their contribution to biogeochemical cycling has gradually increased. Functional groups involved in carbon and nitrogen metabolism are well documented, whereas knowledge about microorganisms involved in the sulfur cycle is still limited. Both sulfate reduction and sulfide oxidation has been detected in the cold water sponge Geodia barretti from Korsfjord in Norway, and with specimens from this site, the present study aims to identify extant versus active sponge-associated microbiota with focus on sulfur metabolism. Comparative analysis of small subunit ribosomal RNA (16S rRNA) gene (DNA) and transcript (complementary DNA (cDNA)) libraries revealed profound differences. The transcript library was predominated by Chloroflexi despite their low abundance in the gene library. An opposite result was found for Acidobacteria. Proteobacteria were detected in both libraries with representatives of the Alpha- and Gammaproteobacteria related to clades with presumably thiotrophic bacteria from sponges and other marine invertebrates. Sequences that clustered with sponge-associated Deltaproteobacteria were remotely related to cultivated sulfate-reducing bacteria. The microbes involved in sulfur cycling were identified by the functional gene aprA (adenosine-5'-phosphosulfate reductase) and its transcript. Of the aprA sequences (DNA and cDNA), 87% affiliated with sulfur-oxidizing bacteria. They clustered with Alphaproteobacteria and with clades of deep-branching Gammaproteobacteria. The remaining sequences clustered with sulfate-reducing Archaea of the phylum Euryarchaeota. These results indicate an active role of yet uncharacterized Bacteria and Archaea in the sponge's sulfur cycle.

Distribution and Composition of Thiotrophic Mats in the Hypoxic Zone of the Black Sea (150-170 m Water Depth, Crimea Margin).

At the Black Sea chemocline, oxygen- and sulfide-rich waters meet and form a niche for thiotrophic pelagic bacteria. Here we investigated an area of the Northwestern Black Sea off Crimea close to the shelf break, where the chemocline reaches the seafloor at around 150–170 m water depth, to assess whether thiotrophic bacteria are favored in this zone. Seafloor video transects were carried out with the submersible JAGO covering 20 km2 on the region between 110 and 200 m depth. Around the chemocline we observed irregular seafloor depressions, covered with whitish mats of large filamentous bacteria. These comprised 25–55% of the seafloor, forming a belt of 3 km width around the chemocline. Cores from the mats obtained with JAGO showed higher accumulations of organic matter under the mats compared to mat-free sediments. The mat-forming bacteria were related to Beggiatoa-like large filamentous sulfur bacteria based on 16S rRNA sequences from the mat, and visual characteristics. The microbial community under the mats was significantly different from the surrounding sediments and enriched with taxa affiliated with polymer degrading, fermenting and sulfate reducing microorganisms. Under the mats, higher organic matter accumulation, as well as higher remineralization and radiotracer-based sulfate reduction rates were measured compared to outside the mat. Mat-covered and mat-free sediments showed similar degradability of the bulk organic matter pool, suggesting that the higher sulfide fluxes and subsequent development of the thiotrophic mats in the patches are consequences of the accumulation of organic matter rather than its qualitative composition. Our observations suggest that the key factors for the distribution of thiotrophic mat-forming communities near to the Crimean shelf break are hypoxic conditions that (i) repress grazers, (ii) enhance the accumulation and degradation of labile organic matter by sulfate-reducers, and (iii) favor thiotrophic filamentous bacteria which are adapted to exploit steep gradients in oxygen and sulfide availability; in addition to a specific seafloor topography which may relate to internal waves at the shelf break.

Comparative analysis of symbiont ratios and gene expression in natural populations of two Bathymodiolus mussel species

Bathymodiolus mussels associated with deep-sea hydrothermal vents and cold seeps harbor chemosynthetic endosymbiotic bacteria in bacteriocytes located in the gill epithelium. Two distinct morphotypes of γ-proteobacteria, sulfur- and methane-oxidizing, have been identified and form a dual symbiosis in B. azoricus mussels from the Mid-Atlantic Ridge and in B. aff. boomerang from cold seeps in the Gulf of Guinea. Thiotrophic bacteria (SOX) are capable of fixing CO2 in the presence of sulfide or thiosulfate and methanotrophic bacteria (MOX) use methane both as a carbon and an energy source. In this study we used quantitative real-time PCR to test whether symbiont abundance and gene expression varied between the two mussel species. Results showed that B. azoricus from two hydrothermal sites had similar ratios and gene expression pattern for both symbiont types. In B. aff. boomerang, SOX ratio and ATP sulfurylase gene expression show differences between specimens collected on the different sites. Analysis of symbiont ratios in both species indicated a clear dominance of MOX symbionts in B. aff. boomerang and SOX symbionts in B. azoricus. We also evidenced that the species from the deeper sites (B. aff. boomerang) and mussels collected from sulfur and methane rich habitats showed higher symbiont ratio suggesting that environmental parameters may have significant impacts on the symbiont ratios in Bathymodiolus mussels.

Hydrothermal sediments are a source of water column Fe and Mn in the Bransfield Strait, Antarctica

Short sediment cores were collected from ∼1100m water depth at the top of Hook Ridge, a submarine volcanic edifice in the Central Basin of the Bransfield Strait, Antarctica, to assess Fe and Mn supply to the water column. Low-temperature hydrothermal fluids advect through these sediments and, in places, subsurface H2S is present at high enough concentrations to support abundant Sclerolinum sp., an infaunal tubeworm that hosts symbiotic thiotrophic bacteria. The water column is fully oxic, and oxygen penetration depths at all sites are 2–5cmbsf. Pore water Fe and Mn content is high within the subsurface ferruginous zone (max. 565μmolFeL−1, >3–7cmbsf)—14–18 times higher than values measured at a nearby, background site of equivalent water depth. Diffusion and advection of pore waters supply significant Fe and Mn to the surface sediment. Sequential extraction of the sediment demonstrates that there is a significant enrichment in a suite of reactive, authigenic Fe minerals in the upper 0–5cm of sediment at one site characterised by weathered crusts at the seafloor. At a site with only minor authigenic mineral surface enrichment we infer that leakage of pore water Fe and Mn from the sediment leads to enriched total dissolvable Fe and Mn in bottom waters. An Eh sensor mounted on a towed package mapped a distinct Eh signature above this coring site which is dispersed over several km at the depth of Hook Ridge. We hypothesise that the main mechanism for Fe and Mn efflux from the sediment is breach of the surface oxic layer by the abundant Sclerolinum sp., along with episodic enhancements by physical mixing and resuspension of sediment in this dynamic volcanic environment. We propose that Hook Ridge sediments are an important source of Fe and Mn to the deep waters of the Central Basin in the Bransfield Strait, where concentrations are sustained by the benthic flux, and Fe is stabilised in the water column as either colloidal phases or ligand-bound dissolved species. Entrainment of this water mass into the Drake Passage and thereby the Antarctic Circumpolar Current could provide a significant metal source to this HNLC region of the Southern Ocean if mixing and upwelling occurs before removal of this metal pool to underlying sediments. Sediment-covered volcanic ridges are common within rifted margins and may play a previously overlooked role in the global Fe cycle.

Morphology, microanatomy and sequence data of Sclerolinum contortum (Siboglindae, Annelida) of the Gulf of Mexico.

Sclerolinum is a small genus of Siboglinidae (Annelida) living in an obligate mutualistic association with thiotrophic bacteria as adults. Its taxonomic position, based on morphology, has been controversial; however, molecular data point to a sister taxa relationship with vestimentiferans. 16S rRNA gene sequencing and comparative morphology revealed that the studied population from deep-sea hydrocarbon seeps of the Gulf of Mexico belongs to Sclerolinum contortum known from the Arctic Sea. Since no anatomical and microanatomical studies have been published yet, we conducted such a study on S. contortum using serial sectioning and light and transmission electron microscopy. We show that the Sclerolinum body, divided into a head, trunk, and opisthosoma, is very similar to that of the vestimentiferans, and therefore we propose that the body regions are homologous in both taxa.

The metatrochophore of a deep-sea hydrothermal vent vestimentiferan (Polychaeta: Siboglinidae).

Vestimentiferans (Siboglinidae, Polychaeta) live as juveniles and adults in an obligate mutualistic association with thiotrophic bacteria. Since their development is aposymbiotic, metatrochophores of vestimentiferans from the East Pacific Rise colonizing deep-sea hydrothermal vents are infected with the specific symbiont, develop the trophosome, and reduce their digestive system. To gain insight into the anatomy and ultrastructure and to compare this stage with metatrochophores from other siboglinids, we serial sectioned and reconstructed three specimens using light and transmission electron microscopy. The metatrochophore was composed of a prostomium, a small peristomium, two chaetigers (or two chaetigers and one additional segment without chaetae), and a minute pygidium. A digestive system and an intraepidermal nervous system were developed. Larval organs such as the prototroch, the neurotroch, and an apical organ were present, along with juvenile/adult organs such as tentacles, uncini, pyriform glands, and the anlage of the nephridial organ. We propose that in vestimentiferans, the vestimentum is the head arising from the prostomium, peristomium, and the anterior part of the first chaetiger. In frenulates, in contrast, the head is composed on the one hand of the cephalic lobe arising from the prostomium and on the other of the forepart developing from the peristomium and the anterior part of the first chaetiger. In frenulates the muscular septum between the forepart and trunk develops later than the first two chaetigers. Since this septum has no counterpart in vestimentiferans, the forepart-trunk border of frenulates is not considered homologous with the vestimentum-trunk border in vestimentiferans. The obturacular region in vestimentiferans does not appear to be a body region but rather the head appendages arising from the first chaetiger. In contrast, the tentacles in frenulates are prostomial head appendages. In both taxa, the trunk is the posterior part of the first chaetiger, and the opisthosoma is the following chaetigers and the pygidium. Comparisons with other polychaetes suggest that two larval segments are autapomorphic for the monophyletic Siboglinidae.

The Transcriptome of Bathymodiolus azoricus Gill Reveals Expression of Genes from Endosymbionts and Free-Living Deep-Sea Bacteria

Deep-sea environments are largely unexplored habitats where a surprising number of species may be found in large communities, thriving regardless of the darkness, extreme cold, and high pressure. Their unique geochemical features result in reducing environments rich in methane and sulfides, sustaining complex chemosynthetic ecosystems that represent one of the most surprising findings in oceans in the last 40 years. The deep-sea Lucky Strike hydrothermal vent field, located in the Mid Atlantic Ridge, is home to large vent mussel communities where Bathymodiolus azoricus represents the dominant faunal biomass, owing its survival to symbiotic associations with methylotrophic or methanotrophic and thiotrophic bacteria. The recent transcriptome sequencing and analysis of gill tissues from B. azoricus revealed a number of genes of bacterial origin, hereby analyzed to provide a functional insight into the gill microbial community. The transcripts supported a metabolically active microbiome and a variety of mechanisms and pathways, evidencing also the sulfur and methane metabolisms. Taxonomic affiliation of transcripts and 16S rRNA community profiling revealed a microbial community dominated by thiotrophic and methanotrophic endosymbionts of B. azoricus and the presence of a Sulfurovum-like epsilonbacterium.

Mats of psychrophilic thiotrophic bacteria associated with cold seeps of the Barents Sea

Abstract. This study investigated the bacterial diversity associated with microbial mats of polar deep-sea cold seeps. The mats were associated with high upward fluxes of sulfide produced by anaerobic oxidation of methane, and grew at temperatures close to the freezing point of seawater. They ranged from small patches of 0.2–5 m in diameter (gray mats) to extensive fields covering up to 850 m2 of seafloor (white mats) and were formed by diverse sulfide-oxidizing bacteria differing in color and size. Overall, both the dominant mat-forming thiotrophs as well as the associated bacterial communities inhabiting the mats differed in composition for each mat type as determined by microscopy, 16S rRNA gene sequencing and automated ribosomal intergenic spacer analysis. While the smaller gray mats were associated with a highly diverse composition of sulfide oxidizers, the larger white mats were composed of only 1–2 types of gliding Beggiatoa filaments. Molecular analyses showed that most of the dominant mat-forming sulfide oxidizers were phylogenetically different from, but still closely related to, thiotrophs known from warmer ocean realms. The psychrophilic nature of the polar mat-forming thiotrophs was tested by visual observation of active mats at in situ temperature compared to their warming to >4 °C. The temperature range of mat habitats and the variation of sulfide and oxygen fluxes appear to be the main factors supporting the diversity of mat-forming thiotrophs in cold seeps at continental margins.

Mechanisms and evolution of oxidative sulfur metabolism in green sulfur bacteria.

Green sulfur bacteria (GSB) constitute a closely related group of photoautotrophic and thiotrophic bacteria with limited phenotypic variation. They typically oxidize sulfide and thiosulfate to sulfate with sulfur globules as an intermediate. Based on genome sequence information from 15 strains, the distribution and phylogeny of enzymes involved in their oxidative sulfur metabolism was investigated. At least one homolog of sulfide:quinone oxidoreductase (SQR) is present in all strains. In all sulfur-oxidizing GSB strains except the earliest diverging Chloroherpeton thalassium, the sulfide oxidation product is further oxidized to sulfite by the dissimilatory sulfite reductase (DSR) system. This system consists of components horizontally acquired partly from sulfide-oxidizing and partly from sulfate-reducing bacteria. Depending on the strain, the sulfite is probably oxidized to sulfate by one of two different mechanisms that have different evolutionary origins: adenosine-5′-phosphosulfate reductase or polysulfide reductase-like complex 3. Thiosulfate utilization by the SOX system in GSB has apparently been acquired horizontally from Proteobacteria. SoxCD does not occur in GSB, and its function in sulfate formation in other bacteria has been replaced by the DSR system in GSB. Sequence analyses suggested that the conserved soxJXYZAKBW gene cluster was horizontally acquired by Chlorobium phaeovibrioides DSM 265 from the Chlorobaculum lineage and that this acquisition was mediated by a mobile genetic element. Thus, the last common ancestor of currently known GSB was probably photoautotrophic, hydrogenotrophic, and contained SQR but not DSR or SOX. In addition, the predominance of the Chlorobium–Chlorobaculum–Prosthecochloris lineage among cultured GSB could be due to the horizontally acquired DSR and SOX systems. Finally, based upon structural, biochemical, and phylogenetic analyses, a uniform nomenclature is suggested for sqr genes in prokaryotes.

Siboglinid-bacteria endosymbiosis

Siboglinid worms are a group of gutless marine annelids which are nutritionally dependent upon endosymbiotic bacteria.1-2 Four major groups of siboglinids are known including vestimentiferans, Osedax spp., frenulates, and moniliferans.3-5 Very little is known about the diversity of bacterial endosymbionts associated with frenulate or monoliferan siboglinids. This lack of knowledge is surprising considering the global distribution of siboglinids; this system is likely among the most common symbioses in the deep sea. At least three distinct clades of endosymbiotic γ-proteobacteria associate with siboglinid annelids.6 Frenulates harbor a clade of γ–proteobacteria that are divergent from both the thiotrophic bacteria of vestimentiferans and monoliferans as well as the heterotrophic bacteria of Osedax spp.6-7 We also discuss priorities for future siboglinid research and the need to move beyond descriptive studies. A promising new method, laser-capture microdissection (LCM), allows for the precise excision of tissue regions of interest.8 This method, when used in concert with molecular and genomic techniques, such as Expressed Sequence Tag surveys using pyrosequencing technology, will likely enable investigations into physiological processes and mechanisms in these symbioses. Furthermore, adopting a comparative approach using different siboglinid groups, such as worms harboring thiotrophic versus methanotrophic endosymbionts, may yield considerable insight into the ecology and evolution of the Siboglinidae.

A multi-isotope study of deep-sea mussels at three different hydrothermal vent sites in the northwestern Pacific

To investigate symbiotic bacterial ecosystems at different deep-sea hydrothermal systems in the northwestern Pacific, compound-specific carbon and hydrogen isotope analyses of lipid biomarkers have been performed in addition to bulk C, H, N, S and O isotope analyses on Bathymodiolus mussels from the Hatoma seamount ( B. platifrons), the Daiyon-Yonaguni (Yonaguni) knoll ( B. platifrons), and the Suiyo seamount ( B. septemdierum). The two B. platifrons contain large amounts of diploptene, while no hopanoid is detected in B. septemdierum, suggesting that B. platifrons and B. septemdierum harbors methanotrophic and thiotrophic bacteria, respectively. In spite of the same symbiont, the large bulk δ 13C difference between the Hatoma (− 44.8‰) and Yonaguni (− 24.5‰) mussels reflects isotopically distinct hydrothermal CH 4 (Hatoma: ~ − 48‰, Yonaguni: ~ −26‰) as a carbon source. Fatty acids of the Hatoma and Yonaguni mussels are more enriched in D (− 144 to − 101‰) than the Suiyo mussel (− 265 to − 162‰), suggesting that D-depleted magmatic water or D-enriched hydrogen derived from CH 4 could be a partial hydrogen source for methanotrophy or thiotrophy, respectively. Apparent positive correlations are observed between δ 13C and δD of the bulk and biomarkers for each mussel due to similar biochemical processes during de novo synthesis. The compound-specific δ 13C and δD variations have provided much information on not only distinct carbon and hydrogen sources but also the lipid synthesis with respect to different symbiotic bacterial ecosystems.

Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem

Observations from the first submersible reconnaissance of the Blake Ridge Diapir provide the geological and ecological contexts for chemosynthetic communities established in close association with methane seeps. The seeps mark the loci of focused venting of methane from the gas hydrate reservoir, and, in one location (Hole 996D of the Ocean Drilling Program), methane emitted at the seafloor was observed forming gas hydrate on the underside of a carbonate overhang. Megafaunal elements of a chemosynthetically based community mapped onto dive tracks provide a preliminary overview of faunal distributions and habitat heterogeneity. Dense mussel beds were prominent and covered 20×20 m areas. The nearly non-overlapping distributions of mussels and clams indicate that there may be local (meter-scale) variations in fluid flux and chemistry within the seep site. Preliminary evidence suggests that the mussels are host to two symbiont types (sulfide-oxidizing thiotrophs and methanotrophs), while the clams derive their nutrition only from thiotrophic bacteria. Invertebrate biomass is dominated by mussels ( Bathymodiolus heckerae) that reach lengths of up to 364 mm and, to a lesser extent, by small (22 mm length) vesicomyid clams ( Vesicomya cf. venusta). Taking into account biomass distributions among taxa, symbiont characteristics of the bivalves, and stable-isotope analyses, the relative importance of methanotrophic vs thiotrophic bacteria in the overall nutrition of the invertebrate assemblage is on the order of 60% vs 40% (3:2).