Molecular phylogenetic and isotopic evidence of two lineages of chemoautotrophic endosymbionts distinct at the subdivision level harbored in one host-animal type: The genusAlviniconcha(Gastropoda: Provannidae)

Multi-element stable isotope geochemistry and arsenic speciation of hydrothermal vent fauna (Alviniconcha sp., Ifremeria nautilei and Eochionelasmus ohtai manusensis), Manus Basin, Papua New Guinea

Deep-sea hydrothermal vent communities, revealing patterns of niche partitioning, live in a limited area characterised by sharp physico-chemical gradients. In this study, we investigated carbon, sulfur, nitrogen stable isotopes as well as arsenic (As) speciations and concentrations for two snails (Alviniconcha sp. and Ifremeria nautilei) and a crustacean, (Eochionelasmus ohtai manusensis), occupying distinct niches in the hydrothermal vent field of the Vienna Woods, Manus Basin, Western Pacific. δ¹³C values of Alviniconcha sp. (foot), I. nautilei (foot and chitin) and E. o. manusensis (soft tissue) are similar, from -28 to -33‰ (V-PDB). The δ¹⁵N values of Alviniconcha sp. (foot and chitin), I. nautilei (foot and chitin) and E. o. manusensis (soft tissue) range from 8.4 to 10.6‰. The δ³⁴S values of Alviniconcha sp. (foot and chitin), I. nautilei (foot) and E. o. manusensis (soft tissue) range from 5.9 to 11.1‰. Using stable isotopes, for the first time, we inferred a Calvin-Benson (RuBisCo) metabolic pathway for Alviniconcha sp. along with the presence of γ-Proteobacteria symbionts for the Vienna Woods communities. For I. nautilei, a feeding pattern is proposed with γ-Proteobacteria symbiosis and a Calvin-Benson-Bassham diet with mixotrophic feeding. E. ohtai manusensis is filtering bacteria with a CBB feeding strategy, with δ¹⁵N values indicating possible higher position in the trophic chain. Arsenic concentrations in the dry tissue of Alviniconcha (foot), I. nautilei (foot) and E. o. manusensis (soft tissue) are high, from 4134 to 8478 μg/g, with inorganic As concentrations of 607, 492 and 104 μg/g, respectively and dimethyl arsenic (DMA) concentrations of 11.12, 0.25 and 11.2 μg/g, respectively. Snails occurring in a vent proximal position have higher As concentration than barnacles, a pattern not observed for S concentrations. Arsenosugars were not put in evidence indicating that the available organic material for the vent organisms are not surface derived.

Allopatric and Sympatric Drivers of Speciation in Alviniconcha Hydrothermal Vent Snails

Despite significant advances in our understanding of speciation in the marine environment, the mechanisms underlying evolutionary diversification in deep-sea habitats remain poorly investigated. Here, we used multigene molecular clocks and population genetic inferences to examine processes that led to the emergence of the six extant lineages of Alviniconcha snails, a key taxon inhabiting deep-sea hydrothermal vents in the Indo-Pacific Ocean. We show that both allopatric divergence through historical vicariance and ecological isolation due to niche segregation contributed to speciation in this genus. The split between the two major Alviniconcha clades (separating A. boucheti and A. marisindica from A. kojimai, A. hessleri, and A. strummeri) probably resulted from tectonic processes leading to geographic separation, whereas the splits between co-occurring species might have been influenced by ecological factors, such as the availability of specific chemosynthetic symbionts. Phylogenetic origin of the sixth species, Alviniconcha adamantis, remains uncertain, although its sister position to other extant Alviniconcha lineages indicates a possible ancestral relationship. This study lays a foundation for future genomic studies aimed at deciphering the roles of local adaptation, reproductive biology, and host–symbiont compatibility in speciation of these vent-restricted snails.

The Microbial Community Associated with Rhizostoma pulmo: Ecological Significance and Potential Consequences for Marine Organisms and Human Health

Jellyfish blooms are frequent and widespread in coastal areas worldwide, often associated with significant ecological and socio-economic consequences. Recent studies have also suggested cnidarian jellyfish may act as vectors of bacterial pathogens. The scyphomedusa Rhizostoma pulmo is an outbreak-forming jellyfish widely occurring across the Mediterranean basin. Using combination of culture-based approaches and a high-throughput amplicon sequencing (HTS), and based on available knowledge on a warm-affinity jellyfish-associated microbiome, we compared the microbial community associated with R. pulmo adult jellyfish in the Gulf of Taranto (Ionian Sea) between summer (July 2016) and winter (February 2017) sampling periods. The jellyfish-associated microbiota was investigated in three distinct compartments, namely umbrella, oral arms, and the mucus secretion. Actinobacteria, Bacteroidetes, Chlamydiae, Cyanobacteria, Deinococcus-Thermus, Firmicutes, Fusobacteria, Planctomycetes, Proteobacteria, Rhodothermaeota, Spirochaetes, Tenericutes, and Thaumarchaeota were the phyla isolated from all the three R. pulmo compartments in the sampling times. In particular, the main genera Mycoplasma and Spiroplasma, belonging to the class Mollicutes (phylum Tenericutes), have been identified in all the three jellyfish compartments. The taxonomic microbial data were coupled with metabolic profiles resulting from the utilization of 31 different carbon sources by the BIOLOG Eco-Plate system. Microorganisms associated with mucus are characterized by great diversity. The counts of culturable heterotrophic bacteria and potential metabolic activities are also remarkable. Results are discussed in terms of R. pulmo ecology, the potential health hazard for marine and human life as well as the potential biotechnological applications related to the associated microbiome.

Dual energy metabolism of the Campylobacterota endosymbiont in the chemosynthetic snail Alviniconcha marisindica

Some deep-sea chemosynthetic invertebrates and their symbiotic bacteria can use molecular hydrogen (H₂) as their energy source. However, how much the chemosynthetic holobiont (endosymbiont-host association) physiologically depends on H₂ oxidation has not yet been determined. Here, we demonstrate that the Campylobacterota endosymbionts of the gastropod Alviniconcha marisindica in the Kairei and Edmond fields (kAlv and eAlv populations, respectively) of the Indian Ocean, utilize H₂ in response to their physical and environmental H₂ conditions, although the 16S rRNA gene sequence of both the endosymbionts shared 99.6% identity. A thermodynamic calculation using in situ H₂ and hydrogen sulfide (H₂S) concentrations indicated that chemosynthetic symbiosis could be supported by metabolic energy via H₂ oxidation, particularly for the kAlv holobiont. Metabolic activity measurements showed that both the living individuals and the gill tissues consumed H₂ and H₂S at similar levels. Moreover, a combination of fluorescence in situ hybridization, quantitative transcript analyses, and enzymatic activity measurements showed that the kAlv endosymbiont expressed the genes and enzymes for both H₂- and sulfur-oxidations. These results suggest that both H₂ and H₂S could serve as the primary energy sources for the kAlv holobiont. The eAlv holobiont had the ability to utilize H₂, but the gene expression and enzyme activity for hydrogenases were much lower than for sulfur-oxidation enzymes. These results suggest that the energy acquisitions of A. marisindica holobionts are dependent on H₂- and sulfur-oxidation in the H₂-enriched Kairei field and that the mechanism of dual metabolism is controlled by the in situ H₂ concentration.

Jellyfish summer outbreaks as bacterial vectors and potential hazards for marine animals and humans health? The case of Rhizostoma pulmo (Scyphozoa, Cnidaria)

Jellyfish represent an important component of marine food webs characterized by large fluctuations of population density, with the ability to abruptly form outbreaks, followed by rarity periods. In spite of considerable efforts to investigate how jellyfish populations are responding globally to anthropogenic change, available evidence still remains unclear. In the last 50 years, jellyfish are seemingly on the rise in a number of coastal areas, including the Mediterranean Sea, where jellyfish blooms periodically become an issue to marine and maritime human activities. Their impacts on marine organism welfare have been poorly quantified. The jellyfish, Rhizostoma pulmo, is an outbreak-forming scyphomedusa whose large populations spread across the Mediterranean, with increasing periodicity and variable abundance. Studies on cnidarian jellyfish suggested being important vectors of bacterial pathogens. In the present study, by combination of conventional culture-based methods and a high-throughput amplicon sequencing (HTS) approach, we characterized the diversity of the bacterial community associated with this jellyfish during their summer outbreak. Three distinct jellyfish compartments, namely umbrella, oral arms, and the mucus secretion obtained from whole specimens were screened for specifically associated microbiota. A total of 17 phyla, 30 classes, 73 orders, 146 families and 329 genera of microbial organisms were represented in R. pulmo samples with three major clades (i.e. Spiroplasma, Mycoplasma and Wolinella) representing over 90% of the retrieved total sequences. The taxonomic microbial inventory was then combined with metabolic profiling data obtained from the Biolog Eco-Plate system. Significant differences among the jellyfish compartments were detected in terms of bacterial abundance, diversity and metabolic utilization of 31 different carbon sources with the highest value of abundance and metabolic potential in the mucus secretion compared to the umbrella and oral arms. Results are discussed in the framework of the species ecology as well as the potential health hazard for marine organisms and humans.

The Bacterial Symbionts of Closely Related Hydrothermal Vent Snails With Distinct Geochemical Habitats Show Broad Similarity in Chemoautotrophic Gene Content

Symbiosis has evolved between a diversity of invertebrate taxa and chemosynthetic bacterial lineages. At the broadest level, these symbioses share primary function: the bacterial symbionts use the energy harnessed from the oxidation of reduced chemicals to power the fixation of inorganic carbon and/or other nutrients, providing the bulk of host nutrition. However, it is unclear to what extent the ecological niche of the host species is influenced by differences in symbiont traits, particularly those involved in chemoautotrophic function and interaction with the geochemical environment. Hydrothermal vents in the Lau Basin (Tonga) are home to four morphologically and physiologically similar snail species from the sister genera Alviniconcha and Ifremeria. Here, we assembled nearly complete genomes from their symbionts to determine whether differences in chemoautotrophic capacity exist among these symbionts that could explain the observed distribution of these snail species into distinct geochemical habitats. Phylogenomic analyses confirmed that the symbionts have evolved from four distinct lineages in the classes γ-proteobacteria or Campylobacteria. The genomes differed with respect to genes related to motility, adhesion, secretion, and amino acid uptake or excretion, though were quite similar in chemoautotrophic function, with all four containing genes for carbon fixation, sulfur and hydrogen oxidation, and oxygen and nitrate respiration. This indicates that differences in the presence or absence of symbiont chemoautotrophic functions does not likely explain the observed geochemical habitat partitioning. Rather, differences in gene expression and regulation, biochemical differences among these chemoautotrophic pathways, and/or differences in host physiology could all influence the observed patterns of habitat partitioning.

The uptake and excretion of partially oxidized sulfur expands the repertoire of energy resources metabolized by hydrothermal vent symbioses

Symbiotic associations between animals and chemoautotrophic bacteria crowd around hydrothermal vents. In these associations, symbiotic bacteria use chemical reductants from venting fluid for the energy to support autotrophy, providing primary nutrition for the host. At vents along the Eastern Lau Spreading Center, the partially oxidized sulfur compounds (POSCs) thiosulfate and polysulfide have been detected in and around animal communities but away from venting fluid. The use of POSCs for autotrophy, as an alternative to the chemical substrates in venting fluid, could mitigate competition in these communities. To determine whether ESLC symbioses could use thiosulfate to support carbon fixation or produce POSCs during sulfide oxidation, we used high-pressure, flow-through incubations to assess the productivity of three symbiotic mollusc genera—the snails Alviniconcha spp. and Ifremeria nautilei, and the mussel Bathymodiolus brevior—when oxidizing sulfide and thiosulfate. Via the incorporation of isotopically labelled inorganic carbon, we found that the symbionts of all three genera supported autotrophy while oxidizing both sulfide and thiosulfate, though at different rates. Additionally, by concurrently measuring their effect on sulfur compounds in the aquaria with voltammetric microelectrodes, we showed that these symbioses excreted POSCs under highly sulfidic conditions, illustrating that these symbioses could represent a source for POSCs in their habitat. Furthermore, we revealed spatial disparity in the rates of carbon fixation among the animals in our incubations, which might have implications for the variability of productivity in situ. Together, these results re-shape our thinking about sulfur cycling and productivity by vent symbioses, demonstrating that thiosulfate may be an ecologically important energy source for vent symbioses and that they also likely impact the local geochemical regime through the excretion of POSCs.

Intracellular Oceanospirillales inhabit the gills of the hydrothermal vent snail Alviniconcha with chemosynthetic, γ-Proteobacterial symbionts

Associations between bacteria from the γ-Proteobacterial order Oceanospirillales and marine invertebrates are quite common. Members of the Oceanospirillales exhibit a diversity of interactions with their various hosts, ranging from the catabolism of complex compounds that benefit host growth to attacking and bursting host nuclei. Here, we describe the association between a novel Oceanospirillales phylotype and the hydrothermal vent snail Alviniconcha. Alviniconcha typically harbour chemoautotrophic γ- or ε-Proteobacterial symbionts inside their gill cells. Via fluorescence in situ hybridization and transmission electron microscopy, we observed an Oceanospirillales phylotype (named AOP for ‘Alviniconcha Oceanospirillales phylotype’) in membrane-bound vacuoles that were separate from the known γ- or ε-Proteobacterial symbionts. Using quantitative polymerase chain reaction, we surveyed 181 Alviniconcha hosting γ-Proteobacterial symbionts and 102 hosting ε-Proteobacterial symbionts, and found that the population size of AOP was always minor relative to the canonical symbionts (median 0.53% of the total quantified 16S rRNA genes). Additionally, we detected AOP more frequently in Alviniconcha hosting γ-Proteobacterial symbionts than in those hosting ε-Proteobacterial symbionts (96% and 5% of individuals respectively). The high incidence of AOP in γ-Proteobacteria hosting Alviniconcha implies that it could play a significant ecological role either as a host parasite or as an additional symbiont with unknown physiological capacities.

Sulfurovum aggregans sp. nov., a hydrogen-oxidizing, thiosulfate-reducing chemolithoautotroph within the Epsilonproteobacteria isolated from a deep-sea hydrothermal vent chimney, and an emended description of the genus Sulfurovum

A novel mesophilic, strictly hydrogen-oxidizing, sulfur-, nitrate- and thiosulfate-reducing bacterium, designated strain Monchim33T, was isolated from a deep-sea hydrothermal vent chimney at the Central Indian Ridge. The non-motile, rod-shaped cells were Gram-stain-negative and non-sporulating. Growth was observed between 15 and 37 °C (optimum 33 °C; 3.2 h doubling time) and between pH 5.4 and 8.6 (optimum pH 6.0). The isolate was a strictly anaerobic chemolithoautotroph capable of using molecular hydrogen as the sole energy source and carbon dioxide as the sole carbon source. The G+C content of the genomic DNA was 42.6 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the novel isolate belonged to the genus Sulfurovum and was closely related to Sulfurovum sp. NBC37-1 and Sulfurovum lithotrophicum 42BKT (95.6 and 95.4 % similarity, respectively). DNA–DNA hybridization demonstrated that the novel isolate could be differentiated genotypically from Sulfurovum sp. NBC37-1 and Sulfurovum lithotrophicum . On the basis of the molecular and physiological traits of the new isolate, the name Sulfurovum aggregans sp. nov. is proposed, with the type strain Monchim33T ( = JCM 19824T = DSM 27205T).

Optimization of a peptide nucleic acid fluorescence in situ hybridization (PNA-FISH) method for the detection of bacteria and disclosure of a formamide effect

Despite the fact that fluorescence in situ hybridization (FISH) is a well-established technique to identify microorganisms, there is a lack of understanding concerning the interaction of the different factors affecting the obtained fluorescence. In here, we used flow cytometry to study the influence of three essential factors in hybridization - temperature, time and formamide concentration - in an effort to optimize the performance of a Peptide Nucleic Acid (PNA) probe targeting bacteria (EUB338). The PNA-FISH optimization was performed with bacteria representing different families employing response surface methodology. Surprisingly, the optimum concentration of formamide varied according to the bacterium tested. While hybridization on the bacteria possessing the thickest peptidoglycan was more successful at nearly 50% (v/v) formamide, hybridization on all other microorganisms appeared to improve with much lower formamide concentrations. Gram staining and transmission electron microscopy allowed us to confirm that the overall effect of formamide concentration on the fluorescence intensity is a balance between a harmful effect on the bacterial cell envelope, affecting cellular integrity, and the beneficial denaturant effect in the hybridization process. We also conclude that microorganisms belonging to different families will require different hybridization parameters for the same FISH probe, meaning that an optimum universal PNA-FISH procedure is non-existent for these situations.

Spatial differences in East scotia ridge hydrothermal vent food webs: influences of chemistry, microbiology and predation on trophodynamics

The hydrothermal vents on the East Scotia Ridge are the first to be explored in the Antarctic and are dominated by large peltospiroid gastropods, stalked barnacles (Vulcanolepas sp.) and anomuran crabs (Kiwa sp.) but their food webs are unknown. Vent fluid and macroconsumer samples were collected at three vent sites (E2, E9N and E9S) at distances of tens of metres to hundreds of kilometres apart with contrasting vent fluid chemistries to describe trophic interactions and identify potential carbon fixation pathways using stable isotopes. δ(13)C of dissolved inorganic carbon from vent fluids ranged from -4.6‰ to 0.8‰ at E2 and from -4.4‰ to 1.5‰ at E9. The lowest macroconsumer δ(13)C was observed in peltospiroid gastropods (-30.0‰ to -31.1‰) and indicated carbon fixation via the Calvin-Benson-Bassham (CBB) cycle by endosymbiotic gamma-Proteobacteria. Highest δ(13)C occurred in Kiwa sp. (-19.0‰ to -10.5‰), similar to that of the epibionts sampled from their ventral setae. Kiwa sp. δ(13)C differed among sites, which were attributed to spatial differences in the epibiont community and the relative contribution of carbon fixed via the reductive tricarboxylic acid (rTCA) and CBB cycles assimilated by Kiwa sp. Site differences in carbon fixation pathways were traced into higher trophic levels e.g. a stichasterid asteroid that predates on Kiwa sp. Sponges and anemones at the periphery of E2 assimilated a proportion of epipelagic photosynthetic primary production but this was not observed at E9N. Differences in the δ(13)C and δ(34)S values of vent macroconsumers between E2 and E9 sites suggest the relative contributions of photosynthetic and chemoautotrophic carbon fixation (rTCA v CBB) entering the hydrothermal vent food webs vary between the sites.

Evidence for the role of endosymbionts in regional-scale habitat partitioning by hydrothermal vent symbioses

Deep-sea hydrothermal vents are populated by dense communities of animals that form symbiotic associations with chemolithoautotrophic bacteria. To date, our understanding of which factors govern the distribution of host/symbiont associations (or holobionts) in nature is limited, although host physiology often is invoked. In general, the role that symbionts play in habitat utilization by vent holobionts has not been thoroughly addressed. Here we present evidence for symbiont-influenced, regional-scale niche partitioning among symbiotic gastropods (genus Alviniconcha) in the Lau Basin. We extensively surveyed Alviniconcha holobionts from four vent fields using quantitative molecular approaches, coupled to characterization of high-temperature and diffuse vent-fluid composition using gastight samplers and in situ electrochemical analyses, respectively. Phylogenetic analyses exposed cryptic host and symbiont diversity, revealing three distinct host types and three different symbiont phylotypes (one ε-proteobacteria and two γ-proteobacteria) that formed specific associations with one another. Strikingly, we observed that holobionts with ε-proteobacterial symbionts were dominant at the northern fields, whereas holobionts with γ-proteobacterial symbionts were dominant in the southern fields. This pattern of distribution corresponds to differences in the vent geochemistry that result from deep subsurface geological and geothermal processes. We posit that the symbionts, likely through differences in chemolithoautotrophic metabolism, influence niche utilization among these holobionts. The data presented here represent evidence linking symbiont type to habitat partitioning among the chemosynthetic symbioses at hydrothermal vents and illustrate the coupling between subsurface geothermal processes and niche availability.

Phylogenetic characterization of episymbiotic bacteria hosted by a hydrothermal vent limpet (lepetodrilidae, vetigastropoda)

Marine invertebrates hosting chemosynthetic bacterial symbionts are known from multiple phyla and represent remarkable diversity in form and function. The deep-sea hydrothermal vent limpet Lepetodrilus fucensis from the Juan de Fuca Ridge complex hosts a gill symbiosis of particular interest because it displays a morphology unique among molluscs: filamentous bacteria are found partially embedded in the host's gill epithelium and extend into the fluids circulating across the lamellae. Our objective was to investigate the phylogenetic affiliation of the limpet's primary gill symbionts for comparison with previously characterized bacteria. Comparative 16S rRNA sequence analysis identified one γ- and three ε-Proteobacteria as candidate symbionts. We used fluorescence in situ hybridization (FISH) to test which of these four candidates occur with the limpet's symbiotic gill bacteria. The γ-proteobacterial probes consistently hybridized to the entire area where symbiotic bacteria were found, but fluorescence signal from the ε-proteobacterial probes was generally absent. Amplification of the γ-proteobacterial 16S rRNA gene using a specific forward primer yielded a sequence similar to that of limpets collected from different ridge sections. In total, direct amplification or FISH identified a single γ-proteobacterial lineage from the gills of 23 specimens from vents separated by a distance up to about 200 km and collected over the course of 2 years, suggesting a highly specific and widespread symbiosis. Thus, we report the first filamentous γ-proteobacterial gill symbiont hosted by a mollusc.

Isolation and physiological characterization of two novel, piezophilic, thermophilic chemolithoautotrophs from a deep-sea hydrothermal vent chimney

Two novel, thermophilic piezophiles, capable of chemolithoautotrophic growth, are successfully cultivated and isolated from a black smoker chimney at the TAG field (Mid Atlantic Ridge: MAR) by using a piezophilic cultivation technique. Both strains (strains 106 and 108) represent dominant cultivated populations of the microbial communities in the chimney surface habitat. Strain 106 represents typically thin, long spiral cells under the piezophilic growth condition but short bent cells under the non-piezophilic condition. It is a strictly chemolithoautotrophic gammaproteobacterium using reduced sulfur compounds as the electron donors, and nitrate and O(2) as the electron acceptors. Based on the 16S rRNA gene sequence, strain 106 would represent a novel genus of the previously uncultivated group (Symbiont Group I; a potentially novel family) within the Gammaproteobacteria, and 'Thioprofundum lithotrophica' gen. nov., sp. nov. is proposed. Strain 108 is a short, oval rod at any of the growth pressures. It is a facultative chemoautotroph, capable of both chemolithoautotrophic growth with H(2) and S oxidations and organotrophic growth with complex organics or organic acids using nitrate and O(2) as the electron acceptors. The chemolithoautotrophic growth is strictly piezophilic and under the organotrophic growth condition, it grows at conventional pressures (0.1 MPa). Strain 108 is phylogenetically distinctive from any of the previously described genera of the family Rhodobacteraceae within the Alphaproteobacteria, and 'Piezobacter thermophilus' gen. nov., sp. nov. is proposed. The piezophilic cultivation technique can be a powerful tool to isolate and characterize the previously uncultivated phylotypes in the deep-sea hydrothermal vent environments.

Sulfurivirga caldicuralii gen. nov., sp. nov., a novel microaerobic, thermophilic, thiosulfate-oxidizing chemolithoautotroph, isolated from a shallow marine hydrothermal system occurring in a coral reef, Japan

Novel thermophilic bacteria, designated strains VW1 and MM1(T), were isolated from hydrothermal fluid and microbial mat samples, respectively, collected from a shallow marine hydrothermal system (water depth 22 m) occurring in coral reefs off Taketomi Island, Okinawa, Japan. Cells of the two novel strains were motile rods with a single polar flagellum in the exponential growth phase. In a medium that included elemental sulfur, cells of the two strains became non-motile with oval to spherical cell shapes. For both strains, growth occurred at between 30 and 60 degrees C (optimum temperature of 50-55 degrees C; 60-80 min doubling time) and between pH 5.5 and 7.1 (optimum pH 6.0). The isolates were microaerobic chemolithoautotrophs capable of using thiosulfate or tetrathionate as the sole energy source, O(2) as the sole electron acceptor and CO(2) as the sole carbon source. Organic substrates, such as yeast extract and tryptone, inhibited growth of both strains. The G+C contents of genomic DNA were 51.3 and 49.5 mol% for strains VW1 and MM1(T), respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the two strains were closely related to each other (99.9 % sequence similarity) and were distantly related to other previously described genera within the Gammaproteobacteria. The novel isolates could also be differentiated from other gammaproteobacterial genera on the basis of their physiological properties. It is suggested that the novel isolates represent the type species of a new genus, for which the name Sulfurivirga caldicuralii gen. nov., sp. nov. (type strain MM1(T)=JCM 13439(T)=DSM 17737(T)) is proposed.

Host-symbiont relationships in hydrothermal vent gastropods of the genus Alviniconcha from the Southwest Pacific

Hydrothermal vent gastropods of the genus Alviniconcha are unique among metazoans in their ability to derive their nutrition from chemoautotrophic gamma- and epsilon-proteobacterial endosymbionts. Although host-symbiont relationships in Alviniconcha gastropods from the Central Indian Ridge in the Indian Ocean and the Mariana Trough in the Western Pacific have been studied extensively, host-symbiont relationships in Alviniconcha gastropods from the Southwest Pacific remain largely unknown. Phylogenetic analysis using mitochondrial cytochrome c oxidase subunit I gene sequences of host gastropods from the Manus, North Fiji, and Lau Back-Arc Basins in the Southwest Pacific has revealed a new host lineage in a Alviniconcha gastropod from the Lau Basin and the occurrence of the host lineage Alviniconcha sp. type 2 in the Manus Basin. Based on 16S rRNA gene sequences of bacterial endosymbionts, two gamma-proteobacterial lineages and one epsilon-proteobacterial lineage were identified in the present study. The carbon isotopic compositions of the biomass and fatty acids of the gastropod tissues suggest that the gamma- and epsilon-proteobacterial endosymbionts mediate the Calvin-Benson cycle and the reductive tricarboxylic acid cycle, respectively, for their chemoautotrophic growth. Coupling of the host and symbiont lineages from the three Southwest Pacific basins revealed that each of the Alviniconcha lineages harbors different bacterial endosymbionts belonging to either the gamma- or epsilon-Proteobacteria. The host specificity exhibited in symbiont selection provides support for the recognition of each of the host lineages as a distinct species. The results from the present study also suggest the possibility that Alviniconcha sp. types 1 and 2 separately inhabit hydrothermal vent sites approximately 120 m apart in the North Fiji Basin and 500 m apart in the Manus Basin.