Mariana Back-Arc Basin Research Articles

Geography, not lifestyle, explains the population structure of free-living and host-associated deep-sea hydrothermal vent snail symbionts

BackgroundMarine symbioses are predominantly established through horizontal acquisition of microbial symbionts from the environment. However, genetic and functional comparisons of free-living populations of symbionts to their host-associated counterparts are sparse. Here, we assembled the first genomes of the chemoautotrophic gammaproteobacterial symbionts affiliated with the deep-sea snail Alviniconcha hessleri from two separate hydrothermal vent fields of the Mariana Back-Arc Basin. We used phylogenomic and population genomic methods to assess sequence and gene content variation between free-living and host-associated symbionts.ResultsOur phylogenomic analyses show that the free-living and host-associated symbionts of A. hessleri from both vent fields are populations of monophyletic strains from a single species. Furthermore, genetic structure and gene content analyses indicate that these symbiont populations are differentiated by vent field rather than by lifestyle.ConclusionTogether, this work suggests that, despite the potential influence of host-mediated acquisition and release processes on horizontally transmitted symbionts, geographic isolation and/or adaptation to local habitat conditions are important determinants of symbiont population structure and intra-host composition.11R7XF31NC9oJy9jTm658cVideo

Late Neoarchean plate subduction in Western North China Craton: Evidence from ca. 2.51 Ga to 2.46 Ga basement rocks in Northern Ordos Basin

The most poorly exposed and least understood in the North China Craton (NCC) lies largely hidden beneath the Ordos Basin, severely limiting the cognition of the Precambrian tectonic evolution in the western NCC and the tectonic division. Diverse orthogneiss samples collected from four boreholes drilled into the basement of the north side of Hangjinqi Fault Zone (HFZ) in the North Ordos Basin (NOB) are investigated by integrating zircon U-Pb dating, Hf-Nd isotopes, and whole-rock geochemical analyses. The TTG gneiss, quartz-monzonite gneiss, granodiorite gneiss, and amphibolite were emplaced at ca. 2.51–2.46 Ga, of which the amphibolite also recorded the metamorphic age of ca. 1.95 Ga. The TTG gneisses display low MgO, Mg#, Cr, and Ni, high HREE and Rb/Sr, enrichment of LILEs and depletion of Nb, Ta, and Ti, and positive εNd(t) (2.04–4.06) and εHf(t) (3.04–7.59) values, suggesting that they were mainly originated from partial melting of the tectonically thickened mafic island-arc crust. In contrast, the quartz monzonite gneisses and granodiorite gneisses show the geochemical features of Archean sanukitoids with higher MgO, Mg#, Cr, and Ni, and similar εNd(t) (1.34–3.82) and εHf(t) values (3.94–7.32), suggesting that they were derived from the partial melting of the mantle metasomatized by fluids and melts of the subduction. The amphibolites exhibit a mixed characteristic of E-MORB and arc-like geochemical affinities resembling the Mariana back-arc basin, which was derived from the partial melting of spinel lherzolite. The arc and arc-back lithological assemblages suggest that the north margin of the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca. 2.51–2.46 Ga. Combined with previous analytical results, the northern margin of the NOB was most likely in a long-term active continental margin setting during 2.51–1.95 Ga and involved in the collision event between the Yinshan Block and Ordos Block at ca. 1.95 Ga.

Age and tectonic setting of the Quinebaug-Marlboro belt and implications for the history of Ganderian crustal fragments in southeastern New England, USA

Abstract Crustal fragments underlain by high-grade rocks represent a challenge to plate reconstructions, and integrated mapping, geochronology, and geochemistry enable the unravelling of the temporal and spatial history of exotic crustal blocks. The Quinebaug-Marlboro belt (QMB) is an enigmatic fragment on the trailing edge of the peri-Gondwanan Ganderian margin of southeastern New England. SHRIMP U-Pb geochronology and geochemistry indicate the presence of Ediacaran to Cambrian metamorphosed volcanic and intrusive rocks dated for the first time between ca. 540–500 Ma. The entire belt may preserve a cryptic, internal stratigraphy that is truncated by subsequent faulting. Detrital zircons from metapelite in the overlying Nashoba and Tatnic Hill Formations indicate deposition between ca. 485–435 Ma, with provenance from the underlying QMB or Ganderian crust. The Preston Gabbro (418 ± 3 Ma) provides a minimum age for the QMB. Mafic rocks are tholeiitic with trace elements that resemble arc and E-MORB sources, and samples with negative Nb-Ta anomalies are similar to arc-like rocks, but others show no negative Nb-Ta anomaly and are similar to rocks from E-MORB to OIB or backarc settings. Geochemistry points to a mixture of sources that include both mantle and continental crust. Metamorphic zircon, monazite, and titanite ages range from 400 to 305 Ma and intrusion of granitoids and migmatization occurred between 410 and 325 Ma. Age and chemistry support correlations with the Ellsworth terrane in Maine and the Penobscot arc and backarc system in Maritime Canada. The arc-rifting zone where the Mariana arc and the Mariana backarc basin converge is a possible modern analog.

A new deep-sea hot vent stalked barnacle from the Mariana Trough with notes on the feeding ecology of Vulcanolepas

A new species of deep-sea neolepadid stalked barnacle, Vulcanolepas verenaesp. nov., is described from Hafa Adai hydrothermal vent field in the Mariana back-arc basin, northwest Pacific. Vulcanolepas verenaesp. nov. is differentiated from other described species in the family morphologically by a mandible with a very small first tooth and small, non-protruding peduncular scales. Its gross external morphology exhibits a range of plasticity, similar to those previously reported from other neolepadid stalked barnacles. This is the first report of the genus Vulcanolepas in the northern hemisphere, and we discuss the phylogeography of Neolepadidae with a phylogenetic reconstruction based on the cytochrome c oxidase subunit I gene. We also discuss the relationship between key morphological characteristics and the feeding habits in the family, showing that, a number of Vulcanolepas species including Vulcanolepas verenaesp. nov. are specialized in culturing filamentous bacteria on their cirri.

Late Neoarchean crustal growth under paired continental arc-back arc system in the North China Craton

The late Archean (~3.0–2.5 Ga) was a key period of continental growth globally, which is widely considered to reflect the onset of vigorous plate tectonic activity, although related continental growth modes remain contentious. Here we investigate a suite of late Neoarchean metavolcanic rocks from the southwest Qixia area of the Jiaobei terrane in the North China Craton. The rocks in this suite include amphibolites, clinopyroxene amphibolites, and hornblende plagioclase gneisses. We present zircon U-Pb isotopic data which indicate that the protoliths of these rocks formed during ~2549–2511 Ma.The (clinopyroxene) amphibolites correspond to meta-basaltic rocks, with some containing high modal content of titanite. These rocks show moderate to high FeOT (8.96–13.62 wt.%) and TiO2 (0.59–1.59 wt.%), flat to less fractionated REE patterns, and mildly negative Th, Nb, and Ta anomalies, resembling those of Fe-tholeiites. In addition, they display positive zircon ƐHf(t) values (+2.6 to +8.7), and are devoid of crustal contamination or fractional crystallization. Combined with the low Nb/Yb (mostly < 1.60) and (Hf/Sm)N (mostly < 0.95), low to moderate Th/Yb (0.08–0.54), and low V/Sc (5.53–9.19) ratios, these basaltic rocks are interpreted to have been derived from a relatively reduced and depleted mantle source that was mildly metasomatized by hydrous fluids. The hornblende plagioclase gneisses are meta-andesitic rocks, and occur interlayered with the basaltic rocks. They are transitional between tholeiitic and calc-alkaline rock series, and show fractionated REE patterns with evidently negative Th, Nb, and Ta anomalies. The depleted zircon ƐHf(t) values (+2.4 to +8.4) and quantitative chemical modeling suggest that the andesitic rocks were most likely generated by injection and mixing of juvenile felsic magmas with the tholeiitic basaltic magmas.In general, the chemical features and genesis of late Neoarchean meta-basaltic rocks in our study area resemble those of Mariana back-arc basin basalts. Combined with regional geological data, it is proposed that the Jiaobei terrane witnessed late Neoarchean crustal growth under a paired continental arc-back arc setting. On a regional context, we propose two distinct geodynamic mode of late Neoarchean continental growth across North China Craton (particularly the Eastern Block), i.e., (1) arc-continent accretion along northwestern part of the Eastern Block; and (2) paired continental arc-back arc system surrounding the ~3.8–2.7 Ga continental nuclei to the southeast.

First Discovery of a Nonophiolite-Type Spinel Lherzolite Xenolith in the Back-Arc Basin of the Mariana Island Arc System

The first sampling of mantle horizons underlying the ophiolite peridotite complex of the Mariana back-arc basin was carried out based on a spinel lherzolite xenolith in basanites from the submarine Esmeralda Volcano. The paper shows significant differences between the bulk, petrological, geochemical, and mineralogical compositions of this xenolith and similar characteristics of ophiolite peridotites dredged from the Central Trough of the north part of the Mariana Trench, on the west wall of one of the fault zones of the Santa Rosa Bank Fault (southern part of the trough), and ultramafic rocks dredged from the Conical and Torishima seamounts in the fore-arc part of the Mariana and Idzu-Bonin Island Arc Systems. Using different geothermometers and geobarometers, we revealed the PT conditions for the formation of Esmeralda xenolith’s mineral paragenesis. The optimal temperature is 950–1050°C and the pressure is 13–15 kbar. These parameters significantly exceed those suggested for the compared ophiolite peridotites.

Active subseafloor microbial communities from Mariana back-arc venting fluids share metabolic strategies across different thermal niches and taxa

There are many unknowns regarding the distribution, activity, community composition, and metabolic repertoire of microbial communities in the subseafloor of deep-sea hydrothermal vents. Here we provide the first characterization of subseafloor microbial communities from venting fluids along the central Mariana back-arc basin (15.5–18°N), where the slow-spreading rate, depth, and variable geochemistry along the back-arc distinguish it from other spreading centers. Results indicated that diverse Epsilonbacteraeota were abundant across all sites, with a population of high temperature Aquificae restricted to the northern segment. This suggests that differences in subseafloor populations along the back-arc are associated with local geologic setting and resultant geochemistry. Metatranscriptomics coupled to stable isotope probing revealed bacterial carbon fixation linked to hydrogen oxidation, denitrification, and sulfide or thiosulfate oxidation at all sites, regardless of community composition. NanoSIMS (nanoscale secondary ion mass spectrometry) incubations at 80 °C show only a small portion of the microbial community took up bicarbonate, but those autotrophs had the highest overall rates of activity detected across all experiments. By comparison, acetate was more universally utilized to sustain growth, but within a smaller range of activity. Together, results indicate that microbial communities in venting fluids from the Mariana back-arc contain active subseafloor communities reflective of their local conditions with metabolisms commonly shared across geologically disparate spreading centers throughout the ocean.

The complete mitochondrial genome of Paralvinella hessleri: an endemic species of deep-sea hydrothermal vent

Paralvinella hessleri is a terrebellomorphy polychaetae worm lives on the hydrothermal vents in the Okinawa trough and Mariana Back-Arc basin. We report the complete mitochondrial genome of P. hessleri, which is 16,978 bp in length, containing 13 protein-coding genes, two ribosomal proteins, and 22 tRNAs. The gene order is identical to that of reported terebellids. The overall base composition is AT-biased. Our results of P. hessleri mt genome may provide useful genetic information for phylogenetic and population connectivity studies of the deep-sea alvinellid worms.

マリアナ背弧海盆の熱水噴出孔にすむ腹足類の 1 新種

マリアナ背弧海盆の熱水噴出孔にすむ腹足類の 1 新種

Molecular taxonomy and naming of five cryptic species of Alviniconcha snails (Gastropoda: Abyssochrysoidea) from hydrothermal vents

Large symbiont-hosting snails of the genus Alviniconcha (Gastropoda: Abyssochrysidae) are among the dominant inhabitants of hydrothermal vents in the Western Pacific and Indian oceans. The genus was originally described as monotypic, but unique DNA sequences for mitochondrial genes revealed six distinct evolutionary lineages that we could not distinguish based on external morphology. Subsumed under the name Alviniconcha hessleri Okutani & Ohta, the distinct allopatric and sympatric lineages have been assigned placeholder epithets that complicate scientific communications. Based on the present multi-gene sequence data, we hereby describe five Alviniconcha species (in the order of their discovery) – A. kojimai sp. nov., A. boucheti sp. nov., A. marisindica sp. nov., A. strummeri sp. nov. and A. adamantis sp. nov. Thus, we restrict application of the name A. hessleri to specimens that are genetically similar (≥95% for COI) to those found at localities in the Mariana Trough. Single distinct Alviniconcha species inhabit vent fields along the Central Indian Ridge, the Mariana volcanic arc, and the Mariana back-arc basin, whereas vents in the Manus, Fiji and Lau back-arc basins may host two or three additional species. Formal recognition of these species facilitates future attempts to assess their physiological differences and symbiont associations. Furthermore, their reported distributions have significant biogeographic implications, affecting estimates of the diversity within and overlap among Indo-Pacific vent localities.http://zoobank.org/urn:lsid:zoobank.org:pub:1E4B2E71-9F1D-479E-9A9A-22A9E303AAE5

Geochronology and geochemistry of volcanic rocks from the Shaojiwa Formation and Xingzi Group, Lushan area, SE China: Implications for Neoproterozoic back-arc basin in the Yangtze Block

We present a systematic SHRIMP zircon U–Pb geochronological, geochemical and Nd isotopic study on the Shaojiwa basaltic–rhyolitic suite and Xingzi amphibolites in Lushan area, Yangtze Block. The protoliths of the amphibolites are low potassium tholeiitic basalts. Rhyolite and amphibolite yield SHRIMP zircon U–Pb ages of 828±6Ma and 811±12Ma respectively. These ages suggest that the Xingzi Group formed in the Neoproterozoic rather than Paleoproterozoic as previously suggested. The basalts from the Shaojiwa Formation have slightly positive ɛNd(t) values (0 to +0.7) and exhibit significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. These geochemical and isotopic signatures suggest that the basalts might have been derived from a slightly depleted mantle source metasomatized by fluid/melt released from a newly subducted slab with high degree of mantle melting. The rhyolites exhibit similar Nd isotopic ratios as the associated basalts and show subparallel REE patterns which can be explained by fractional crystallization of the basalt magma. In contrast, the amphibolites from the Xingzi Group have relatively high ɛNd(t) values (+3.4 to +4.0) and are characterized by nearly flat REE distributions with slight LREE depletion and generally similar contents of moderately incompatible elements to those of the average N-MORB, but with relative enrichment in LILE, similar to the Mariana back-arc basin basalt (BABB). The precursor magma of the amphibolites might have been originated from an N-MORB source modified by slab-derived fluids with relatively low degree of mantle melting. The geochemical affinity to arc (or intra-continental BABB) of the Shaojiwa basalts and intra-oceanic BABB of the Xingzi amphibolites appears to support the development of a Neoproterozoic incipient to mature back-arc basin in the southeastern Yangtze Block during the 828±6 to 811±12Ma period in response to the rollback of subducted slab. Widespread Neoproterozoic igneous rocks in the Jiangnan Orogen with ages older than 800Ma most likely occurred in an arc-back-arc environment

New species ofMunidopsis(Decapoda: Anomura: Galatheidae) from hydrothermal vent areas of Indian and Pacific Oceans

Two new species ofMunidopsisfrom the hydrothermal vent area, Kairei Field, Central Indian Ridge in the Indian Ocean and Forecast Vent Field, Mariana Back Arc Basin in the west Pacific are described and illustrated. Their affinities to closely related species are discussed. The number ofMunidopsisspecies associated to hydrothermal vents in the world oceans has increased to 16. The habitat of new species is briefly described and the pattern of abundance of vent associatedMunidopsisis briefly discussed.

Copepods of the family Dirivultidae (Siphonostomatoida) from deep-sea hydrothermal vent fields on the Mid-Atlantic Ridge at 14ºN and 5ºS

Five species of three genera of Dirivultidae Humes and Dojiri, 1980 were found at deep-sea hydrothermal vent fields on the Mid-Atlantic Ridge, at the Logachev-1 field at 14ºN and at two new sites (Turtle Pits and Red Lion) explored at 5ºS. The copepods were collected with a Remotely Operated Vehicle (ROV Quest 4000) and Video Controlled Grab, operated from the R/V Meteor during two cruises (M60/3 and M64/1) conducted in 2004 and 2005 at depths 2992–3048 m. The male of Stygiopontius lomonosovi n. sp. from the Logachev field shares a medioventrally prolonged syncoxa of the maxilliped with S. mirus Humes, 1996 and S. latulus Humes, 1996 from the Snake Pit at 23ºN of the Mid-Atlantic Ridge, but differs from them in having two, instead of three, outer spines on the distal exopodal segment of leg 4. Aphotopontius atlanteus Humes, 1996 previously known from the Lucky Strike and the Menez Gwen sites at 37°N and Rimipontius mediospinifer Humes, 1996 known from three sites (Logachev, Snake Pit, and Broken Spur at 29°N) are recorded from the Logachev field. Stygiopontius pectinatus Humes, 1987 previously recorded from the Mid-Atlantic Ridge hydrothermal vent fields TAG (26°N), Snake Pit, Broken Spur (29°N), and from the Mariana Back-Arc Basin of the Pacific Ocean and Stygiopontius cladarus Humes, 1996 previously known only from Snake Pit and Broken Spur, were found associated with alvinocaridid shrimps (Rimicaris sp.) from the hydrothermal vent field at 5ºS. The discovery of dirivultids at 5ºS represents the first record of copepods from a deep-sea hydrothermal vent southward of the Logachev field in the Atlantic Ocean.

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

The hydrothermal-vent gastropod Alviniconcha hessleri from the Alice Springs deep-sea hydrothermal field in the Mariana Back-Arc Basin in the Western Pacific houses an intracellular bacterial endosymbiont in its gill. Although enzymatic analysis has revealed that the endosymbiont is a sulfur-oxidizing chemoautotroph using the Calvin–Benson cycle for the fixation of carbon dioxide, the phylogenetic affiliation of, and the trophic relationship of A. hessleri with, the chemoautotrophic endosymbiont remains undetermined. A single 16S rRNA gene sequence was obtained from the DNA extract of the gill, and phylogenetic analysis placed the source organism within the lineage of the gamma subdivision of the Proteobacteria that consists of many chemoautotrophic endosymbionts of marine invertebrates. Fluorescence in situ hybridization analysis showed the bacterium densely colonizing the gill filaments. The fatty acid profile of the symbiont-free mantle contains the high level of the 16:1 fatty acid originating from the endosymbiont, which indicates that the endosymbiont cells are digested by, and incorporated into, the host. Compound-specific carbon isotopic analysis revealed that fatty acids from the gastropod tissues are all 13C-depleted relative to the gastropod biomass. This fractionation pattern is consistent with chemoautotrophy based on the Calvin–Benson cycle and subsequent fatty-acid biosynthesis from 13C-depleted acetyl coenzyme A. The results from the present study are clearly different from those from our previous study for A. aff. hessleri from the Indian Ocean that harbors a chemoautotrophic endosymbiont belonging to the epsilon subdivision of the Proteobacteria, which mediates the reductive tricarboxylic acid cycle for carbon fixation. Thus, it is concluded here that two lineages of chemoautotrophic bacteria, phylogenetically distinct at the subdivision level, occur as the primary endosymbiont in one host-animal type, which is unknown for the other metazoans.

Faulting and volcanism in the axial valley of the slow-spreading center of the Mariana back arc basin from Wadatsumi side-scan sonar images

Faulting and volcanism in the axial valley of the slow-spreading center of the Mariana back arc basin from Wadatsumi side-scan sonar images

Faulting and volcanism in the axial valley of the slow‐spreading center of the Mariana back arc basin from Wadatsumi side‐scan sonar images

We analyzed in detail the geology of the median valley floor of the Mariana Basin slow‐spreading ridge using sea surface geophysical data and a high‐resolution deep‐tow side‐scan sonar survey over one spreading segment. Analysis of surface magnetic data indicates highly asymmetric accretion, with the half‐spreading rate on the western side of the basin being two to three times larger than on the eastern side. Surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of ∼10 km of amplitude. Deep‐tow backscatter data indicate along‐axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end, respectively. This variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end. Concerning tectonic processes, we find a power law distribution of the fractures, with an exponent of 1.74. This suggests that within the inner valley floor, fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8% of the strain. According to our calculation based on a ratio of 0.02 to 0.03 between the vertical displacement and the length of faults, the amount of tectonic strain accommodated in the inner valley floor would consistently be ∼1.1–3.4%. Data also show two distinct sets of fractures. One trend is parallel to the rift direction at the segment center (∼N160°E) and perpendicular to the plate separation direction. Another set trends ∼17° oblique to this direction (∼N175°E) and is located over the eastern part of the valley, in the vicinity of a major bounding fault also trending ∼N175°E, that is, obliquely to the direction of plate motion. We modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three‐dimensional boundary element approach. We found that the orientation of the predicted fissuring near the oblique fault is locally rotated by ∼15° due to a flexure of the bending plate close to this fault.

A new species of the shrimp genus Chorocaris (Decapoda: Caridea: Alvinocarididae) from hydrothermal vents in the eastern Pacific Ocean

Chorocaris paulexa, new species, the first member of the genus Chorocaris Martin & Hessler, 1990 reported from the eastern Pacific, is described based primarily on two specimens, one of which is ovigerous, collected by the DSV Alvin at the Homer hydrothermal (black smoker) vent site (17°S) on the southern East Pacific Rise (SEPR). Seven additional (non-type) specimens from other SEPR sites (Rapa Nui, Brandon vents) are also described and compared to the types. The new species is compared to its congeners C. vandoverae Martin & Hessler, 1990, from the Mariana Back-Arc Basin in the western Pacific, and C. chacei (Williams & Rona, 1986), from the Mid-Atlantic Ridge), as well as to Mirocaris fortunata (Martin & Christiansen, 1995) (Mid-Atlantic Ridge). The new species is markedly similar in morphology to both Chorocaris vandoverae and C. chacei, but it can be distinguished from either of those species by a combination of the relatively acute branchiostegal projection of the carapace border, extent of the rostrum, and shape of the antennal scale. The finding extends the known range of the genus Chorocaris ~11,000 km eastward (from the Mariana Back-Arc Basin) and stimulates evolutionary and biogeographic hypotheses to explain this Pacific-wide distribution of a shrimp genus specialized to inhabit hydrothermal vent systems.

化学合成がうみだすもうひとつの深海生態系 太平洋西部・インド洋のシンカイヒバリガイ類の分子系統と分岐年代 （予報）

There are unique biological communities around deep-sea hydrothermal vents and cold-water seeps throughout the world. Mussels of the genus Bathymodiolus (Bivalvia, Mytilidae) are one of the dominant macroorganisms in these communities. Bathymodiolus mussels live in a reductive environment, depending on energy supplied by bacterial endosymbionts employing chemosynthesis. Although Bathymodiolus mussels are taxonomically classified by conchological traits, their evolution is poorly understood. In this study, phylogenetic relationships of nine Bathymodiolus species obtained from the Western Pacific and Indian Oceans were investigated by comparing partial nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI). These species were divided into two clusters comprising Bathymodiolus japonicus, B. platifrons, B. sp. (short-type, Kuroshima Knoll), and B. sp. (long-type, Kuroshima Knoll) on the one hand (group 1) and B. septemdierum, B. aduloides, B. marisindicus, B. sp. (Mariana Back-Arc Basin), and B. sp. (Manus Basin) on the other (group 2). The former is a lineage containing methanotrophic endosymbionts and the latter is a lineage containing thioautotrophic endosymbionts (no data for B. sp. from the Mariana Back-Arc Basin). Bathymodiolus septemdierum, B. marisindicus, and B. sp. (Mariana Back-Arc Basin) are closely related with small genetic distances among them (cluster A). Divergence time calculated based on the estimated evolutionary rate of COI (0.014 of sequence divergence/million years) was 12 to 14 million years ago between the two groups and 0.3 to 2.9 million years ago among the three species in cluster A.

Phylogenetic characterization and biomass estimation of bacterial endosymbionts associated with invertebrates dwelling in chemosynthetic communities of hydrothermal vent and cold seep fields

Molecular phylogenetics for endosymbiotic bacteria recovered from vestimentiferan tubeworm Lamellibrachia satsuma from Kagoshima Bay, vesicomyid clam Calyptogena laubieri from the Nankai Trough and mytilid mussel Bathymodiolus sp. from the Mariana Back-arc Basin, were examined by PCR-aided 16S rDNA cloning and sequencing, and quinone profiling. The 16SrRNA clones of the endosymbionts from the 3 organisms fall within γ-Proteobacteria and showed distinct lines of descent specific to their respective host. The 16S rRNA gene phylogeny confirms the host- endosymbiont specificity in the co-evolutionary process. Ubiquinones with 9 isoprene units (Q-9) or Q-10 were found as the major quinones in all test tissues of the host. Larger amounts of Q-8 were detectable only in those host body parts (gill or trophosome) harboring the endosymbiotic bacteria. These observations suggest that Q-8 is the major quinone of the endosymbionts. Based on the bac- terial quinone concentration, the population densities of the bacteria present were estimated to be 10 10 to 10 11 cells g -1 wet wt of host tissue. The quantitative determination of quinones may provide information about physiological activity of the chemosynthetic communities as well as their biomass production.

Hemoglobin in the symbiont-harboring gill of the marine gastropod Alviniconcha hessleri.

Hydrogen sulfide of geochemical origin, mixing at oceanic hydrothermal vents with oxygen from oceanic seawater, supports dense populations of chemoautrophic, sulfur-oxidizing bacteria. Those animals, the vestimentiferan worm Riftia pachyptila, certain bivalve molluscs, and the recently discovered Pacific gastropod Alviniconcha hessleri, that interiorize the bacteria as intracellular symbionts dominate the vent fauna (1, 2). The immense size of these animals, the large standing crop represented in their dense communities, and the rapid growth of individuals all attest to the effective use of an abundant food base. Dense concentrations of the mesogastropod Alviniconcha hessleri (2, 3) were recently discovered at deep-sea hydrothermal vents at the spreading center in the Mariana Back-Arc Basin of the Western Pacific. These animals house chemoautrophic, sulfide-oxidizing bacteria within specialized cells (bacteriocytes) of their modified gills (2). They are the only reported example of a symbiotic association between a gastropod mollusc and intracellular chemoautotrophic bacteria. We now show that the modified gill of Alviniconcha contains hemoglobin at a concentration of about 65 mumol/kg wet weight gill. This value falls within the range, 20-250 mumol hemoglobin per kilogram, encountered in the modified symbiont-harboring gills of many of the sulfide-dependent clams examined but is short of the very high concentrations, 550 and 1200 mumol/kg, found in Myrtea spinifera and Lucina pectinata respectively (4). Accordingly, bacteriocyte hemoglobin is a feature common to both gastropod and bivalve symbioses.