Size-dependent variations on the nutritional pathway of Bathymodiolus azoricus demonstrated by a C-flux model

Using a food web model to predict the effects of Hazardous and Noxious Substances (HNS) accidental spills on deep-sea hydrothermal vents from the Mid-Atlantic Ridge (MAR) region

Deep-sea hydrothermal vents host unique ecosystems but face risks of incidents with Hazardous and Noxious Substances (HNS) along busy shipping lanes such as the transatlantic route. We developed an Ecopath with Ecosim (EwE) model of the Menez Gwen (MG) vent field (MG-EwE) (Mid-Atlantic Ridge) to simulate ecosystem effects of potential accidental spills of four different HNS, using a semi-Lagrangian Dispersion Model (sLDM) coupled with the Regional Ocean Modelling System (ROMS) calibrated for the study area. Food web modelling revealed a simplified trophic structure with low energy efficiency. The MG ecosystem was vulnerable to disruptions caused by all tested HNS, yet it revealed some long-term resilience. Understanding these impacts is vital for enhancing Spill Prevention, Control, and Countermeasure plans (SPCC) in remote marine areas and developing tools to assess stressors effects on these invaluable habitats.

Gene expression profiles provide insights into the survival strategies in deep-sea mussel (Bathymodiolus platifrons) of different developmental stages

Background

Deep-sea mussels living in the cold seeps with enormous biomass act as the primary consumers. They are well adapted to the extreme environment where light is absent, and hydrogen sulfide, methane, and other hydrocarbon-rich fluid seepage occur. Despite previous studies on diversity, role, evolution, and symbiosis, the changing adaptation patterns during different developmental stages of the deep-sea mussels remain largely unknown.

Results

The deep-sea mussels (Bathymodiolus platifrons) of two developmental stages were collected from the cold seep during the ocean voyage. The gills, mantles, and adductor muscles of these mussels were used for the Illumina sequencing. A total of 135 Gb data were obtained, and subsequently, 46,376 unigenes were generated using de-novo assembly strategy. According to the gene expression analysis, amounts of genes were most actively expressed in the gills, especially genes involved in environmental information processing. Genes encoding Toll-like receptors and sulfate transporters were up-regulated in gills, indicating that the gill acts as both intermedium and protective screen in the deep-sea mussel. Lysosomal enzymes and solute carrier responsible for nutrients absorption were up-regulated in the older mussel, while genes related to toxin resistance and autophagy were up-regulated in the younger one, suggesting that the older mussel might be in a vigorous stage while the younger mussel was still paying efforts in survival and adaptation.

Conclusions

In general, our study suggested that the adaptation capacity might be formed gradually during the development of deep-sea mussels, in which the gill and the symbionts play essential roles.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08505-9.

Life in the Dark: Phylogenetic and Physiological Diversity of Chemosynthetic Symbioses

Possibly the last discovery of a previously unknown major ecosystem on Earth was made just over half a century ago, when researchers found teaming communities of animals flourishing two and a half kilometers below the ocean surface at hydrothermal vents. We now know that these highly productive ecosystems are based on nutritional symbioses between chemosynthetic bacteria and eukaryotes and that these chemosymbioses are ubiquitous in both deep-sea and shallow-water environments. The symbionts are primary producers that gain energy from the oxidation of reduced compounds, such as sulfide and methane, to fix carbon dioxide or methane into biomass to feed their hosts. This review outlines how the symbiotic partners have adapted to living together. We first focus on the phylogenetic and metabolic diversity of these symbioses and then highlight selected research directions that could advance our understanding of the processes that shaped the evolutionary and ecological success of these associations. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Biochemical and metabolic responses of the deep-sea mussel Bathymodiolus platifrons to cadmium and copper exposure

Greater interest in commercial deep-sea mining has been accompanied by mounting environmental concerns, including metal contamination resulting from mining activities. However, little is known about the toxic effects of metal exposure on deep-sea life. Given its ability to accumulate metals from the surrounding environment, its wide distribution at both vents and seeps, and its high abundance, the deep-sea mussel Bathymodiolus platifrons could serve as an ideal model to investigate the toxicological responses of deep-sea organisms to metal exposure. Here, we evaluated metal accumulation, traditional metal-related biomarkers, namely acid phosphatase (ACP), alkaline phosphatase (AKP), superoxide dismutase, catalase, reduced glutathione, metallothioneins, and malondialdehyde, as well as metabolic profiles in the gills of B. platifrons after a 7-day exposure to copper (100 μg/L), cadmium (500 μg/L), or copper-plus-cadmium treatments (100 μg/L Cu and 500 μg/L Cd). Metal exposure concentrations selected in this study can be found in deep-sea hydrothermal environments. Metal exposure resulted in significant metal accumulation in the gills of the mussel, indicating that B. platifrons has promise for use as an indicator of deep-sea metal pollution levels. Traditional biomarkers (AKP, ACP, and measured antioxidants) revealed cellular injury and oxidative stress in mussels following metal exposure. Metabolic responses in the three treatment groups indicated that metal exposure perturbed osmoregulation, energy metabolism, and nucleotide metabolism in mussels, in a response marked by differentially altered levels of amino acids, hypotaurine, betaine, succinate, glucose 6-phosphate, fructose 6-phosphate, guanosine, guanosine 5'-monophosphate, and inosine. Nevertheless, several uniquely altered metabolites were found in each treatment exposure group, suggesting dissimilar modes of toxicity between the two metal types. In the Cd-exposed group, the monosaccharide D-allose, which is involved in suppressing mitochondrial ROS production, was downregulated, a response consistent with oxidative stress in Cd-exposed B. platifrons. In the Cu-exposed group, the detected alterations in dopamine, dopamine-related, and serotonin-related metabolites together suggest disturbed neurotransmission in Cu-exposed B. platifrons. In the Cu-plus-Cd group, we detected a decline in fatty acid levels, implying that exposure to both metals jointly exerted a negative influence on the physiological functioning of the mussel. To the best of our knowledge, this is the first study to investigate changes in metabolite profiles in Bathymodiolus mussels exposed to metal. The findings reported here advance our understanding of the adverse impact of metal exposure on deep-sea life and can inform deep-sea mining assessments through the use of multiple biomarkers.

Zinc source differentiation in hydrothermal vent mollusks: Insight from Zn isotope ratios

Hydrothermal vent represents an extreme environment where metal-enriched fluids are in contact with chemosymbiotic animals. In the present study, Zn isotopic compositions were determined in multiple tissues of three dominant hydrothermal vent mollusks (the mussel Bathymodiolus marisindicus and two gastropods Chrysomallon squamiferum and Gigantopelta aegis) collected from a hydrothermal vent field (Southwest Indian Ridge in the Indian Ocean). We found approximately 1.78‰ differences in the δ⁶⁶Zn values among the three vent mollusks despite of their similar range of Zn concentrations. The significant variation in the δ⁶⁶Zn values was considered to be indicative of different Zn uptake sources among the three species as a result of their morphological adaptations. Zinc uptake associated with symbiotic activities may be more relevant in the vent gastropods, whereas Zn uptake from hydrothermal fluids during filter-feeding may also play a role in the vent mussels. However, no significant difference in δ⁶⁶Zn values was observed among tissues of any of the mollusks, showing the absence of Zn isotope fractionation during internal Zn transport. Our results demonstrated that variable Zn uptake pathways existed among different hydrothermal vent mollusks and could be differentiated by determining the Zn isotopic compositions in their tissues. We also highlight that Zn isotope ratios can be used to track Zn sources to the vent mollusks.

Metal adaptation strategies of deep-sea Bathymodiolus mussels from a cold seep and three hydrothermal vents in the West Pacific

Deep-sea Bathymodiolus mussels are ubiquitous in most cold seeps and hydrothermal fields, where they have adapted to various toxic environments including high metal exposure. However, there is scarce knowledge of metal accumulation and metal-related biomarkers in B. mussels. Here, we present data for metal concentrations (Ag, Cd, Cr, Cu, Fe, Mn, Pb, and Zn) and metal related biomarkers (superoxide dismutase-SOD, catalase-CAT, glutathione peroxidase-GPX, glutathione-GSH, metallothioneins-MTs, and lipid peroxidation-LPO) in different tissues of B. mussels from four different deep-sea geochemical settings: one cold seep and three vent fields in the West Pacific Ocean. Results showed that mussel gills generally exhibited higher metal enrichment than the mantle. Mussels from hydrothermal vents usually had higher metal concentrations (Fe, Cr, Cd, and Pb) than those from cold seep, which could be related to their higher contents in fluids or sediments. However, despite quite different metals loads among the geochemical environment settings, Mn, Zn, and Cu concentrations varied over a smaller range across the sampling sites, implying biological regulation by deep-sea mussels for these elements. Several statistically significant correlations were observed between SOD, CAT, GSH, MTs, and metal levels in analyzed tissues. Although the vent ecosystem is harsher than the cold seep ecosystem, according to our results their mussels' biomarker levels were not so different. This finding suggests that some adaptive or compensatory mechanisms may occur in chronically polluted deep-sea mussels. Principal component analysis allowed for distinguishing different deep-sea settings, indicating that B. mussels are robust indicators of their living environments. We also compared our results with those reported for coastal mussels. To our best knowledge, this is the first integrated study to report metal accumulation and metal-related biomarkers in the deep-sea B. mussels from the West Pacific.

Comparative proteomics of related symbiotic mussel species reveals high variability of host-symbiont interactions

Deep-sea Bathymodiolus mussels and their chemoautotrophic symbionts are well-studied representatives of mutualistic host-microbe associations. However, how host-symbiont interactions vary on the molecular level between related host and symbiont species remains unclear. Therefore, we compared the host and symbiont metaproteomes of Pacific B. thermophilus, hosting a thiotrophic symbiont, and Atlantic B. azoricus, containing two symbionts, a thiotroph and a methanotroph. We identified common strategies of metabolic support between hosts and symbionts, such as the oxidation of sulfide by the host, which provides a thiosulfate reservoir for the thiotrophic symbionts, and a cycling mechanism that could supply the host with symbiont-derived amino acids. However, expression levels of these processes differed substantially between both symbioses. Backed up by genomic comparisons, our results furthermore revealed an exceptionally large repertoire of attachment-related proteins in the B. thermophilus symbiont. These findings imply that host-microbe interactions can be quite variable, even between closely related systems.

Development of physical modelling tools in support of risk scenarios: A new framework focused on deep-sea mining

Deep-sea mining has gained international interest to provide materials for the worldwide industry. European oceans and, particularly, the Portuguese Exclusive Economic Zone present a recognized number of areas with polymetallic sulphides rich in metals used in high technology developments. A large part of these resources are in the vicinity of sensitive ecosystems, where the mineral extraction can potentially damage deep-ocean life services. In this context, technological research must be intensified, towards the implementation of environmental friendly solutions that mitigate the associated impacts. To reproduce deep-sea dynamics and evaluate the effects of the mining activities, reliable numerical modelling tools should be developed. The present work highlights the usefulness of a new framework for risk and impact assessment based on oceanographic numerical models to support the adoption of good management practices for deep-sea sustainable exploitation. This tool integrates the oceanic circulation model ROMS-Agrif with the semi-Lagrangian model ICHTHYOP, allowing the representation of deep-sea dynamics and particles trajectories considering the sediments physical properties. Numerical simulations for the North Mid-Atlantic Ridge region, revealed the ability of ROMS-Agrif to simulate real deep-sea dynamics through validation with in situ data. Results showed a strong diversity in the particle residence time, with a dependency on their density and size but also on local ocean conditions and bottom topography. The highest distances are obtained for the smaller and less dense particles, although they tend to be confined by bathymetric constrains and deposited in deepest regions. This work highlights the potential of this modelling tool to forecast laden plume trajectories, allowing the definition of risk assessment scenarios for deep-sea mining activities and the implementation of sustainable exploitation plans. Furthermore, the coupling of this numerical solution with models of biota inhabiting deep-sea vent fields into ecosystem models is discussed and outlined as cost-effective tools for the management of these remote ecosystems.

Gill chamber and gut microbial communities of the hydrothermal shrimp Rimicaris chacei Williams and Rona 1986: A possible symbiosis

Rimicaris chacei Williams and Rona 1986, formerly named as Chorocaris chacei, is a caridean shrimp living in deep-sea hydrothermal ecosystems. This shrimp is endemic to the Mid Atlantic Ridge (MAR) and lives at the periphery of aggregates of its well-known congeneric R. exoculata Williams and Rona 1986. Contrasting with the very dense and mobile clusters formed by R. exoculata, R. chacei lives in small groups of several individuals that are not very mobile. Although devoid of the characteristic hypertrophied cephalothorax of R. exoculata, which harbors the ectosymbionts, a microbial community has also been reported in the cephalothorax of R. chacei. Previous data on morphology, behavior and isotopic values indicate a diet based on a combination of feeding on its epibiotic bacteria and scavenging or occasional predation. In this study, our objective was to describe, for the first time, the distribution, morphology and phylogeny of the microbial communities associated with R. chacei. This species is significantly less studied than R. exoculata, but nevertheless represents the only other known example of symbiosis in crustaceans of MAR hydrothermal vent sites. Microbial communities have been observed at the same locations as in R. exoculata (mouthparts, branchiostegites and digestive tract). However, in R. chacei, the surfaces occupied by the bacteria are smaller. The main lineages are affiliated to Epsilon and Gammaproteobacteria in the cephalothorax and to Deferribacteres, Mollicutes, Epsilon and Gammaproteobacteria in the digestive tract. Comparison with the well-described bacterial communities of R. exoculata and hypotheses about the role of these communities in R. chacei are discussed.

Cryptic niche switching in a chemosymbiotic gastropod

Life stages of some animals, including amphibians and insects, are so different that they have historically been seen as different species. 'Metamorphosis' broadly encompasses major changes in organism bodies and, importantly, concomitant shifts in trophic strategies. Many marine animals have a biphasic lifestyle, with small pelagic larvae undergoing one or more metamorphic transformations before settling into a permanent, adult morphology on the benthos. Post-settlement, the hydrothermal vent gastropod Gigantopelta chessoia experiences a further, cryptic metamorphosis at body sizes around 5-7 mm. The terminal adult stage is entirely dependent on chemoautotrophic symbionts; smaller individuals do not house symbionts and presumably depend on grazing. Using high-resolution X-ray microtomography to reconstruct the internal organs in a growth series, we show that this sudden transition in small but sexually mature individuals dramatically reconfigures the organs, but is in no way apparent from external morphology. We introduce the term 'cryptometamorphosis' to identify this novel phenomenon of a major body change and trophic shift, not related to sexual maturity, transforming only the internal anatomy. Understanding energy flow in ecosystems depends on the feeding ecology of species; the present study highlights the possibility for adult animals to make profound shifts in biology that influence energy dynamics.

Metal interactions between the polychaete Branchipolynoe seepensis and the mussel Bathymodiolus azoricus from Mid-Atlantic-Ridge hydrothermal vent fields

The vent blood-red commensal polynoid polychaete Branchipolynoe seepensis is commonly found in the pallial cavity of the vent mussel Bathymodiolus azoricus, the dominant bivalve species along the Mid-Atlantic-Ridge (MAR) and is known to be kleptoparasitic. Mussels were collected from three hydrothermal vent fields in the MAR: Menez Gwen (850 m depth, MG2, MG3 and MG4), Lucky Strike (1700 m depth, Montségur-MS and Eiffel Tower-ET) and Rainbow (2300 m depth). Polychaetes were absent in all Menez Gwen vent mussels, while the highest percentage was detected in mussels from Lucky Strike, where more than 70% of the mussels had at least one polychaete in their mantle cavity, followed by Rainbow with 33% of mussels with polychaetes. Total metal concentrations (Ag, Cd, Co, Cu, Fe, Mn, Ni and Zn) were determined in polychaetes whole body and in the mussel tissues (gills, digestive gland and mantle). To understand the possible metal interactions between symbiont and host, the activity of antioxidant defence (catalase (CAT), metallothioneins (MTs)), biotransformation enzymes (glutathione-s-transferases (GST)) activities and lipid peroxidation (LPO) were determined in polychaete whole soft tissues and in mussel tissues (gills, digestive gland and mantle). Metal concentrations in polychaetes and mussels tissues indicated that the accumulation patterns were species specific and also influenced by, and possibly dependent upon, the inter- and intra-variation of vent physico-chemistry between hydrothermal fields. Despite not detecting any strong correlations between metal and enzymes activities in polychaetes and mussels, when in presence of polychaetes, mussels presented less metal concentrations in the gills and digestive gland and lower activity of enzymatic biomarkers. This leads to infer that the polychaete plays a role on the detoxification process, and the interaction between the polychaete mussel association is probably an adaptation to metals concentrations at the vent sites.

Metabolic and physiological interdependencies in the Bathymodiolus azoricus symbiosis

The hydrothermal vent mussel Bathymodiolus azoricus lives in an intimate symbiosis with two types of chemosynthetic Gammaproteobacteria in its gills: a sulfur oxidizer and a methane oxidizer. Despite numerous investigations over the last decades, the degree of interdependence between the three symbiotic partners, their individual metabolic contributions, as well as the mechanism of carbon transfer from the symbionts to the host are poorly understood. We used a combination of proteomics and genomics to investigate the physiology and metabolism of the individual symbiotic partners. Our study revealed that key metabolic functions are most likely accomplished jointly by B. azoricus and its symbionts: (1) CO₂ is pre-concentrated by the host for carbon fixation by the sulfur-oxidizing symbiont, and (2) the host replenishes essential biosynthetic TCA cycle intermediates for the sulfur-oxidizing symbiont. In return (3), the sulfur oxidizer may compensate for the host's putative deficiency in amino acid and cofactor biosynthesis. We also identified numerous ‘symbiosis-specific' host proteins by comparing symbiont-containing and symbiont-free host tissues and symbiont fractions. These proteins included a large complement of host digestive enzymes in the gill that are likely involved in symbiont digestion and carbon transfer from the symbionts to the host.

Adapted to change: The rapid development of symbiosis in newly settled, fast-maturing chemosymbiotic mussels in the deep sea

Symbioses between microbiota and marine metazoa occur globally at chemosynthetic habitats facing imminent threat from anthropogenic disturbance, yet little is known concerning the role of symbiosis during early development in chemosymbiotic metazoans: a critical period in any benthic species' lifecycle. The emerging symbiosis of Idas (sensu lato) simpsoni mussels undergoing development is assessed over a post-larval-to-adult size spectrum using histology and fluorescence in situ hybridisation (FISH). Post-larval development shows similarities to that of both heterotrophic and chemosymbiotic mussels. Data from newly settled specimens confirm aposymbiotic, planktotrophic larval development. Sulphur-oxidising (SOX) symbionts subsequently colonise multiple exposed, non-ciliated epithelia shortly after metamorphosis, but only become abundant on gills as these expand with greater host size. This wide-spread bathymodiolin recorded from sulphidic wood, bone and cold-seep habitats, displays a suite of adaptive traits that could buffer against anthropogenic disturbance.

Proteomic responses to metal-induced oxidative stress in hydrothermal vent-living mussels, Bathymodiolus sp., on the Southwest Indian Ridge

Bathymodiolin mussels are amongst the dominant fauna occupying hydrothermal vent ecosystems throughout the World's oceans. This subfamily inhabits a highly ephemeral and variable environment, where exceptionally high concentrations of reduced sulphur species and heavy metals necessitate adaptation of specialised detoxification mechanisms. Whilst cellular responses to common anthropogenic pollutants are well-studied in shallow-water species, they remain limited in deep-sea vent fauna. Bathymodiolus sp. were sampled from two newly-discovered vent sites on the Southwest Indian Ridge (Tiamat and Knuckers Gaff) by the remotely operated vehicle (ROV) Kiel 6000 during the RRS James Cook cruise, JC 067 in November 2011. Here, we use redox proteomics to investigate the effects of tissue metal accumulation on protein expression and thiol oxidation in gill. Following 2D PAGE, we demonstrate a significant difference in intensity in 30 protein spots in this organ between the two vent sites out of 205 matched spots. We also see significant variations in thiol oxidation in 15 spots, out of 143 matched. At Tiamat, 23 protein spots are up-regulated compared to Knuckers Gaff and we identify 5 of these with important roles in metabolism, cell structure, stress response, and redox homeostasis. We suggest that increased metal exposure triggers changes in the proteome, regulating tissue uptake. This is evident both between vent sites and across a chemical gradient within the Knuckers Gaff vent site. Our findings highlight the importance of proteomic plasticity in successful adaptation to the spatially and temporally fluctuating chemical environments that are characteristic of hydrothermal vent habitats.

Metal concentrations in the tissues of the hydrothermal vent mussel Bathymodiolus: reflection of different metal sources

Hydrothermal vent mussels of the genus Bathymodiolus are ideally positioned for the use of recording hydrothermal fluxes at the hydrothermal vent sites they inhabit. Barium, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Pb, Sr, and U concentrations in tissue sections of Bathymodiolus mussels from several hydrothermal fields between 15°N and 9°S at the Mid-Atlantic Ridge were determined and compared to the surrounding fluids and solid substrates in the habitats. Elements generally enriched in hydrothermal fluids, such as Fe, Cu, Zn, Pb and Cd, were significantly enriched in the gills and digestive glands of the hydrothermal mussels. The rather small variability of Zn (and Mn) and positive correlation with K and earth alkaline metals may indicate a biological regulation of accumulation. Enrichments of Mo and U in many tissue samples indicate that particulate matter such as hydrothermal mineral particles from the plumes can play a more important role as a metal source than dissolved metals. Highest enrichments of Cu in mussels from the Golden Valley site indicate a relation to the ≥400 °C hot heavy-metal rich fluids emanating in the vicinity. In contrast, mussels from the low-temperature Lilliput field are affected by the Fe oxyhydroxide sediment of their habitat. In a comparison of two different sites within the Logatchev field metal distributions in the tissues reflected small-scale local variations in the metal content of the fluids and the particulate material.

Biological data extraction from imagery - How far can we go? A case study from the Mid-Atlantic Ridge

In the past few decades, hydrothermal vent research has progressed immensely, resulting in higher-quality samples and long-term studies. With time, scientists are becoming more aware of the impacts of sampling on the faunal communities and are looking for less invasive ways to investigate the vent ecosystems. In this perspective, imagery analysis plays a very important role. With this study, we test which factors can be quantitatively and accurately assessed based on imagery, through comparison with faunal sampling. Twelve instrumented chains were deployed on the Atlantic Eiffel Tower hydrothermal edifice and the corresponding study sites were subsequently sampled. Discrete, quantitative samples were compared to the imagery recorded during the experiment. An observer-effect was tested, by comparing imagery data gathered by different scientists. Most factors based on image analyses concerning Bathymodiolus azoricus mussels were shown to be valid representations of the corresponding samples. Additional ecological assets, based exclusively on imagery, were included.

Arsenic speciation in food chains from mid-Atlantic hydrothermal vents

Arsenic concentration and speciation were determined in benthic fauna collected from the Mid-Atlantic Ridge hydrothermal vents. The shrimp species, Rimicaris exoculata, the vent chimney-dwelling mussel, Bathymodiolus azoricus, Branchipolynoe seepensis, a commensal worm of B. azoricus, and the gastropod Peltospira smaragdina showed variations in As concentration and in stable isotope (δ¹³C and δ¹⁵N) signature between species, suggesting different sources of As uptake. Arsenic speciation showed arsenobetaine to be the dominant species in R. exoculata, whereas in B. azoricus and B. seepensis arsenosugars were most abundant, although arsenobetaine, dimethylarsinate, and inorganic arsenic were also observed, along with several unidentified species. Scrape samples from outside the vent chimneys, covered with microbial mat, which is a presumed food source for many vent organisms, contained high levels of total As, but organic species were not detectable. The formation of arsenosugars in pelagic environments is typically attributed to marine algae, and the pathway to arsenobetaine is still unknown. The occurrence of arsenosugars and arsenobetaine in these deep sea organisms, where primary production is chemolithoautotrophic and stable isotope analyses indicate food sources are of vent origin, suggests that organic arsenicals can occur in a food web without algae or other photosynthetic life.

Reproductive traits of the cold-seep symbiotic mussel Idas modiolaeformis: gametogenesis and larval biology

We describe the first reproductive features of a chemosynthetic mussel collected at cold seeps from the eastern Mediterranean Sea. Idas modiolaeformis (Bivalvia, Mytilidae) is a hermaphroditic species in which production of male and female gametes likely alternates, a feature regarded as an adaptation to patchy and ephemeral habitats. By using fluorescent in situ hybridization, we demonstrate that bacterial symbionts, while present within the gills, are absent within acini that enclose female gametes and male gametes. This supports the hypothesis of environmental acquisition of symbionts in chemosynthetic mytilids. Prodissoconch I (PI) is relatively small compared to prodissoconch II (PII), suggesting a planktotrophic larval stage. Diameters of the two larval shells are in the range of sizes reported for mytilids, with a PII size between that of the shallow Mytilus edulis and that of the cold-seep mussel "Bathymodiolus" childressi.

Relative abundances of methane- and sulphur-oxidising symbionts in the gills of a cold seep mussel and link to their potential energy sources

Bathymodiolus mussels are key species in many deep-sea chemosynthetic ecosystems. They often harbour two types of endosymbiotic bacteria in their gills, sulphur- and methane oxidisers. These bacteria take up sulphide and methane from the environment and provide energy to their hosts, supporting some of the most prolific ecosystems in the sea. In this study, we tested whether symbiont relative abundances in Bathymodiolus gills reflect variations in the highly spatially dynamic chemical environment of cold seep mussels. Samples of Bathymodiolus aff. boomerang were obtained from two cold seeps of the deep Gulf of Guinea, REGAB (5°47.86S, 9°42.69E, 3170 m depth) and DIAPIR (6°41.58S, 10°20.94E, 2700 m depth). Relative abundances of both symbiont types were measured by means of 3D fluorescence in situ hybridisation and image analysis and compared considering the local sulphide and methane concentrations and fluxes assessed via benthic chamber incubations. Specimens inhabiting areas with highest methane content displayed higher relative abundances of methane oxidisers. The bacterial abundances correlated also with carbon stable isotope signatures in the mussel tissue, suggesting a higher contribution of methane-derived carbon to the biomass of mussels harbouring higher densities of methane-oxidising symbionts. A dynamic adaptation of abundances of methanotrophs and thiotrophs in the gill could be a key factor optimising the energy yield for the symbiotic system and could explain the success of dual symbiotic mussels at many cold seeps and hydrothermal vents of the Atlantic and Gulf of Mexico.

(210)Po and (210)Pb in the tissues of the deep-sea hydrothermal vent mussel Bathymodiolus azoricus from the Menez Gwen field (Mid-Atlantic Ridge)

The hydrothermal deep-sea vent fauna is naturally exposed to a highly specific environment enriched in potentially toxic species such as sulfides, metals and natural radionuclides due to the convective seawater circulation inside the oceanic crust and its interaction with basaltic or ultramafic host rocks. However, data on radionuclides in biota from such environment are very limited. An investigation was carried out on tissue partitioning of (210)Po and (210)Pb, two natural radionuclides within the (238)U decay chain, in Bathymodiolus azoricus specimens from the Mid-Atlantic Ridge (Menez Gwen field). These two elements showed different distributions with high (210)Pb levels in gills and high (210)Po levels in both gills and especially in the remaining parts of the body tissue (including the digestive gland). Various factors that may explain such partitioning are discussed. However, (210)Po levels encountered in B. azoricus were not exceptionally high, leading to weighted internal dose rate in the range 3 to 4 μGy h⁻¹. These levels are slightly higher than levels characterizing coastal mussels (~1 μGy h⁻¹).

The influence of nutritional conditions on metal uptake by the mixotrophic dual symbiosis harboring vent mussel Bathymodiolus azoricus

The vent mussel Bathymodiolus azoricus, host thioautotrophic and methanotrophic bacteria, in their gills and complementary, is able to digest suspended organic matter. But the involvement of nutritional status in metal uptake and storage remains unclear. The influence of B. azoricus physiological condition on its response to the exposure of a mixture of metals in solution is addressed. Mussels from the Menez Gwen field were exposed to 50 μgL(-1) Cd, plus 25 μgL(-1) Cu and 100 μgL(-1) Zn for 24 days. Four conditions were tested: (i) mussels harboring both bacteria but not feed, (ii) harboring only methanotrophic bacteria, (iii) without bacteria but fed during exposure and (iv) without bacteria during starvation. Unexposed mussels under the same conditions were used as controls. Eventual seasonal variations were assessed. Metal levels were quantified in subcellular fractions in gills and digestive gland. Metallothionein levels and condition indices were also quantified. Gill sections were used for fluorescence in situ hybridization (FISH) to assess the temporal distribution of symbiotic associations. Starvation damages metal homeostasis mechanisms and increase the intracellular Zn and MT levels function. There is a clear metallic competition for soluble and insoluble intracellular ligands at each condition. Seasonal variations were observed at metal uptake and storage.

Variation in physiological indicators in Bathymodiolus azoricus (Bivalvia: Mytilidae) at the Menez Gwen Mid-Atlantic Ridge deep-sea hydrothermal vent site within a year

Bathymodiolus azoricus, thriving at Mid-Atlantic Ridge deep vents, benefits from a symbiosis with methane- and sulphide-oxidising (MOX and SOX) bacteria, and feeds on particulate and dissolved organic matter. To investigate the temporal evolution in their nutrition adult mussels were collected from one location at the Menez Gwen vent site (817 m depth) on four occasions between 2006 and 2007 and studied using different techniques, including stable isotope analyses and FISH. Gill and mantle tissues delta13C and delta15N signatures varied by 2-3 per thousand during the year and these variations were linked to fluctuations in tissue condition index, C and N contents and SOX/MOX volume ratios as quantified by 3D-FISH. October and January mussels presented a particularly poor condition, possibly related with the prolonged summer period of low sea-surface primary production and/or with the stress of the transplant to acoustically retrievable cages for the October mussels, and with their reproductive state in January mussels, since they were spawning. Our results point to the possibility that May mussels benefited from a pulse of sinking sea-surface plankton material. Results underline the dependency of stable isotopic signatures on the physiological state of the mussel at the time of collection, and on the type of tissue analyzed.

Methanotrophic symbioses in marine invertebrates

Symbioses between marine animals and aerobic methane-oxidizing bacteria are found at hydrothermal vents and cold seeps in the deep sea where reduced, methane-rich fluids mix with the surrounding oxidized seawater. These habitats are 'oases' in the otherwise nutrient-poor deep sea, where entire ecosystems are fueled by microbial chemosynthesis. By associating with bacteria that gain energy from the oxidation of CH4 with O2 , the animal host is indirectly able to gain nutrition from methane, an energy source that is otherwise only available to methanotrophic microorganisms. The host, in turn, provides its symbionts with continuous access to both electron acceptors and donors that are only available at a narrow oxic - anoxic interface for free-living methanotrophs. Symbiotic methane oxidizers have resisted all attempts at cultivation, so that all evidence for these symbiotic associations comes from ultrastructural, enzymatic, physiological, stable isotope and molecular biological studies of the symbiotic host tissues. In this review, we present an overview of the habitats and invertebrate hosts in which symbiotic methane oxidizers have been found, and the methods used to investigate these symbioses, focusing on the symbioses of bathymodiolin mussels that have received the most attention among methanotrophic associations.