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The deep ocean absorbs vast amounts of heat and carbon dioxide, providing a critical buffer to climate change but exposing vulnerable ecosystems to combined stresses of warming, ocean acidification, deoxygenation, and altered food inputs. Resulting changes may threaten biodiversity and compromise key ocean services that maintain a healthy planet and human livelihoods. There exist large gaps in understanding of the physical and ecological feedbacks that will occur. Explicit recognition of deep-ocean climate mitigation and inclusion in adaptation planning by the United Nations Framework Convention on Climate Change (UNFCCC) could help to expand deep-ocean research and observation and to protect the integrity and functions of deep-ocean ecosystems.

The two closely related deep-sea tubeworms Riftia pachyptila and Tevnia jerichonana both rely exclusively on a single species of sulfide-oxidizing endosymbiotic bacteria for their nutrition. They do, however, thrive in markedly different geochemical conditions. A detailed proteogenomic comparison of the endosymbionts coupled with an in situ characterization of the geochemical environment was performed to investigate their roles and expression profiles in the two respective hosts. The metagenomes indicated that the endosymbionts are genotypically highly homogeneous. Gene sequences coding for enzymes of selected key metabolic functions were found to be 99.9% identical. On the proteomic level, the symbionts showed very consistent metabolic profiles, despite distinctly different geochemical conditions at the plume level of the respective hosts. Only a few minor variations were observed in the expression of symbiont enzymes involved in sulfur metabolism, carbon fixation and in the response to oxidative stress. Although these changes correspond to the prevailing environmental situation experienced by each host, our data strongly suggest that the two tubeworm species are able to effectively attenuate differences in habitat conditions, and thus to provide their symbionts with similar micro-environments.

The Rimicaris exoculata dominates the megafauna of some of the Mid Atlantic ridge hydrothermal vent sites. This species harbors a rich community of bacterial epibionts inside its gill chamber. Literature data indicate that a single 16S rRNA phylotype dominates this epibiotic community, and is assumed to be a sulfide-oxidizing bacteria. However attempts of cultivation were not successful and did not allow to confirm it. The aim of our study was to test the hypothesis of sulfide oxidation in the gill chamber, by a multidisciplinary approach, using in vivo experiments at in situ pressure in the presence of sulfide, microscopic observations and a molecular survey. Morphology of microorganisms, before and after treatment, was analyzed to test the effect of sulfide depletion and re-exposure. Our observations, as well as molecular data indicate a wider diversity than previously described for this shrimp's epibiotic community. We observed occurrence of bacterial intracellular sulfur-and iron-enriched granules and some methanotrophic-like bacteria cells for the first time. Genes that are characteristic of methaneoxidizing (pmoA) and sulfide-oxidizing (APS) bacteria were identified. These results suggest that three metabolic types (iron, sulfide and methane oxidation) may co-occur within the epibiont community associated with Rimicaris exoculata. As this shrimp colonizes chemically contrasted environments, the relative abundance of each metabolic type could vary according to the local availability of reduced compounds.

The deep ocean below 200 m water depth is the least observed, but largest habitat on our planet by volume and area. Over 150 years of exploration has revealed that this dynamic system provides critical climate regulation, houses a wealth of energy, mineral, and biological resources, and represents a vast repository of biological diversity. A long history of deep-ocean exploration and observation led to the initial concept for the Deep-Ocean Observing Strategy (DOOS), under the auspices of the Global Ocean Observing System (GOOS). Here we discuss the scientific need for globally

The shrimp Rimicaris exoculata dominates the megafauna of some Mid-Atlantic Ridge hydrothermal vent fields. This species harbours a rich bacterial epibiosis inside its gill chamber. At the 'Rainbow' vent site (36°14.0' N), the epibionts are associated with iron oxide deposits. Investigation of both bacteria and minerals by scanning electron microscopy (SEM) and X-ray microanalysis (EDX) revealed 3 distinct compartments in the gill chamber: (1) the lower pre-branchial chamber, housing bacteria but devoid of minerals; (2) the 'true' branchial chamber, containing the gills and devoid of both bacteria and minerals; and (3) the upper pre-branchial chamber, housing the main ectosymbiotic bacterial community and associated mineral deposits. Our chemical and temperature data indicated that abiotic iron oxidation appears to be kinetically inhibited in the environment of the shrimps, which would explain the lack of iron oxide deposits in the first 2 compartments. We propose that iron oxidation is microbially promoted in the third area. The discrepancy between the spatial distribution of bacteria and minerals suggests that different bacterial metabolisms are involved in the first and third compartments. A possible explanation lies in the modification of physico-chemical conditions downstream of the gills that would reduce the oxygen content and favours the development of bacterial iron-oxidizers in this Fe II -rich environment. A potential role of such iron-oxidizing symbionts in the shrimp diet is suggested. This would be unusual for hydrothermal ecosystems, in which most previously described symbioses rely on sulphide or methane as an energy source.

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The vestimentiferan tubeworm Riftia pachyptila (Polychaeta: Sibloglinidae) often dominates early succession stages and high productivity habitats at low-temperature hydrothermal vents on the East Pacific Rise. We collected 8 aggregations of R. pachyptila and the associated epifaunal community at 2 discrete sites of diffuse hydrothermal activity, in December 2001 and December 2002. Because of the high spatial and temporal variability of the biotic and abiotic factors related to hydrothermal vent activity, significant differences in the structure and the composition of the community were expected to occur at the scale of either 1 yr or 500 m distance between very different sites. There was no significant difference in the temperature ranges of the diffuse flow between sites or years, even though the environmental conditions were very different at the 2 sites. At 1 site (Riftia Field), the diffuse hydrothermal fluids had relatively low concentrations of sulfide, low pH, and high concentrations of iron. At the other site (Tica), the diffuse hydrothermal fluids had higher sulfide concentrations, the pH was closer to neutral, and iron was undetectable. The physiological condition of R. pachyptila appeared to reflect the availability of sulfide at each site. However, the structure and the composition of the epifaunal community were remarkably similar between sites and years, with the exception of a few species. Aggregations of R. pachyptila support high local species diversity relative to the surrounding seafloor and high community similarity in different hydrothermal vent habitats.