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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., (© 2011 Blackwell Publishing Ltd.)

The Amon mud volcano (MV), located at 1250m water depth on the Nile Deep Sea Fan, is known for its active emission of methane and non-methane hydrocarbons into the hydrosphere. Previous investigations showed a low efficiency of hydrocarbon-degrading anaerobic microbial communities inhabiting the Amon MV center in the presence of sulphate and hydrocarbons in the seeping subsurface fluids. By comparing spatial and temporal patterns of in situ biogeochemical fluxes, temperature gradients, pore water composition and microbial activities over three years, we investigated why the activity of anaerobic hydrocarbon degraders can be low despite high energy supplies. We found that the central dome of the Amon MV, as well as a lateral mud flow at its base, showed signs of recent exposure of hot subsurface muds lacking active hydrocarbon degrading communities. In these highly disturbed areas, anaerobic degradation of methane was less than 2% of the methane flux. Rather high oxygen consumption rates compared to low sulphide production suggest a faster development of more rapidly growing aerobic hydrocarbon degraders in highly disturbed areas. In contrast, the more stabilized muds surrounding the central gas and fluid conduits hosted active anaerobic hydrocarbon-degrading microbial communities. Furthermore, within three years, cell numbers and hydrocarbon degrading activity increased at the gas-seeping sites. The low microbial activity in the hydrocarbon-vented areas of Amon mud volcano is thus a consequence of kinetic limitations by heat and mud expulsion, whereas most of the outer mud volcano area is limited by hydrocarbon transport. [ABSTRACT FROM AUTHOR]

The giant pockmark REGAB (West African mar- gin, 3160m water depth) is an active methane-emitting cold seep ecosystem, where the energy derived from microbially mediated oxidation of methane supports high biomass and diversity of chemosynthetic communities. Bare sediments interspersed with heterogeneous chemosynthetic assemblagesof mytilid mussels, vesicomyid clams and siboglinid tube-worms form a complex seep ecosystem. To better understand if benthic bacterial communities reflect the patchy distribution of chemosynthetic fauna, all major chemosynthetic habitats at REGAB were investigated using an interdisciplinary approach combining pore water geochemistry, in situ quantification of fluxes and consumption of methane, as well as bacterial community fingerprinting. This study revealed that sediments populated by different fauna assemblages show distinct biogeochemical activities and are associated with distinct sediment bacterial communities. The methane consumption rates and methane effluxes ranged over one to two orders of magnitude across habitats, and reached highest values at the mussel habitat, which hosted a different bacterial community compared to the other habitats. Clam assemblages had a profound impact on the sediment geochemistry, but less so on the bacterial community structure. Moreover, all clam assemblages at REGAB were restricted to sediments characterized by complete methane consumption in the seafloor, and intermediate biogeochemical activity. Overall, variations in the sediment geochemistry were reflected in the distribution of both fauna and microbial communities; and were mostly determined by methane flux. [ABSTRACT FROM AUTHOR]

The Håkon Mosby Mud Volcano is a highly active methane seep hosting different chemosynthetic communities such as thiotrophic bacterial mats and siboglinid tubeworm assemblages. This study focuses on in situ measurements of methane fluxes to and from these different habitats, in comparison to benthic methane and oxygen consumption rates. By quantifying in situ oxygen, methane, and sulfide fluxes in different habitats, a spatial budget covering areas of 10-1000-m diameter was established. The range of dissolved methane efflux (770-2 mmol m-2 d-1) from the center to the outer rim was associated with a decrease in temperature gradients from 46° Cm-1 to < 1°C m-1, indicating that spatial variations in fluid flow control the distribution of benthic habitats and activities. Accordingly, total oxygen uptake (TOU) varied between the different habitats by one order of magnitude from 15 mmol m-2 d-1 to 161 mmol m-2 d-1. High fluid flow rates appeared to suppress benthic activities by limiting the availability of electron acceptors. Accordingly, the highest TOU was associated with the lowest fluid flow and methane efflux. This was most likely due to the aerobic oxidation of methane, which may be more relevant as a sink for methane as previously considered in submarine ecosystems. [ABSTRACT FROM AUTHOR]