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Vesicomyidae clams harbor sulfide-oxidizing endosymbionts and are typical members of cold seep communities where active venting of fluids and gases takes place. We investigated the central biogeochemical processes that supported a vesicomyid clam colony as part of a locally restricted seep community in the Japan Trench at 5346 m water depth, one of the deepest seep settings studied to date. An integrated approach of biogeochemical and molecular ecological techniques was used combining in situ and ex situ measurements. In sediment of the clam colony, low sulfate reduction rates (maximum 128 nmol mL−1 day−1) were coupled to the anaerobic oxidation of methane. They were observed over a depth range of 15 cm, caused by active transport of sulfate due to bioturbation of the vesicomyid clams. A distinct separation between the seep and the surrounding seafloor was shown by steep horizontal geochemical gradients and pronounced microbial community shifts. The sediment below the clam colony was dominated by anaerobic methanotrophic archaea ( ANME-2c) and sulfate-reducing Desulfobulbaceae ( SEEP- SRB-3, SEEP- SRB-4). Aerobic methanotrophic bacteria were not detected in the sediment, and the oxidation of sulfide seemed to be carried out chemolithoautotrophically by Sulfurovum species. Thus, major redox processes were mediated by distinct subgroups of seep-related microorganisms that might have been selected by this specific abyssal seep environment. Fluid flow and microbial activity were low but sufficient to support the clam community over decades and to build up high biomasses. Hence, the clams and their microbial communities adapted successfully to a low-energy regime and may represent widespread chemosynthetic communities in the Japan Trench. In this regard, they contributed to the restricted deep-sea trench biodiversity as well as to the organic carbon availability, also for non-seep organisms, in such oligotrophic benthic environment of the dark deep ocean. [ABSTRACT FROM AUTHOR]

The giant pockmark REGAB (West African margin, 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 assemblages of mytilid mussels, vesicomyid clams and siboglinid tubeworms 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 porewater geochemistry, in situ quantification of fluxes and consumption of methane, as well 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 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]

Sediment oxygen concentration profiles and benthic microbial oxygen consumption rates were investigated during an IODP site survey in the South Pacific Gyre. Primary production, particle fluxes and sedimentation rates are extremely low in this ultra-oligotrophic oceanic region. We derived O2 consumption rates from vertical oxygen profiles in sediments obtained on different spatial scales ex situ (in piston cores and multi cores), and in situ (using a benthic lander equipped with a microelectrode profiler). Along a transect in the area 24 to 46° S and 165 to 117°W, cores from 10 out of 11 sites were oxygenated over their entire length (as much as 8m below seafloor), with deep O2 concentrations >150 μmol L-1. This represents the deepest oxygen penetration ever measured in marine sediments. High-resolution microprofiles from the surface sediment layer revealed a diffusive oxygen uptake between 0.1 and 1.3 mmolm-2 d-1, equal to a carbon mineralization rate of ∼0.4-4.5 gCm-2 yr-1. This is in the lower range of previously reported fluxes for oligotrophic sediments but corresponds well to the low surface water primary production. Half of the pool of reactive organic matter was consumed in the top 1.5-6mm of the sediment. Because of the inert nature of the deeper sediment, oxygen that is not consumed within the top centimeters diffuses downward to much greater depth. In deeper zones, a small O2 flux between 0.05 and 0.3 μmol m-2 d-1 was still present. This flux was nearly constant with depth, indicating extremely low O2 consumption rates. Modeling of the oxygen profiles suggests that the sediment is probably oxygenated down to the basalt, suggesting an oxygen flux from the sediment into the basaltic basement. [ABSTRACT FROM AUTHOR]

More than 50% of the Earth' s surface is sea floor below 3,000?m of water. Most of this major reservoir in the global carbon cycle and final repository for anthropogenic wastes is characterized by severe food limitation. Phytodetritus is the major food source for abyssal benthic communities, and a large fraction of the annual food load can arrive in pulses within a few days. Owing to logistical constraints, the available data concerning the fate of such a pulse are scattered and often contradictory, hampering global carbon modelling and anthropogenic impact assessments. We quantified (over a period of 2.5 to 23 days) the response of an abyssal benthic community to a phytodetritus pulse, on the basis of 11 in situ experiments. Here we report that, in contrast to previous hypotheses, the sediment community oxygen consumption doubled immediately, and that macrofauna were very important for initial carbon degradation. The retarded response of bacteria and Foraminifera, the restriction of microbial carbon degradation to the sediment surface, and the low total carbon turnover distinguish abyssal from continental-slope ‘deep-sea’ sediments. [ABSTRACT FROM AUTHOR]

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists. Our understanding of sediment biogeochemistry and exchange fluxes was revolutionized by the introduction of benthic chambers and in situ micro-electrode profilers that allow for the direct measurement of chemical fluxes between sediment and water at the sea floor and for porewater composition. Since then, rapid progress in the technology of in situ sensors and benthic chambers (such as the introduction of gel probes, voltammetric electrodes or one- and two-dimensional optodes) have yielded major breakthroughs in the scientific understanding of benthic biogeochemistry. This paper is a synthesis of discussions held during the workshop on sediment biogeochemistry at the “Benthic Dynamics: in situ surveillance of the sediment–water interface” international conference (Aberdeen, UK—March 25–29, 2002). We present a review of existing in situ technologies for the study of benthic biogeochemistry dynamics and related scientific applications. Limitations and possible improvement (e.g., technology coupling) of these technologies and future development of new sensors are discussed. There are countless important scientific and technical issues that lend themselves to investigation using in situ benthic biogeochemical assessment. While the increasing availability of these tools will lead research in yet unanticipated directions, a few emerging issues include greater insight into the controls on organic matter (OM) mineralization, better models for the understanding of benthic fluxes to reconcile microelectrode and larger-scale chamber measurements, insight into the impacts of redox changes on trace metal behavior, new insights into geochemical reaction pathways in surface sediments, and a better understanding of contaminant fate in nearshore sediments. [Copyright &y& Elsevier]