A large fraction of globally produced methane is converted to CO2 by anaerobic oxidation in marine sediments. Strong geochemical evidence for net methane consumption in anoxic sediments is based on methane profiles, radiotracer experiments and stable carbon isotope data. But the elusive microorganisms mediating this reaction have not yet been isolated, and the pathway of anaerobic oxidation of methane is insufficiently understood. Recent data suggest that certain archaea reverse the process of methanogenesis by interaction with sulphate-reducing bacteria. Here we provide microscopic evidence for a structured consortium of archaea and sulphate-reducing bacteria, which we identified by fluorescence in situ hybridization using specific 16S rRNA-targeted oligonucleotide probes. In this example of a structured archaeal-bacterial symbiosis, the archaea grow in dense aggregates of about 100 cells and are surrounded by sulphate-reducing bacteria. These aggregates were abundant in gas-hydrate-rich sediments with extremely high rates of methane-based sulphate reduction, and apparently mediate anaerobic oxidation of methane.

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From the Hå kon Mosby Mud Volcano (HMMV) on the southwest Barents Sea shelf, gas and fluids are expelled by active mud volcanism. We studied the mass transfer phenomena and microbial conversions in the surface layers using in situ microsensor measurements and on retrieved cores. The HMMV consists of three concentric habitats: a central area with gray mud, a surrounding area covered by white mats of big sulfide oxidizing filamentous bacteria (Beggiatoa), and a peripheral area colonized by symbiontic tube worms (Pogonophora). A fourth habitat comprised gray microbial mats near gas seeps. The differences between these four methane-fueled habitats are best explained by different transport rates of sulfate into the sediments and porewater upflow rates. The upflow velocities were estimated by two independent methods at 3-6 m yr 21 in the central area and 0.3-1 m yr 21 in Beggiatoa mats. In the central area no sulfide was found, indicating that the rapidly rising sulfate-free fluids caused sulfate limitation that inhibited anaerobic oxidation of methane (AOM). Under Beggiatoa mats a steep sulfide peak was found at 2 to 3 cm below the seafloor (bsf), most likely due to AOM. All sulfide was oxidized anaerobically, possibly through nitrate reduction by Beggiatoa. The Beggiatoa mats were dominated by a single filamentous morphotype with a diameter of 10 mm and abundant sulfur inclusions. A high diversity of sulfide oxidizer morphotypes was observed in a grayish microbial mat near gas vents, where aerobic sulfide oxidation was important. The sediments colonized by Pogonophora were influenced by bioventilation, allowing sulfate penetration and AOM to 70 cm bsf. The HMMV is a unique and diverse ecosystem, the structure and functioning of which is mainly controlled by pore-water flow.Interest in anaerobic oxidation of methane (AOM) and its linkage to sulfate reduction was strongly stimulated by the recent discovery of the microorganisms involved (Boetius et al. 2000;Michaelis et al. 2002;Orphan et al. 2002). Evidence was presented that consortia of methanotrophic archaea and sulfate-reducing bacteria are responsible for the process. These microorganisms were found in high abundance in methane-rich sediments above gas hydrates and various types of cold seeps. The microbial conversion of methane and sulfate to CO 2 and sulfide in surface sediments is usually accompanied by sulfide oxidation (SO) by free-living and symbiotic bacteria.1 Corresponding author (dbeer@mpi-bremen.de).

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A series of in situ enrichment experiments was carried out at 1265 m water depth in the Sognefjord on the west coast of Norway in order to follow the short-term fate of freshly settled phytodetritus in a deep-sea sediment. For all experiments, a deep-sea benthic chamber lander system was used. In the lander chambers, a settling spring bloom was simulated by the injection of 0.2 g of freeze-dried Thalassiosira rotula, an equivalent of 1 g organic C m -2 . The algae were 98% 13 C-labeled, thus enabling us to follow the processing of the carbon by bacteria and macrofauna. Experiment duration varied from 8 h to 3 d. The total oxygen consumption of the sediments increased by approximately 25% due to particulate organic matter (POM) enrichment. Macrofauna organisms became immediately labeled with 13 C. After 3 d, 100% of the individuals sampled down to 10 cm sediment depth had taken up 13 C from the phytodetritus added. Bacterial uptake of the tracer was fast too, and even bacteria in deeper sediment layers had incorporated the fresh material within 3 d. Our study documents the rapid downward mixing of labile organic matter and the importance of macrofauna for this process. We present the first evidence for the immediate breakdown and incorporation of POM by bacteria even in deep sediment layers. Surprisingly, the initial processing of carbon was dominated by macrofauna, although the group comprises < 5% of the benthic biomass.Altogether, approximately 5% of the carbon added had been processed within 3 d, with the majority being released from the sediment as CO 2 . Due to the good comparability of our study site with midslope settings at continental margins, in general, we propose that the processes we observed are widespread at continental margins and are significant for the biogeochemical cycling of particulate matter on the slope.

This contribution presents total oxygen uptake (TOU) rates and nutrient fluxes of organically poor permeable shelf sands of the German Bight. Measurements have been made in situ with the novel autonomous benthic chamber system Sandy under controlled conditions of advective pore-water exchange. Average oxygen consumption rates of 31.3 Ϯ 18.2 mmol m Ϫ2 d Ϫ1 measured in this study were relatively high as compared with rates reported from shelf sediments with much higher organic contents. TOU of highly permeable medium and coarse grained sands was substantially enhanced in the presence of advection. This indicates that advective oxygen supply contributed significantly to respiration in these sediments and that advection has to be considered when assessing oxygen consumption and organic matter mineralization in shelf areas. In fine-grained, less permeable sands, no effect of advection could be measured. A lower advective oxygen supply in these sediments is in agreement with a release of ammonium instead of nitrate and a shallower oxygen penetration depth. Scaled up to the entire German Bight, the results imply that in 40% of the area an effect of advection on benthic oxygen uptake and other advectionrelated processes can be largely excluded, while in the remaining 60% significant pore-water advection potentially takes place. However, because permeabilities of the sediments investigated in this study were widely spaced, a significant effect on oxygen supply was only verified for highly permeable sands that are likely to cover approximately 3% of the area.Until the 1980s the biogeochemistry of coastal sands has received relatively little attention. Because organic matter content generally decreases with increasing grain size, it seemed that coarse grained sandy deposits lack the substrate to sustain significant biological activity. Sands were therefore considered to be biogeochemical deserts that did not significantly contribute to the cycling of organic matter (Boudreau et al. 2001).A new perception of sandy sediments developed when 1 Corresponding author (fjanssen@mpi-bremen.de).

The deep-sea includes over 90% of the world oceans and is thought to be one of the most diverse ecosystems in the World. It supplies society with valuable ecosystem services, including the provision of food, the regeneration of nutrients and the sequestration of carbon. Technological advancements in the second half of the 20th century made large-scale exploitation of mineral-, hydrocarbon-and fish resources possible. These economic activities, combined with climate change impacts, constitute a considerable threat to deep-sea biodiversity. Many governments, including that of the UK, have therefore decided to implement additional protected areas in their waters of national jurisdiction. To support the decision process and to improve our understanding for the acceptance of marine conservation plans across the general public, a choice experiment survey asked Scottish households for their willingness-to-pay for additional marine protected areas in the Scottish deep-sea. This study is one of the first to use valuation methodologies to investigate public preferences for the protection of deep-sea ecosystems. The experiment focused on the elicitation of economic values for two aspects of biodiversity: (i) the existence value for deep-sea species and (ii) the option-use value of deep-sea organisms as a source for future medicinal products. Acknowledgments:We thank Mirko Moro, Dugald Tinch and Neil Odam for their invaluable input on survey and experimental design. Funding for this project was provided by MASTS (Marine Alliance for Science and Technology Scotland).

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