In situ fluxes and zonation of microbial activity in surface sediments of the Håkon Mosby Mud Volcano

Abstract

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).

Physiology and behaviour of marine Thioploca

“…Oxygen uptake rates measured on marine Beggiatoa (trichome diameter 20 mm) in a setup similar to the one of this study showed a 10 times higher biovolume-specific oxygen consumption (32000 mmol dm À3 h À1 , LP Nielsen et al, 2006 (unpublished data)). Thus, it seems that Thioploca and Thiomargarita, which populate OMZs and which rarely experience oxygenated conditions, have a metabolic potential less optimized for the use of oxygen than large marine Beggiatoa, which thrive in sediments underlying oxygenated bottom water (for example, Mussman et al, 2003;Preisler et al, 2007;de Beer et al, 2006). Denitrification by Thioploca was not detectable, either by microelectrodes (o0.2 % of the sulphide uptake rates) or by 15 N experiments.…”

“…Sulphide oxidation by the large marine sulphur bacteria affects mineral cycling in many organic-rich surface sediments (for example, Ferdelman et al, 1997;Mussmann et al, 2003;de Beer et al, 2006;Preisler et al, 2007). Only one study has addressed the biomass-specific sulphide oxidation rate of Thioploca (Otte et al, 1999).…”

Metabolically active microbial communities in marine sediment under high-CO2 and low-pH extremes

“…The profiler module (Wenzhö fer et al, 2000;de Beer et al, 2006) was equipped with microsensors for dissolved oxygen, temperature (Pt100, UST Umweltsensortechnik GmbH, Geschwenda, Germany), pH, H 2 S, redox and pCO 2 (Microelectrodes Ltd, Ottawa, Canada). The temperature and CO 2 minisensors had a diameter of 2 mm.…”

Novel observations of Thiobacterium, a sulfur-storing Gammaproteobacterium producing gelatinous mats

“…The white color of the gelatinous spheres is an indication for the presence of elemental sulfur, and microscopic analyses later revealed a high density of light-reflecting granules typical for the sulfur inclusions of giant sulfide-oxidizing bacteria (Teske and Nelson, 2006). The white filaments were also observed surrounding the gelatinous masses on top of the sediment and were morphologically attributed to bacteria of the genus Beggiatoa, mat-forming giant sulfide oxidizers known to occur on highly reduced sulfidic sediments (Ahmad et al, 1999;Mills et al, 2004;de Beer et al, 2006). The underlying sediment was characterized by a beige-brown top layer above a dark gray to blackish horizon.…”