ALBERT

Description: Methylated amines and sulfides are ubiquitous organic nitrogen and sulfur compounds in the marine environment and could serve as important energy substrates to methanogens inhabiting anoxic sediments. However, their abundance and isotopic values remain largely unconstrained in marine sediments. In this study, we investigated the distribution of trimethylamine (TMA), dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in Aarhus Bay, Denmark and provided the first report for their stable carbon isotopic composition. Simultaneous measurement of those two compounds in small volumes of pore waters and sediments was accomplished with gas chromatography in combination with either a purge and trap system for quantification or a headspace method for carbon isotopic analysis. TMA in the solid phase (exchangeable pool, 0.3-6.6 µmol/kg wet sediment; base-extractable pool, 2-18 µmol/kg) was much more abundant than the dissolved pool (〈 20 nM), indicating strong adsorption of TMA to sediments. Likewise, total base-hydrolyzable DMS(P)t (including DMS and base-released DMS from DMSP) in sediment was at least three orders of magnitude higher (11-65 µmol/kg) than the dissolved pool of DMS(P)d in the pore water (including DMS and dissolved DMSP; 1-12 nM). TMA and DMS(P) contents in the solid phase peaked in the surface sediment, consistent with their phytodetrital origin. TMA was more 13C-depleted than DMS(P) (TMA: -36.4 per mil to -39.2 per mil; DMS: -18.6 per mil to -23.4 per mil), presumably due to different biological or biosynthetic origins of the respective methyl groups. Both compounds showed a downcore decrease in their solid-phase concentration, a feature that was attributed to microbial degradation, but progressive enrichment in 13C (up to 4 per mil) with depth was observed only for DMS(P). The considerable pool size of TMA and DMS(P) outlined in this study and geochemical evidence of their degradability suggested these two compounds could be potentially important substrates for methane production in sulfate-reducing environments.

Description: Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40° to 60°C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from 〈10 to ~10**4 cells cm**-3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.

Description: Marine sediments host an unexpectedly large microbial biosphere, suggesting unique microbial mechanisms for surviving burial and slow metabolic turnover. Although dormancy is generally considered an important survival strategy, its specific role in subsurface sediments remains unclear. We quantified dormant bacterial endospores in 331 marine sediment samples from diverse depositional types and geographical origins. The abundance of endospores relative to vegetative cells increased with burial depth and endospores became dominant below 25 m, with an estimated population of 2.5 × 1028 to 1.9 × 1029 endospores in the uppermost kilometer of sediment and a corresponding biomass carbon of 4.6 to 35 Pg surpassing that of vegetative cells. Our data further identify distinct endospore subgroups with divergent resistance to burial and aging. Endospores may shape the deep biosphere by providing a core population for colonization of new habitats and/or through low-frequency germination to sustain slow growth in this environment.

Description: Sulfate reduction is a globally important yet poorly quantified redox process in marine sediments. We developed an artificial neural network trained with 199 sulfate profiles, constrained with geomorphological and geochemical maps to estimate global sulfate reduction rate distributions. Globally, 11.3 Tmol sulfate are reduced yearly, ~15% of previous estimates, accounting for the oxidation of 12-29% of the organic carbon flux to the sea floor. Combined with global cell distributions in marine sediments, these results indicate a strong contrast in sub–sea-floor prokaryote habitats: in continental margins global cell numbers in sulfate-depleted sediment exceed those in the overlying sulfate-bearing sediment by an order of magnitude, whereas in the abyss most life occurs in oxic and/or sulfate-reducing sediments.

Description: Sulfate is the dominant terminal electron acceptor in marine sediments. Sulfate reduction proceeds under anoxic conditions and is supported by a variety of electron donors (e.g. hydrogen, acetate, methane, propane, and butane), most of which are supplied by the decomposition of sedimentary organic matter. Consequently, a combination of primary productivity and water column depth is often thought to control sulfate reduction throughout most of the ocean’s seafloor [1, 2]. However, global models of sulfate reduction do not resolve the many different physical and ecological parameters that are encountered on a global scale, and that ultimately play a major role in driving local and regional sulfate reduction rates. We sought to better determine sulfate reduction rates on a global scale, irrespective of region or location by 1) including sulfate profiles from diverse settings and 2) compiling multiple geochemical parameters that are relevant to sulfate reduction and can help discern the magnitude of sulfate reduction rates. All available sulfate concentration profiles from DSDP/ODP/IODP (to Exp. 312) and additionally those in the database Pangaea (www.pangaea.de) were compiled reaching a total 〉600 nonrepetitive concentration profiles. Basic metadata describing the cores was included, such as water depth and distance to shore. Water column data such as minimum percent O2 saturation, bottom water O2, NO3 -, PO4 3-, and concentrations of surface water chlorophyll a and POC [3, 4] were included as additional variables that describe the biogeochemical setting of the cores. All compiled data and concentration profiles were applied to a training algorithm to estimate global sulfate reduction rates. The result was the most precise depiction of global sulfate reduction rates at the highest resolution to date. Our model serves as a platform for the examination of biogeochemical processes on the global scale and lets us predict energetic constraints for microbial metabolism in the subseafloor. [1] Canfield (1991) AJOS 291, 177-188. [2] Middelburg et al. (1997) DSR 44, 327-344. [3] Levitus & Boyer (1994) NOAA Atlas NESDIS [4] NASA, Aqua-MODIS