The geochemical fingerprint of microbial long-distance electron transport in the seafloor

Recently, a novel “electrogenic” type of sulfur oxidation has been documented in marine sediments, whereby long filamentous cable bacteria are generating electrical currents over centimeter-scale distances. Here we propose a numerical model description that is capable of quantitatively simulating the solute depth profiles and biogeochemical transformations in such electro-active marine sediments. The model is based on a conventional reactive transport description of marine sediments, which is extended with a new model formulation for the long-distance electron transport induced by the cable bacteria. The mechanism of electron hopping is implemented to describe the electron transport along the longitudinal axis of the microbial filaments. We demonstrate that this model is capable of reproducing the observed geochemical fingerprint of electrogenic sulfur oxidation, which consists of a characteristic set of O2, pH and H2S depth profiles. Our simulation results suggest that the cable bacteria must have a high affinity for both oxygen and sulfide, and that intensive cryptic sulfur cycling takes place within the suboxic zone. A sensitivity analysis shows how electrogenic sulfur oxidation strongly impacts the biogeochemical cycling of sulfur, iron, carbon and calcium in marine sediments

The impact of electrogenic sulfide oxidation on elemental cycling and solute fluxes in coastal sediment

Filamentous sulfide oxidizing cable bacteria are capable of linking the oxidation of free sulfide in deep anoxic layers of marine sediments to the reduction of oxygen or nitrate in surface sediments by conducting electrons over centimeter-scale distances. Previous studies have shown that this newly discovered microbial process, referred to as electrogenic sulfide oxidation (e-SOx), may alter elemental cycling in sediments, but the nature and rates of the resulting biogeochemical transformations and their influence on benthic-pelagic coupling remain largely unknown. Here we quantify changes in sediment geochemistry and solute fluxes at the sediment–water interface as e-SOx develops and declines over time in laboratory incubations of organic-rich sediments from a seasonally hypoxic coastal basin (Marine Lake Grevelingen, The Netherlands).Our results show that e-SOx enhanced sediment O₂ consumption and acidified subsurface sediment, resulting in the dissolution of calcium carbonate and iron sulfide minerals in deeper sediment horizons and the associated accumulation of dissolved iron, manganese, and calcium in porewater. Remobilized Fe diffusing upward was reoxidized at the sediment–water interface, producing an amorphous Fe oxide crust, while dissolved Fe diffusing downward was reprecipitated in the form of FeS as it encountered the free sulfide horizon. The development of e-SOx enhanced the diffusive release of dissolved Mn at the sediment–water interface, capped the phosphate efflux, generated a buildup of organic matter in surface sediments, and strongly stimulated the release of alkalinity from the sediment. About 75% of this alkalinity production was associated with net CaCO₃ dissolution, while the remaining 25% was attributed to a pumping mechanism that transfers alkalinity from anodic H₂S oxidation (an alkalinity sink) in deeper sediments to cathodic O₂ reduction (an alkalinity source) near the sediment–water interface. The resulting sediment alkalinity efflux buffers the release of dissolved inorganic carbon at the sediment–water interface, and may therefore counteract the influence of benthic respiration on coastal ocean pH. Overall, our results demonstrate that e-SOx development strongly affects the biogeochemical cycles of C, P, Ca, Fe, Mn, and S in coastal sediments

Natural occurrence of microbial sulphur oxidation by long-range electron transport in the seafloor

Recently, a novel mode of sulphur oxidation was described in marine sediments, in which sulphide oxidation in deeper anoxic layers was electrically coupled to oxygen reduction at the sediment surface. Subsequent experimental evidence identified that long filamentous bacteria belonging to the family Desulfobulbaceae likely mediated the electron transport across the centimetre-scale distances. Such long-range electron transfer challenges some long-held views in microbial ecology and could have profound implications for sulphur cycling in marine sediments. But, so far, this process of electrogenic sulphur oxidation has been documented only in laboratory experiments and so its imprint on the seafloor remains unknown. Here we show that the geochemical signature of electrogenic sulphur oxidation occurs in a variety of coastal sediment environments, including a salt marsh, a seasonally hypoxic basin, and a subtidal coastal mud plain. In all cases, electrogenic sulphur oxidation was detected together with an abundance of Desulfobulbaceae filaments. Complementary laboratory experiments in intertidal sands demonstrated that mechanical disturbance by bioturbating fauna destroys the electrogenic sulphur oxidation signal. A survey of published geochemical data and 16S rRNA gene sequences identified that electrogenic sulphide oxidation is likely present in a variety of marine sediments with high sulphide generation and restricted bioturbation, such as mangrove swamps, aquaculture areas, seasonally hypoxic basins, cold sulphide seeps and possibly hydrothermal vent environments. This study shows for the first time that electrogenic sulphur oxidation occurs in a wide range of marine sediments and that bioturbation may exert a dominant control on its natural distribution

Polysaccharide hydrolysis in the presence of oil and dispersants: Insights into potential degradation pathways of exopolymeric substances (EPS) from oil-degrading bacteria

Oceanic oil-degrading bacteria produce copious amounts of exopolymeric substances (EPS) that facilitate their access to oil. The fate of EPS in the water column is in part determined by activities of heterotrophic microbes capable of utilizing EPS compounds as carbon and energy sources. To evaluate the potential of natural microbial communities to degrade EPS produced during oil degradation, we measured potential hydrolysis rates of six structurally distinct polysaccharides in two roller bottle experiments, using water from a natural oil seep in the northern Gulf of Mexico. The suite of polysaccharides used to measure the initial step in carbon degradation is indicative of polymers within microbial EPS. The treatments included (i) unamended surface or deep waters (whole water), and water amended with (ii) a water-accommodated fraction of oil (WAF), (iii) oil dispersant Corexit 9500, and (iv) WAF chemically-enhanced with Corexit (CEWAF). The oil and Corexit treatments were employed to simulate conditions during the Deepwater Horizon oil spill. Polysaccharide hydrolysis rates in the surface-water treatments were lowest in the WAF treatment, despite elevated levels of EPS in the form of transparent exopolymer particles (TEP). In contrast, the three deep-water treatments (WAF, Corexit, CEWAF) showed enhanced hydrolysis rates and TEP levels (WAF) compared to the whole water. We also observed variations in the spectrum of polysaccharide-hydrolyzing enzyme activities among the treatments. These substrate specificities were likely driven by activities of oil-degrading bacteria, shaping the pool of EPS and TEP as well as degradation products of hydrocarbons and Corexit compounds. A model calculation of potential turnover rates of organic carbon within the TEP pool suggests extended residence times of TEP in oil-contaminated waters, making them prone to serve as the sticky matrix for oily aggregates known as marine oil snow

Microbial carbon metabolism associated with electrogenic sulphur oxidation in coastal sediments

Recently, a novel electrogenic type of sulphur oxidation was documented in marine sediments, whereby filamentous cable bacteria (Desulfobulbaceae) are mediating electron transport over cm-scale distances. These cable bacteria are capable of developing an extensive network within days, implying a highly efficient carbon acquisition strategy. Presently, the carbon metabolism of cable bacteria is unknown, and hence we adopted a multidisciplinary approach to study the carbon substrate utilization of both cable bacteria and associated microbial community in sediment incubations. Fluorescence in situ hybridization showed rapid downward growth of cable bacteria, concomitant with high rates of electrogenic sulphur oxidation, as quantified by microelectrode profiling. We studied heterotrophy and autotrophy by following 13C-propionate and -bicarbonate incorporation into bacterial fatty acids. This biomarker analysis showed that propionate uptake was limited to fatty acid signatures typical for the genus Desulfobulbus. The nanoscale secondary ion mass spectrometry analysis confirmed heterotrophic rather than autotrophic growth of cable bacteria. Still, high bicarbonate uptake was observed in concert with the development of cable bacteria. Clone libraries of 16S complementary DNA showed numerous sequences associated to chemoautotrophic sulphur-oxidizing Epsilon- and Gammaproteobacteria, whereas 13C-bicarbonate biomarker labelling suggested that these sulphur-oxidizing bacteria were active far below the oxygen penetration. A targeted manipulation experiment demonstrated that chemoautotrophic carbon fixation was tightly linked to the heterotrophic activity of the cable bacteria down to cm depth. Overall, the results suggest that electrogenic sulphur oxidation is performed by a microbial consortium, consisting of chemoorganotrophic cable bacteria and chemolithoautotrophic Epsilon- and Gammaproteobacteria. The metabolic linkage between these two groups is presently unknown and needs further study

Rapid redox signal transmission by "Cable Bacteria" beneath a photosynthetic biofilm

Recently, long filamentous bacteria, belonging to the family Desulfobulbaceae, were shown to induce electrical currents over long distances in the surface layer of marine sediments. These “cable bacteria” are capable of harvesting electrons from free sulfide in deeper sediment horizons and transferring these electrons along their longitudinal axes to oxygen present near the sediment-water interface. In the present work, we investigated the relationship between cable bacteria and a photosynthetic algal biofilm. In a first experiment, we investigated sediment that hosted both cable bacteria and a photosynthetic biofilm and tested the effect of an imposed diel light-dark cycle by continuously monitoring sulfide at depth. Changes in photosynthesis at the sediment surface had an immediate and repeatable effect on sulfide concentrations at depth, indicating that cable bacteria can rapidly transmit a geochemical effect to centimeters of depth in response to changing conditions at the sediment surface. We also observed a secondary response of the free sulfide at depth manifest on the time scale of hours, suggesting that cable bacteria adjust to a moving oxygen front with a regulatory or a behavioral response, such as motility. Finally, we show that on the time scale of days, the presence of an oxygenic biofilm results in a deeper and more acidic suboxic zone, indicating that a greater oxygen supply can enable cable bacteria to harvest a greater quantity of electrons from marine sediments. Rapid acclimation strategies and highly efficient electron harvesting are likely key advantages of cable bacteria, enabling their success in high sulfide generating coastal sediments

Molecular and optical characterization reveals the preservation and sulfurization of chemically diverse porewater dissolved organic matter in oligohaline and brackish Chesapeake Bay sediments.

In this study, we conducted a detailed analysis of porewater downcore chemical properties and porewater dissolved organic matter (PDOM) composition using elemental C, N and S analysis, fluorescence spectroscopy, and ultrahigh resolution mass spectrometry (FT-ICR MS) at two contrasting sites in Chesapeake Bay. The sites, situated in the oligohaline upper bay and in the seasonally hypoxic mesohaline mid bay, receive fundamentally different detrital inputs predominantly from allochthonous and autochthonous sources, respectively. Unsurprisingly, we observed greater molecular oxygenation and degree of aromaticity in downcore PDOM profiles from the upper bay. At the mid bay station, PDOM composition was more indicative of non-aromatic algal-derived material. Unexpectedly, this autochthonous PDOM had lower C:S ratios. Hence, algal-derived organic matter appeared to be readily sulfurized, which was confirmed by quantification of dissolved organic sulfur as well as by qualitative interpretation of FT-ICR MS data. This finding suggests addition reactions of hydrogen sulfide to double bonds in unsaturated, but non-aromatic, organic molecules in autochthonous PDOM. Intriguingly, we also observed increases in humic-like fluorescence and dissolved organic carbon (DOC) concentrations in downcore PDOM profiles from both sites. Given the differences in molecular composition between sites, these results show that humic-like fluorescence can arise from different sources and biogeochemical processes. In the upper bay, we infer that these fluorescence signals reflect solubilization of terrestrially derived organic matter with a high aromatic and polyphenolic composition. By contrast, in the mid bay, these fluorescence peaks correlated negatively with hydrogen sulfide and are more likely linked to bacterial sulfate reduction

On Material Removal Regimes for the Shaping of Glass Edges: Force Analysis, Surface Topography and Damage Mechanisms

Glass shaping, which corresponds to the removal of the edges of a specimen, is the last finishing operation in glass manufacturing. This process has several functions on the final shaped glass including removing sharp edges, improving mechanical resistance, decreasing surface damage and giving it an aesthetical aspect. This article addresses the effects of working parameters, including grinding forces and consumed power, on surface edge finishing and damage mechanism induced during glass grinding. Microscopic observations and multi-scale analysis have also been conducted to investigate the surface edge characteristics. Experimental results show three damage regimes. The first (regime I) is a partial ductile regime with cutting action accompanied by chip formation. The second (regime II) is a crushing (or fragmentation) regime. The last (regime III) is also a partial ductile regime but by ploughing action with displaced material. The shaped surface obtained in the regime II has a better roughness than that obtained in regime I and III. However, regimes I and III include streaks and form defects which are not present in regime II. Similar to metallic materials, the evolution of force components show a linear relationship between normal and tangential forces. This implicates a constant average contact pressure and friction coefficient (l) between the flat grains and the workpiece

Thermal and magnetic properties and optical spectroscopy of Cr<inf>3</inf> BO<inf>3</inf><inf>4</inf>

We measured the heat capacity, magnetic susceptibility, magnetization, magnetoelectric effect, and optical spectra of samarium chromium borate crystals. It was found that, in single crystals, structures with the space groups and may coexist. Magnetic phase transitions were discovered at temperatures , , and , and assumptions about their nature were made. Based on the high-resolution optical spectra and magnetometry data, the crystal-field parameters in the and positions and the parameters of and exchange interactions were determined. The magnetic properties of the quasi-one-dimensional chromium subsystem were analyzed in the frame of the previously developed self-consistent four-particle cluster model. Despite a strong suppression of magnetic moments by the interchain antiferromagnetic exchange interactions, the contributions of the chromium subsystem to the bulk magnetization are dominant