Molybdenum dynamics in sediments of a seasonally-hypoxic coastal marine basin

Peer reviewe

Rheological and Microstructural Changes of Oat and Barley Starches During Heating and Cooling

Microstructural and rheological changes in barley and oat starch dispersions during heating and cooling were studied by light microscopy and dynamic viscoelastic measurements. The two starch pastes showed similar viscoelastic properties after gelatinization, but during cooling the 20% barley starch pastes heated at 95°C underwent a sharp transition in viscoelastic behaviour probably due to the gelation of amylose. This transition was shifted to lower temperatures at 10% starch concentration. Microstructural studies of an 8% barley starch dispersion heated to 90°C using the smear technique showed amylose to form a network structure around the granules. The granules in starch paste heated to 95°C were poorly stained and amylopectin was fragmented. Microscopic examination of an embedded section of the cooled barley starch gel showed amylose to form a continuous phase in which starch granules were dispersed. G\u27 increased below 80°C during cooling of 10% oat starch dispersions preheated at 95 °C. No rheological changes occurred when they were preheated at only 90°C. Microstructural studies of an 8% oat starch dispersion heated to 90°C using the smear technique showed amylose to form a network structure around the granules. Part of the granule structure had already broken down. Heating to 95°C induced considerable changes in the granule structure of oat starch gels. Amylopectin formed a very fine network. Microscopic examination of embedded sections of the cooled, stored gel showed a much coarser structure compared with that of the smear

Phosphorus cycling and burial in sediments of a seasonally hypoxic marine basin

Recycling of phosphorus (P) from sediments contributes to the development of bottom-water hypoxia in many coastal systems. Here, we present results of a year-long assessment of P dynamics in sediments of a seasonally hypoxic coastal marine basin (Lake Grevelingen, the Netherlands) in 2012. Sequential phosphorus extractions (SEDEX) and X-ray absorption spectroscopy (XAS) indicate that P was adsorbed to Fe-(III)-(oxyhydr)oxides when cable bacteria were active in the surface sediments in spring. With the onset of summer hypoxia, sulphide-induced dissolution of the Fe-(III)-(oxyhydr)oxides led to P release to the pore water and overlying water. The similarity in authigenic Ca-P concentrations in the sediment and suspended matter suggest that Ca-P is not formed in situ. The P burial efficiency was ≤ 32%. Hypoxia-driven sedimentary P recycling had a major impact on the water-column chemistry in the basin in 2012. Water-column monitoring data indicate up to ninefold higher surface water concentrations of phosphate in the basin in the late 1970s and a stronger hypoxia-driven seasonal P release from the sediment. The amplified release of P from the sediment in the past is attributed to the presence of a larger pool of Fe-bound P in the basin prior to the first onset of hypoxia. Given that P is not limiting, primary production in the basin has not been affected by the decadal changes in P availability and recycling over the past 40 years. The changes in P dynamics on decadal time scales wer

Anaerobic oxidation of methane alters sediment records of sulfur, iron and phosphorus in the Black Sea

The surface sediments in the Black Sea are underlain by extensive deposits of iron (Fe)-oxide-rich lake sediments that were deposited prior to the inflow of marine Mediterranean Sea waters ca. 9000 years ago. The subsequent downward diffusion of marine sulfate into the methane-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ), including anaerobic oxidation of methane (AOM) with sulfate. While the sedimentary cycles of sulfur (S), methane and Fe in the SMTZ have been extensively studied, relatively little is known about the diagenetic alterations of the sediment record occurring below the SMTZ. Here we combine detailed geochemical analyses of the sediment and porewater with multicomponent diagenetic modeling to study the diagenetic alterations below the SMTZ at two sites in the western Black Sea. We focus on the dynamics of Fe, S and phosphorus (P), and demonstrate that diagenesis has strongly overprinted the sedimentary burial records of these elements. In line with previous studies in the Black Sea, we show that sulfate-mediated AOM substantially enhances the downward diffusive flux of sulfide into the deep limnic deposits. During this downward sulfidization, Fe oxides, Fe carbonates and Fe phosphates (e.g., vivianite) are converted to sulfide phases, leading to an enrichment in solid-phase S and the release of phosphate to the porewater. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe2+) lead to sequestration of downward-diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front. Our model results further demonstrate that downward-migrating sulfide becomes partly re-oxidized to sulfate due to reactions with oxidized Fe minerals, fueling a cryptic S cycle and thus stimulating slow rates of sulfate-driven AOM (similar to 1-100 pmol cm(-3) d(-1)) in the sulfate-depleted limnic deposits. However, this process is unlikely to explain the observed release of dissolved Fe2+ below the SMTZ. Instead, we suggest that besides organoclastic Fe oxide reduction and reactivation of less reactive Fe oxides by methanogens, AOM coupled to the reduction of Fe oxides may also provide a possible mechanism for the high concentrations of Fe2+ in the porewater at depth. Our results reveal that methane plays a key role in the diagenetic alterations of Fe, S and P records in Black Sea sediments. The downward sulfidization into the limnic deposits is enhanced through sulfate-driven AOM with sulfate, and AOM with Fe oxides may provide a deep source of dissolved Fe2+ that drives the sequestration of P in vivianite below the sulfidization front.Peer reviewe

Cable bacteria generate a firewall against euxinia in seasonally hypoxic basins

Seasonal oxygen depletion (hypoxia) in coastal bottom waters can lead to the release and persistence of free sulfide (euxinia), which is highly detrimental to marine life. Although coastal hypoxia is relatively common, reports of euxinia are less frequent, which suggests that certain environmental controls can delay the onset of euxinia. However, these controls and their prevalence are poorly understood. Here we present field observations from a seasonally hypoxic marine basin (Grevelingen, The Netherlands), which suggest that the activity of cable bacteria, a recently discovered group of sulfur-oxidizing microorganisms inducing long-distance electron transport, can delay the onset of euxinia in coastal waters. Our results reveal a remarkable seasonal succession of sulfur cycling pathways, which was observed over multiple years. Cable bacteria dominate the sediment geochemistry in winter, whereas, after the summer hypoxia, Beggiatoaceae mats colonize the sediment. The specific electrogenic metabolism of cable bacteria generates a large buffer of sedimentary iron oxides before the onset of summer hypoxia, which captures free sulfide in the surface sediment, thus likely preventing the development of bottom water euxinia. As cable bacteria are present in many seasonally hypoxic systems, this euxinia-preventing firewall mechanism could be widely active, and may explain why euxinia is relatively infrequently observed in the coastal ocean

Ocean circulation in the Toarcian (Early Jurassic), a key control on deoxygenation and carbon burial on the European Shelf

The Toarcian Oceanic Anoxic Event (T-OAE, ∼183 My) was a long-lasting episode of ocean deoxygenation during the Early Jurassic. The event is related to a period of global warming and characterized by major perturbations to the hydrological and carbon cycles with high rates of organic matter burial in shelf seas. Ocean circulation during the Toarcian and its influence on marine biogeochemical cycles are still not fully understood. Here,we assess the spatial extent of anoxia in the NW Tethys Ocean during the T-OAE, the relationship with ocean circulation and the impact on organic carbon burial, using new and existing sedimentary records from the European Epicontinental Shelf (EES) in combination with general circulation model results. We demonstrate that bottom waters on the southwestern part of the shelf were mainly oxic during the T-OAE, while those in the northeastern basins were mostly anoxic or even sulfidic. Results for two ocean-atmosphere models (FOAM and MITgcm) suggest the presence of a strong clockwise gyre over the EES, which brought oxygenated equatorial waters from the Tethys Ocean to the southern shelf. The northward limb of the gyre was significantly weakened due to the rough bathymetry of the northern shelf, making this relative small region highly sensitive to local ocean stratification. These sluggish ocean dynamics promoted bottom water anoxia and enhanced burial of organic carbon in the northeastern basins, which accounted for 3–5% of the total carbon extracted from the ocean-atmosphere system as recorded by the positive carbon isotope shift

Impact of cable bacteria on biogeochemical cycling in sediments of a seasonally hypoxic marine basin

Oxygen is a key element for life on earth. It can be taken up by ocean waters via air-sea gas exchange, but is also formed through photosynthesis by phytoplankton in the photic zone. In seawater, oxygen can be consumed through aerobic respiration, where it is used as an electron acceptor in the breakdown of organic matter, in a process known as remineralisation. Dissolved oxygen is necessary for the respiration and metabolism of many marine organisms, several types of which are sensitive to concentrations below thresholds as high as 91 µM O2; yet an oxic water body is defined as containing at least 62.5 µM (2 mg L-1) oxygen. An increased oxygen demand or reduction in oxygen supply can result in oxygen-deficient or hypoxic conditions (< 62.5 µM) and may eventually lead to oxygen depletion or anoxia. Hypoxia and anoxia in coastal waters can occur naturally, mainly affecting water bodies in which water exchange and circulation are restricted. However, low oxygen conditions are becoming increasingly prevalent in the bottom waters of many coastal systems, as a direct consequence of eutrophication, caused by human land-use practices. Increased nutrient input from agricultural run-off and anthropogenic waste disposal can lead to increased primary production in surface waters, where a consequent increase in oxygen demand upon sinking of this organic matter into deeper waters can eventually exceed the supply of dissolved oxygen. Oxygen is the most energetically-favourable electron acceptor for microbial respiration in sediments, but in its absence, a cascade of alternative electron acceptors is available for anaerobic respiration. Anaerobic mineralisation dominates in most coastal sediments due to the high organic matter supply. Iron and manganese-(oxyhyr)oxides can serve as electron acceptors for microbial respiration in the absence of oxygen. Many trace metals can be associated with iron and manganese minerals. Variations in trace metal enrichments in coastal surface sediments can be used as indicators of hypoxic and anoxic conditions in these environments. Iron and manganese cycling also plays a pivotal role in nutrient cycling in coastal environments. Increased availability of nutrients like phosphorus, for autotrophic metabolism, can lead to enhanced primary productivity in coastal surface waters. Under low oxygen concentrations, phosphorus is released from surface sediments, liberated from iron- and manganese-(oxyhydr)oxides into overlying waters. Sedimentary phosphorus cycling is very redox-sensitive and phosphorus recycling can be pivotal to the maintenance of low oxygen conditions in coastal systems, by fuelling primary production and organic matter supply to bottom waters. Furthermore, sulphur-oxidising cable bacteria can influence redox conditions, as well as elemental recycling and burial in coastal sediments. Cable bacteria have been observed to link sulphide oxidation to the reduction of oxygen over centimetre-long distances in the sediment via electrogenic sulphur oxidation (e-SOx). Cable bacteria induce a range of secondary biogeochemical reactions including dissolution and precipitation of minerals. For example, proton generation associated with anodic sulphide oxidation can lead to dissolution of sulphide and carbonate minerals and mobilisation of calcium, iron and sulphate ions to the pore water. Cable bacteria have been detected in many aquatic systems worldwide and are now known to occur in environments ranging from hot vents, freshwater and marine sediments, to mangroves and even aquifers

Impact of cable bacteria on biogeochemical cycling in sediments of a seasonally hypoxic marine basin

Oxygen is a key element for life on earth. It can be taken up by ocean waters via air-sea gas exchange, but is also formed through photosynthesis by phytoplankton in the photic zone. In seawater, oxygen can be consumed through aerobic respiration, where it is used as an electron acceptor in the breakdown of organic matter, in a process known as remineralisation. Dissolved oxygen is necessary for the respiration and metabolism of many marine organisms, several types of which are sensitive to concentrations below thresholds as high as 91 µM O2; yet an oxic water body is defined as containing at least 62.5 µM (2 mg L-1) oxygen. An increased oxygen demand or reduction in oxygen supply can result in oxygen-deficient or hypoxic conditions (< 62.5 µM) and may eventually lead to oxygen depletion or anoxia. Hypoxia and anoxia in coastal waters can occur naturally, mainly affecting water bodies in which water exchange and circulation are restricted. However, low oxygen conditions are becoming increasingly prevalent in the bottom waters of many coastal systems, as a direct consequence of eutrophication, caused by human land-use practices. Increased nutrient input from agricultural run-off and anthropogenic waste disposal can lead to increased primary production in surface waters, where a consequent increase in oxygen demand upon sinking of this organic matter into deeper waters can eventually exceed the supply of dissolved oxygen. Oxygen is the most energetically-favourable electron acceptor for microbial respiration in sediments, but in its absence, a cascade of alternative electron acceptors is available for anaerobic respiration. Anaerobic mineralisation dominates in most coastal sediments due to the high organic matter supply. Iron and manganese-(oxyhyr)oxides can serve as electron acceptors for microbial respiration in the absence of oxygen. Many trace metals can be associated with iron and manganese minerals. Variations in trace metal enrichments in coastal surface sediments can be used as indicators of hypoxic and anoxic conditions in these environments. Iron and manganese cycling also plays a pivotal role in nutrient cycling in coastal environments. Increased availability of nutrients like phosphorus, for autotrophic metabolism, can lead to enhanced primary productivity in coastal surface waters. Under low oxygen concentrations, phosphorus is released from surface sediments, liberated from iron- and manganese-(oxyhydr)oxides into overlying waters. Sedimentary phosphorus cycling is very redox-sensitive and phosphorus recycling can be pivotal to the maintenance of low oxygen conditions in coastal systems, by fuelling primary production and organic matter supply to bottom waters. Furthermore, sulphur-oxidising cable bacteria can influence redox conditions, as well as elemental recycling and burial in coastal sediments. Cable bacteria have been observed to link sulphide oxidation to the reduction of oxygen over centimetre-long distances in the sediment via electrogenic sulphur oxidation (e-SOx). Cable bacteria induce a range of secondary biogeochemical reactions including dissolution and precipitation of minerals. For example, proton generation associated with anodic sulphide oxidation can lead to dissolution of sulphide and carbonate minerals and mobilisation of calcium, iron and sulphate ions to the pore water. Cable bacteria have been detected in many aquatic systems worldwide and are now known to occur in environments ranging from hot vents, freshwater and marine sediments, to mangroves and even aquifers

Bacterially-induced dissolution of calcite: the role of bacteria in limestone weathering

The interaction between microorganisms and the calcite mineral surface in aqueous solutions, under earth surface conditions, was the focus of this study. More specifically, we investigated if bacterial attachment and metabolism increase the dissolution rates of calcite crystals and alter their surfaces in solution. A natural microbial consortium, rather than model organisms, was used in the experiments. Weathered samples from the Trenton carbonates were collected on the flanks of Mount Royal in Montréal (Québec, Canada). The associated bacteria were identified using molecular biology DNA fingerprinting techniques. This information was used to determine the nutrient requirements of suitable growth media. Samples contained typical soil dwelling organisms from the phylum Actinobacteria, gram-positive heterotrophs. Bacteria were combined with cleaved Iceland Spar calcite rhombohedra in a low-ionic strength (10−2 M) NaCl solution at ambient pCO2 , 25°C and 1 atm pressure. The effect of solution chemistry (e.g. the presence of phosphate) on the calcite dissolution kinetics was also investigated. The dissolution rates in the presence of bacteria, did not vary significantly from abiotic conditions, but decreased notably in the presence of phosphate.Cette étude porte sur les interactions entre des micro-organismes et la surface de la calcite en solution aqueuse sous des conditions équivalentes à celles de la surface de la terre. Plus précisément, nous avons étudié si l'attachement des bactéries et leur métabolisme augmentent la vitesse de dissolution des cristaux de calcite et altérent leur surface en solution. Des communautés microbiennes naturelles ont été privilégiées à des organismes types pour les expériences. Des échantillons altérés provenant de carbonates de Trenton ont été récoltés sur les flancs du Mont Royal à Montréal (Québec, Canada). Les bactéries associées ont été identifiées par des techniques de biologie moléculaire utilisant leurs empreintes génétiques d'ADN. Ces informations ont servi à déterminer les besoins en nutriments des milieux de croissance. Les échantillons contenaient des organismes typiques de sols, hétérotrophes, à gram positif, du phylum Actinobacteria. Les bactéries ont été combinées avec des rhombohèdres clivés de calcite provenant de spaths d'Islande dans une solution de NaCl de faible force ionique (10−2 M) à pCO2 ambiante, 25°C et 1 atm de pression. L'effet de la composition chimique de la solution sur la cinétique de dissolution des calcites (en particulier, la présence de phosphates) a également été étudié. Les vitesses de dissolution augmentent en présence de bactéries ne varient pas de façon significative aux échantillons exposés aux conditions abiotiques. En revanche, la présence de phosphate das le milieu de culture masque l'effet des bactéries sur la vitesse de dissolution