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

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

Biogeochemical processes and buffering capacity concurrently affect acidification in a seasonally hypoxic coastal marine basin

Coastal areas are impacted by multiple natural and anthropogenic processes and experience stronger pH fluctuations than the open ocean. These variations can weaken or intensify the ocean acidification signal induced by increasing atmospheric pCO2. The development of eutrophication-induced hypoxia intensifies coastal acidification, since the CO2 produced during respiration decreases the buffering capacity in any hypoxic bottom water. To assess the combined ecosystem impacts of acidification and hypoxia, we quantified the seasonal variation in pH and oxygen dynamics in the water column of a seasonally stratified coastal basin (Lake Grevelingen, the Netherlands). Monthly water-column chemistry measurements were complemented with estimates of primary production and respiration using O2 light–dark incubations, in addition to sediment–water fluxes of dissolved inorganic carbon (DIC) and total alkalinity (TA). The resulting data set was used to set up a proton budget on a seasonal scale. Temperature-induced seasonal stratification combined with a high community respiration was responsible for the depletion of oxygen in the bottom water in summer. The surface water showed strong seasonal variation in process rates (primary production, CO2 air–sea exchange), but relatively small seasonal pH fluctuations (0.46 units on the total hydrogen ion scale). In contrast, the bottom water showed less seasonality in biogeochemical rates (respiration, sediment–water exchange), but stronger pH fluctuations (0.60 units). This marked difference in pH dynamics could be attributed to a substantial reduction in the acid–base buffering capacity of the hypoxic bottom water in the summer period. Our results highlight the importance of acid–base buffering in the pH dynamics of coastal systems and illustrate the increasing vulnerability of hypoxic, CO2-rich waters to any acidifying process

Cable bacteria control iron-phosphorus dynamics in sediments and water column at two sites of a Coastal Hypoxic Basin

Phosphorus is an essential nutrient for life. The release of phosphorus from sediments is critical in sustaining phytoplankton growth in many aquatic systems and is pivotal to eutrophication and the development of bottom water hypoxia. Conventionally, sediment phosphorus release is thought to be controlled by changes in iron oxide reduction driven by variations in external environmental factors, such as organic matter input and bottom water oxygen. Here, we show that internal shifts in microbial communities, and specifically the population dynamics of cable bacteria, can also induce strong seasonality in sedimentary iron-phosphorus dynamics. Field observations in a seasonally hypoxic coastal basin demonstrate that the long-range electrogenic metabolism of cable bacteria leads to a dissolution of iron sulfides in winter and spring. Subsequent oxidation of the mobilized ferrous iron with manganese oxides results in a large stock of iron-oxide-bound phosphorus below the oxic zone. In summer, when bottom water hypoxia develops and cable bacteria are undetectable, the phosphorus associated with these iron oxides is released, strongly increasing phosphorus availability in the water column. Future research should elucidate whether formation of iron-oxide-bound phosphorus driven by cable bacteria, as observed in this study, contributes to the seasonality in iron-phosphorus cycling in aquatic sediments worldwid