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The impact of electrogenic sulfur oxidation on the biogeochemistry of coastal sediments: A field study
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Electro-active sediments distinguish themselves from other sedimentary environments by the presence of microbially induced electrical currents in the surface layer of the sediment. The electron transport is generated by metabolic activity of long filamentous cable bacteria, in a process referred to as electrogenic sulfur oxidation (e-SOx). Laboratory experiments have shown that e-SOx exerts a large impact on the sediment geochemistry, but its influence on the in situ geochemistry of marine sediments has not been previously investigated. Here, we document the biogeochemical cycling associated with e-SOx in a cohesive coastal sediment in the North Sea (Station 130, Belgian Coastal Zone) during three campaigns (January, March and May 2014). Fluorescence in situ hybridization showed that cable bacteria were present in high densities throughout the sampling period, and that filaments penetrated up to 7 cm deep in the sediment, which is substantially deeper than previously recorded. High resolution microsensor profiling (pH, H2S and O2) revealed the typical geochemical fingerprint of e-SOx, with a wide separation (up to 4.8 cm) between the depth of oxygen penetration and the depth of sulfide appearance. The metabolic activity of cable bacteria induced a current density of 25–32 mA m-2 and created an electrical field of 12–17 mV m-1 in the upper centimeters of the sediment. This electrical field created an ionic drift, which strongly affected the depth profiles and fluxes of major cations (Ca2+, Fe2+) and anions (SO42-) in the pore water. The strong acidification of the pore water at depth resulted in the dissolution of calcium carbonates and iron sulfides, thus leading to a strong accumulation of iron, calcium and manganese in the pore water. While sulfate accumulated in the upper centimeters, no significant effect of e-SOx was found on ammonium, phosphate and silicate depth profiles. Overall, our results demonstrate that cable bacteria can strongly modulate the sedimentary biogeochemical cycling under in situ conditions
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The Influence of Bioturbation on Iron and Sulphur Cycling in Marine Sediments: A Model Analysis
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The geochemical cycles of iron and sulphur in marine sediments are strongly intertwined and give rise to a complex network of redox and precipitation reactions. Bioturbation refers to all modes of transport of particles and solutes induced by larger organisms, and in the present-day seafloor, bioturbation is one of the most important factors controlling the biogeochemical cycling of iron and sulphur. To better understand how bioturbation controls Fe and S cycling, we developed reactive transport model of a coastal sediment impacted by faunal activity. Subsequently, we performed a model sensitivity analysis, separately investigating the two different transport modes of bioturbation, i.e. bio-mixing (solid particle transport) and bio-irrigation (enhanced solute transport). This analysis reveals that bio-mixing and bio-irrigation have distinct—and largely opposing effects on both the iron and sulphur cycles. Bio-mixing enhances transport between the oxic and suboxic zones, thus promoting the reduction of oxidised species (e.g. iron oxyhydroxides) and the oxidation of reduced species (e.g. iron sulphides). Through the reoxidation of iron sulphides, bio-mixing strongly enhances the recycling of Fe and S between their reduced and oxidised states. Bio-irrigation on the other hand removes reduced solutes, i.e. ferrous iron and free sulphide, from the sediment pore water. These reduced species are then reoxidised in the overlying water and not recycled within the sediment column, which leads to a decrease in Fe and S recycling. Overall, our results demonstrate that the ecology of the macrofauna (inducing bio-mixing or bio-irrigation, or both) matters when assessing their impact on sediment geochemistry. This finding seems particularly relevant for sedimentary cycling across Cambrian transition, when benthic fauna started colonizing and reworking the seafloor.
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Influence of Natural Oxygenation of Baltic Proper Deep Water on Benthic Recycling and Removal of Phosphorus, Nitrogen, Silicon and Carbon
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At the end of 2014, a Major Baltic Inflow (MBI) brought oxygenated, salty water into the Baltic proper and reached the long-term anoxic Eastern Gotland Basin (EGB) by March 2015. In July 2015, we measured benthic fluxes of phosphorus (P), nitrogen (N) and silicon (Si) nutrients and dissolved inorganic carbon (DIC) in situ using an autonomous benthic lander at deep sites (170–210 m) in the EGB, where the bottom water oxygen concentration was 30–45 μM. The same in situ methodology was used to measure benthic fluxes at the same sites in 2008–2010, but then under anoxic conditions. The high efflux of phosphate under anoxic conditions became lower upon oxygenation, and turned into an influx in about 50% of the flux measurements. The C:P and N:P ratios of the benthic solute flux changed from clearly below the Redfield ratio (on average about 70 and 3–4, respectively) under anoxia to approaching or being well above the Redfield ratio upon oxygenation. These observations demonstrate retention of P in newly oxygenated sediments. We found no significant effect of oxygenation on the benthic ammonium, silicate and DIC flux. We also measured benthic denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) rates at the same sites using isotope-pairing techniques. The bottom water of the long-term anoxic EGB contained less than 0.5 μM nitrate in 2008–2010, but the oxygenation event created bottom water nitrate concentrations of about 10 μM in July 2015 and the benthic flux of nitrate was consistently directed into the sediment. Nitrate reduction to both dinitrogen gas (denitrification) and ammonium (DNRA) was initiated in the newly oxygenated sediments, while anammox activity was negligible. We estimated the influence of this oxygenation event on the magnitudes of the integrated benthic P flux (the internal P load) and the fixed N removal through benthic and pelagic denitrification by comparing with a hypothetical scenario without the MBI. Our calculations suggest that the oxygenation triggered by the MBI in July 2015, extrapolated to the basin-wide scale of the Baltic proper, decreased the internal P load by 23% and increased the total (benthic plus pelagic) denitrification by 18%.
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Early Palaeozoic ocean anoxia and global warming driven by the evolution of shallow burrowing
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The evolution of burrowing animals forms a defining event in the history of the Earth. It has been hypothesised that the expansion of seafloor burrowing during the Palaeozoic altered the biogeochemistry of the oceans and atmosphere. However, whilst potential impacts of bioturbation on the individual phosphorus, oxygen and sulphur cycles have been considered, combined effects have not been investigated, leading to major uncertainty over the timing and magnitude of the Earth system response to the evolution of bioturbation. Here we integrate the evolution of bioturbation into the COPSE model of global biogeochemical cycling, and compare quantitative model predictions to multiple geochemical proxies. Our results suggest that the advent of shallow burrowing in the early Cambrian contributed to a global low-oxygen state, which prevailed for ~100 million years. This impact of bioturbation on global biogeochemistry likely affected animal evolution through expanded ocean anoxia, high atmospheric CO2 levels and global warming.
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Burrowing fauna mediate alternative stable states in the redox cycling of salt marsh sediments
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The East Anglian salt marsh system (UK) has recently generated intriguing data with respect to sediment biogeochemistry. Neighbouring ponds in these salt marshes show two distinct regimes of redox cycling: the sediments are either iron-rich and bioturbated, or they are sulphide-rich and unbioturbated. No conclusive explanation has yet been given for this remarkable spatial co-occurrence. Here, we quantify the geochemical cycling in both pond types, using pore-water analyses and solid-phase speciation. Our results demonstrate that differences in solid-phase carbon and iron inputs are likely small between pond types, and so these cannot act as the direct driver of the observed redox dichotomy. Instead, our results suggest that the presence of bioturbation plays a key role in the transition from sulphur-dominated to iron-dominated sediments. The presence of burrowing fauna in marine sediments stimulates the mineralisation of organic matter, increases the iron cycling and limits the build-up of free sulphide. Overall, we propose that the observed dichotomy in pond geochemistry is due to alternative stable states, which result from non-linear interactions in the sedimentary iron and sulphur cycles that are amplified by bioturbation. This way, small differences in solid phase input can result in very different regimes of redox cycling due to positive feedbacks. This non-linearity in the iron and sulphur cycling could be an inherent feature of marine sediments, and hence, alternative stable states could be present in other systems.
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Quantification of Cable Bacteria in Marine Sediments via qPCR
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Cable bacteria (Deltaproteobacteria, Desulfobulbaceae) are long filamentous sulfur-oxidizing bacteria that generate long-distance electric currents running through the bacterial filaments. This way, they couple the oxidation of sulfide in deeper sediment layers to the reduction of oxygen or nitrate near the sediment-water interface. Cable bacteria are found in a wide range of aquatic sediments, but an accurate procedure to assess their abundance is lacking. We developed a qPCR approach that quantifies cable bacteria in relation to other bacteria within the family Desulfobulbaceae. Primer sets targeting cable bacteria, Desulfobulbaceae and the total bacterial community were applied in qPCR with DNA extracted from marine sediment incubations. Amplicon sequencing of the 16S rRNA gene V4 region confirmed that cable bacteria were accurately enumerated by qPCR, and suggested novel diversity of cable bacteria. The conjoint quantification of current densities and cell densities revealed that individual filaments carry a mean current of ~110 pA and have a cell specific oxygen consumption rate of 69 fmol O2 cell-1 day-1. Overall, the qPCR method enables a better quantitative assessment of cable bacteria abundance, providing new metabolic insights at filament and cell level, and improving our understanding of the microbial ecology of electrogenic sediments.
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Bistability in the redox chemistry of sediments and oceans
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For most of Earth’s history, the ocean’s interior was pervasively anoxic and showed occasional shifts in ocean redox chemistry between iron-buffered and sulfide-buffered states. These redox transitions are most often explained by large changes in external inputs, such as a strongly altered delivery of iron and sulfate to the ocean, or major shifts in marine productivity. Here, we propose that redox shifts can also arise from small perturbations that are amplified by nonlinear positive feedbacks within the internal iron and sulfur cycling of the ocean. Combining observational evidence with biogeochemical modeling, we show that both sedimentary and aquatic systems display intrinsic iron–sulfur bistability, which is tightly linked to the formation of reduced iron–sulfide minerals. The possibility of tipping points in the redox state of sediments and oceans, which allow large and nonreversible geochemical shifts to arise from relatively small changes in organic carbon input, has important implications for the interpretation of the geological rock record and the causes and consequences of major evolutionary transitions in the history of Earth’s biosphere.
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Prospections sur la « villa gallo-romaine du Sart », au lieu-dit La Hamaide, à Hautrage (B).
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Hautrage, une commune belge appartenant à l’entité de Saint-Ghislain, se situe dans la partie occidentale de la province du Hainaut. Plusieurs découvertes archéologiques, la plupart anciennes et souvent mal documentées, sont mentionnées sur son territoire1 . 1. Historique des recherches, le site (J. DUFRASNES) La première mention de ce site archéologique apparaît dans HAUBOUDIN 1898, p. 80. L’auteur, un archéologue amateur très actif habitant Stambruges2 , note alors : Villa du Sart.- Un chemin, qui porte encore le nom de chemin d’El Ville3 , indique la Villa du Sart qui est proche. Le cimetière était la nécropole de cette villa4 . Des notes manuscrites datées de janvier 19415 et un inventaire réalisé à l’occasion de l’entrée d’une partie de la collection d’Edmond Haubourdin aux Musées royaux d’Art et d’Histoire de Bruxelles mentionnent du matériel susceptible de provenir de la villa de la Hamaide à Hautrage, c’est-à-dire de la « villa du Sart » (fig. 1). Cependant, aucun des écrits laissés par cet archéologue ne relate qu’il obtint du matériel archéologique de ce site par acquisition, par des fouilles ou des prospections. Edmond Haubourdin ajoute : « Diverticulum.- Le chemin qui traverse le chemin de fer à la station de la Hamaide porte, au cadastre d’Hautrage, le nom de chaussée romaine. Cette chaussée [dont l’actuelle rue des Bats, à 300 mètres à l’est du site, serait un tronçon selon l’auteur ndlr] passait près de la villa et se dirigeait vers Bavay, probablement vers Chièvres, par Villerot, Sirault et Neufmaison, où elle se confondait avec le diverticulum qui venait de Condé par Stambruges. La grande largeur de cette voie de communication accuse son origine romaine »6
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RBINS Staff Publications 2022
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Bistability in the redox chemistry of sediments and oceans
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For most of Earth’s history, the ocean’s interior was pervasively anoxic and showed occasional shifts in ocean redox chemistry between iron-buffered and sulfide-buffered states. These redox transitions are most often explained by large changes in external inputs, such as a strongly altered delivery of iron and sulfate to the ocean, or major shifts in marine productivity. Here, we propose that redox shifts can also arise from small perturbations that are amplified by nonlinear positive feedbacks within the internal iron and sulfur cycling of the ocean. Combining observational evidence with biogeochemical modeling, we show that both sedimentary and aquatic systems display intrinsic iron–sulfur bistability, which is tightly linked to the formation of reduced iron–sulfide minerals. The possibility of tipping points in the redox state of sediments and oceans, which allow large and nonreversible geochemical shifts to arise from relatively small changes in organic carbon input, has important implications for the interpretation of the geological rock record and the causes and consequences of major evolutionary transitions in the history of Earth’s biosphere
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RBINS Staff Publications 2020
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Cryptophyllium, the hidden leaf insects – descriptions of a new leaf insect genus and thirteen species from the former celebicum species group (Phasmatodea, Phylliidae)
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RBINS Staff Publications 2021
