Sulfate Reduction In Sediments Research Articles

Depth-dependent transformation of ZnO and Ag nanoparticles in sulfate-reducing sediments tracked using scanning transmission electron microscopy

The transformation of Ag and ZnO nanoparticles was investigated along pore water depth gradients in sulfate-reducing sediments. This work highlights the benefits of tailored experimental strategies to study nanoparticle environmental transformations.

Filamentous electroactive microorganisms promote mass transfer and sulfate reduction in sediment microbial electrochemical systems

Electron transfer and mass transfer are critical processes for efficient sediment remediation using sediment microbial electrochemical systems (SMESs). Increasing reports show that filamentous electroactive microorganisms (f-EAMs) that widely exist in heterogeneous sediments play essential roles in biogeochemical cycles and sediment remediation, significantly impacting process efficiencies. However, the knowledge of the mechanism of f-EAMs in biogeochemical cycles and sediment remediation is still limited. This study investigated the spatiotemporal effect of a unicellular Gram-positive f-EAM, Lysinibacillus varians GY32, on the sulfur cycle in SMESs. The results showed that adding L. varians GY32 into sediments could significantly decrease the diffusion resistances of the sediments by up to 49.4% and reduce the porewater sulfate concentrations by up to 46.4%, which validated that L. varians GY32 contributed to enhancing the mass transfer and sulfate reduction in sediments. Meanwhile, the vertical difference in diffusion resistances and porewater sulfate concentrations was reduced. The dynamics of electrochemical and physicochemical properties of sediments coincided well with the enrichment of filamentous microorganisms, sulfate-reducing bacteria, EAMs (especially f-EAMs), and syntrophic bacteria, as well as the reinforcement of the community interactions that took the above microorganisms as core members. Our findings provide novel insights for understanding the underlying mechanism of f-EAMs in biogeochemical cycles and sediment remediation.

The effects of organic matter and anaerobic oxidation of methane on the microbial sulfate reduction in cold seeps

Cold seep sediments are dominated by intensive microbial sulfate reduction coupled to anaerobic oxidation of methane. However, the contribution proportion between this process and the role of organic matter has remained enigmatic. Here, pore water data combined with PROFILE model, fluxes of sulfate and methane concentration calculated from Fick's first law, and δ34SSO4 and δ18OSO4 of pore water sulfate were studied to reconstruct co-occurring microbial organoclastic sulfate reduction and anaerobic oxidation of methane coupled with sulfate reduction in methane seep sediments collected from South China Sea. The sulfate concentration profiles of C9 and C14 in Qiongdongnan Basin generally show quasilinear depletion with depth. Reaction-transport modeling provided close fits to concentration data. δ18OSO4 and δ34SSO4 increase fastest with sediment depth above 400 cmbsf and slowest below that depth. The values of methane flux are always lower than those of total sulfate reduction of sulfate diffusive flux at GC-10, GC-9, GC-11 and HD319 sites in Taixinan Basin. Besides, positions of sulfate methane transition zone in all study sites are approximately ~400 to 800 centimeters below seafloor. These results showed that microbial sulfate reduction in sediments is mainly controlled by intense anaerobic oxidation of methane, but there is a certain relationship with organic matter metabolism process. This emphasizes that traditional redox order of bacterial respiration is highly simplified, where, in sediments such as these seeps, all of these microbial sulfate reduction processes can occur together with complex couplings between them.

Effect of algal blooms outbreak and decline on phosphorus migration in Lake Taihu, China

Algal blooms (ABs) can affect the migration of phosphorus (P) among sediments, interstitial water and overlying water. It is important to analyze the characteristics of P and their interactions in the three media during ABs. A 5-month field study (June to October in 2016) was conducted in Zhushan Bay of Lake Taihu. P fractions, P adsorption characteristics and P diffusion fluxes at the sediment-water interface (SWI) were investigated. During the outbreak period of ABs from June to August, labile P concentrations in the sediment measured by diffusive gradients in thin films (DGT-labile P) and its diffusion fluxes across the SWI increased significantly. The equilibrium P concentration (EPC0) of the sediment was higher than the PO43--P concentration in the overlying water. During the period of decline of ABs from September to October, the concentrations and diffusion fluxes of DGT-labile P sharply decreased. However, the sediment total P (TP), overlying water TP, total dissolved P (TDP) and PO43--P concentrations increased. These results show that the ability of sediment solids to supplement interstitial water labile P was significantly enhanced by the outbreak of ABs. Labile P was then intensively released into the overlying water by interstitial water. Degraded algae became a crucial P source during the period of decline of ABs. P from the degraded algae was re-released to the sediment and overlying water. The observed DGT-labile P and DGT-labile Fe coupling in June, September and October confirmed the Fe redox-driven P release mechanism in sediment during these periods. The decoupling of DGT-labile P and DGT-labile Fe was observed in July and August and was probably caused by algal decomposition, labile organic P degradation and/or sulfate reduction in sediment stimulated by the ABs outbreak.

An Assessment of xsBa Flux as a Paleoproductivity Indicator and Its Water‐Depth Dependence in the Easternmost Equatorial Pacific Ocean

AbstractWe present the first regional‐scale records of biogenic Barium (xsBa) fluxes in the Panama basin of the eastern‐equatorial margin of the Pacific Ocean in order to assess xsBa as a paleoproductivity proxy. Measurements of xsBa from 13 cores that range in water depths from about 700 to 3,000 m show an increase in 230Th‐normalized xsBa mass accumulation rates (MARs) with increasing water depth during both marine oxygen isotope stages (MIS) 1 and 2. The correlation of xsBa MARs with depth are strong despite differences in bulk sediment MARs and differing degrees of sediment redistribution. We interpret the increasing xsBa with water depth as likely due to the continued decomposition and remineralization of falling and/or resuspended biogenic particles. xsBa does not seem to be affected by diagenetic sulfate reduction in most of the cores. Calculated estimates of xsBa preservation in the sediment pile are high and fluctuate between 45% and 52% throughout the last 25 kyr. Although xsBa fluxes can be a robust indicator of paleoproductivity, caution is needed if (a) there is evidence of sulfate reduction in sediments being analyzed, and (b) one is trying to quantify differences in paleoproductivity among sites that are located at different depths in the water column.

Reduction of iron (hydr)oxide-bound arsenate: Evidence from high depth resolution sampling of a reducing aquifer in Yinchuan Plain, China

Sediment in fluvial-deltaic plains with high-As groundwater is heterogenous but its characterization of As and Fe oxidation states lacks resolution, and is rarely attempted for aqueous and solid phases simultaneously. Here, we pair high-resolution (> 1 sample/meter) Fe extended fine-structure spectroscopy (EXAFS, n = 40) and As X-ray absorption near-edge spectroscopy (XANES, n = 49) with groundwater composition and metagenomics measurements for two sediment cores and their associated wells (n = 8) from the Yinchuan Plain in northwest China. At shallower depths, nitrate and Mn/Fe reducing sediment zones are fine textured, contain 9.6 ± 5.6 mg kg−1 of As(V) and 2.3 ± 2.7 mg kg−1 of As(III) with 9.1 ± 8.1 g kg−1 of Fe(III) (hydr)oxides, with bacterial genera capable of As and Fe reduction identified. In four deeper 10-m sections, sulfate-reducing sediments are coarser and contain 2.6 ± 1.3 mg kg−1 of As(V) and 1.1 ± 1.0 mg kg−1 of As(III) with 3.2 ± 2.6 g kg−1 of Fe(III) (hydr)oxides, even though groundwater As concentrations can exceed 200 μg/L, mostly as As(III). Super-enrichment of sediment As (42–133 mg kg−1, n = 7) at shallower depth is due to redox trapping during past groundwater discharge. Active As and Fe reduction is supported by the contrast between the As(III)-dominated groundwater and the As(V)-dominated sediment, and by the decreasing sediment As(V) and Fe(III) (hydr)oxides concentrations with depth.

Methyl-compounds driven benthic carbon cycling in the sulfate-reducing sediments of South China Sea.

Methane is a potent greenhouse gas; methane production and consumption within seafloor sediments has generated intense interest. Anaerobic oxidation of methane (AOM) and methanogenesis (MOG) primarily occur at the depth of the sulfate-methane transition zone or underlying sediment respectively. Methanogenesis can also occur in the sulfate-reducing sediments through the utilization of non-competitive methylated compounds; however, the occurrence and importance of this process are not fully understood. Here, we combined a variety of data, including geochemical measurements, rate measurements and molecular analyses to demonstrate the presence of a cryptic methane cycle in sulfate-reducing sediments from the continental shelf of the northern South China Sea. The abundance of methanogenic substrates as well as the high MOG rates from methylated compounds indicated that methylotrophic methanogenesis was the dominant methanogenic pathway; this conclusion was further supported by the presence of the methylotrophic genus Methanococcoides. High potential rates of AOM were observed in the sediments, indicating that methane produced in situ could be oxidized simultaneously by AOM, presumably by ANME-2a/b as indicated by 16S rRNA gene analysis. A significant correlation between the relative abundance of methanogens and methanotrophs was observed over sediment depth, indicating that methylotrophic methanogenesis could potentially fuel AOM in this environment. In addition, higher potential rates of AOM than sulfate reduction rates at in situ methane conditions were observed, making alternative electron acceptors important to support AOM in sulfate-reducing sediment. AOM rates were stimulated by the addition of Fe/Mn oxides, suggesting AOM could be partially coupled to metal oxide reduction. These results suggest that methyl-compounds driven methane production drives a cryptic methane cycling and fuels AOM coupled to the reduction of sulfate and other electron acceptors.

Pyrite δ34S and Δ33S constraints on sulfur cycling at sublacustrine hydrothermal vents in Yellowstone Lake, Wyoming, USA

Sulfur isotope values (δ34S and Δ33S) of pyrite in sediment from steam-heated hydrothermal vents on the floor of Yellowstone Lake (WY) were measured using secondary ionization mass spectrometry (SIMS). The high resolution of the SIMS data place important constraints on sulfur cycling processes at/near the vent fluid-lake water interface. Pyrite with a distinct mantle-basalt (δ34S = 0‰) isotope composition (δ34S = +0.5 to +3.1‰) replaces pyrrhotite during incipient stages of alteration at moderately high temperature. Disseminated cubic pyrite (δ34S = +2.0 to +5.3‰) occurs in zones where more extensive oxidation is likely. Framboidal pyrite with δ34S values ranging from –5.2 to +4.1‰ and Δ33S up to +0.30‰ suggest formation from low-temperature microbial sulfate reduction in sediments near but not directly in the vent fluid up-flow zone. The co-occurrence of pyrite with S isotope values characteristic of distinct formation processes, coupled with notable intra-crystal S isotope variations, suggests the venting locus is dynamic in time and space.

Spatially Variable Bioturbation and Physical Mixing Drive the Sedimentary Biogeochemical Seascape in the Louisiana Continental Shelf Hypoxic Zone.

Seasonal hypoxia on the Louisiana continental shelf (LCS) has grown to over 22,000 km2 with limited information available on how low oxygen effects the benthos. Benthic macrofaunal colonization and sediment biogeochemical parameters were characterized at twelve stations in waters 10 - 50 m deep along four transects spanning 320 km across the LCS hypoxic zone in the early fall of 2010 when bottom waters typically return to oxic conditions. Chemical data and sediment profile imaging (SPI) support three primary mechanistic pathways of organic matter degradation on the LCS: (i) metal oxide cycling in depositional muds, (ii) infauna-driven bioturbation delivering oxygen below the sediment-water interface, and (iii) sulfate reduction in sediments where iron oxide availability is limited. The transect nearest the Mississippi River delta had the highest concentrations of porewater and solid phase Mn and Fe with SPI images of recently deposited reddish, mixed muddy sediments suggestive of metal cycling. The deepest stations had high oxidized iron concentrations and rust colored sediments with faunal colonization that suggests sediments are oxidized via bioturbation. Many nearshore and central LCS stations had more black sediments, more disturbed clay layers, lower amounts of oxidized iron, and higher sulfate reduction rates than the deepest stations. Sediment mixing coefficients, DB , determined from chlorophyll-a concentration profiles varied between 33 and 183 cm-2 y-1. DB values were highest at the deepest stations where sediments were colonized. DB were not determined at two nearshore stations where chlorophyll-a concentrations were highly variable in surficial sediments, and on the eastern shelf where sedimentation is high. This study provides a regional view of benthic faunal colonization and sediment biogeochemistry on the LCS, describes regions with potentially different pathways of organic matter degradation, and demonstrates the importance of both bioturbation and physical mixing in processing the large amounts of organic matter in river-dominated continental shelf systems.

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

Abstract. We report a data set of biogeochemical variables related to carbon cycling obtained in the three branches (Mỹ Tho, Hàm Luông, Co^´ Chiên) of the Mekong delta (Be^´n Tre province, Vietnam) in December 2003, April 2004, and October 2004. Both the inner estuary (upstream of the mouth) and the outer estuary (river plume) were sampled, as well as side channels. The values of the partial pressure of CO2 (pCO2) ranged between 232 and 4085 ppm, O2 saturation level (%O2) between 63 and 114 %, and CH4 between 2 and 2217 nmol L−1, within the ranges of values previously reported in temperate and tropical meso- and macro-tidal estuaries. Strong seasonal variations were observed. In the upper oligohaline estuary, low pCO2 (479–753 ppm) and high %O2 (98–106 %) values were observed in April 2004 most probably related to freshwater phytoplankton growth owing to low freshwater discharge (1400 m3 s−1) and increase in water residence time; during the two other sampling periods with a higher freshwater discharge (9300–17 900 m3 s−1), higher pCO2 (1895–2664 ppm) and lower %O2 (69–84 %) values were observed in the oligohaline part of the estuary. In October 2004, important phytoplankton growth occurred in the offshore part of the river plume as attested by changes in the contribution of particulate organic carbon (POC) to total suspended matter (TSM) (%POC) and the stable isotope composition of POC (δ13C-POC), possibly related to low TSM values (improvement of light conditions for phytoplankton development), leading to low pCO2 (232 ppm) and high %O2 (114 %) values. Water in the side channels in the Mekong delta was strongly impacted by inputs from the extensive shrimp farming ponds. The values of pCO2, CH4, %O2, and the stable isotope composition of dissolved inorganic carbon (δ13C-DIC) indicated intense organic matter degradation that was partly mediated by sulfate reduction in sediments, as revealed by the slope of total alkalinity (TA) and DIC covariations. The δ13C-POC variations also indicated intense phytoplankton growth in the side channels, presumably due to nutrient enrichment related to the shrimp farming ponds. A data set in the mangrove creeks of the Ca Mau province (part of the Mekong delta) was also acquired in April and October 2004. These data extended the range of variability in pCO2 and %O2 with more extreme values than in the Mekong delta (Be^´n Tre), with maxima and minima of 6912 ppm and 37 %, respectively. Similarly, the maximum CH4 concentration (686 nmol L−1) was higher in the Ca Mau province mangrove creeks than in the Mekong delta (Be^´n Tre, maximum 222 nmol L−1) during the October 2004 cruise (rainy season and high freshwater discharge period). In April 2004 (dry season and low freshwater discharge period), the CH4 values were much lower than in October 2004 (average 19 ± 13 and 210 ± 158 nmol L−1, respectively) in the Ca Mau province mangrove creeks, owing to the higher salinity (average 33.2 ± 0.6 and 14.1 ± 1.2, respectively) that probably led to higher sediment sulfate reduction, leading to inhibition of sediment methanogenesis and higher anaerobic CH4 oxidation. In the inner estuarine region (three branches of the Mekong delta), CO2 emissions to the atmosphere averaged 121 mmol m−2 d−1, and the CH4 emissions averaged 118 µmol m−2 d−1. The CO2 emission to the atmosphere from the Mekong inner estuary was higher than reported in the Yangtze and Pearl river inner estuaries. This was probably due to the lower salinity in the Mekong delta branches, possibly due to different morphology: relatively linear channels in the Mekong delta versus funnel-shaped estuaries for the Yangtze and Pearl river inner estuaries.

Controls on the Variation of Methylmercury Concentration in Seagrass Bed Consumer Organisms of the Big Bend, Florida, USA

Mercury poses both ecosystem and human health concerns that are exacerbated by the production of methylmercury (CH3Hg+; MeHg), which is often associated with sulfate-reducing bacteria (SRB) in marine sediments. We examined the relationship between MeHg concentrations and reduced-sulfur stable isotope δ34S values in four coastal consumer organisms collected from northern and southern regions of Florida’s Big Bend seagrass meadows: pinfish (Lagodon rhomboides), pigfish (Orthopristis chrysoptera), black sea bass (Centropristis striata), and shrimp (Tozeuma carolinense and Tozeuma serratum). We tested the hypothesis that depletion in δ34S, which is an indicator of the extent of sulfate reduction within sediments of seagrass dominated systems, would be related to the concentration of MeHg in fauna within these systems. We found that as δ34S became more depleted, indicating more relative sulfate reduction in sediments, the MeHg concentrations increased significantly (r = 0.52; p < 0.001; total n = 157). Stable δ34S values ranged from 17.4 to 4.8‰ and MeHg concentrations from 37.3 to 832 ng g−1 dry weight (DW). Correlations between tissue MeHg and δ13C were also observed (r = 0.44; p < 0.001; total n = 122), indicating that variations in benthic/water column feeding could also be important in controlling the MeHg distributions in the selected fauna. In addition, we observed spatial differences in δ34S, δ13C, and δ15N isotopic values and MeHg concentrations of consumers. Fauna from the southern region of the study area were 13C enriched, 15N depleted, 34S depleted, and had elevated MeHg concentrations relative to their northern Big Bend counterparts.

Experimental Evidence for Abiotic Sulfurization of Marine Dissolved Organic Matter

Dissolved organic sulfur (DOS) is the largest pool of organic sulfur in the oceans, and as such it is an important component of the global sulfur cycle. DOS in the ocean is resistant against microbial degradation and turns over on a millennium time scale. However, sources and mechanisms behind its stability are largely unknown. Here, we hypothesize that in sulfate-reducing sediments sulfur is abiotically incorporated into dissolved organic matter (DOM) and released to the ocean. We exposed natural seawater and the filtrate of a plankton culture to sulfidic conditions. Already after one hour at 20 °C, DOS concentrations had increased 4-fold in these experiments, and 14-fold after 4 weeks at 50 °C, indicating that organic matter does not need long residence times in natural sulfidic environments to be affected by sulfurization. Molecular analysis via ultrahigh-resolution mass spectrometry showed that sulfur was covalently and unselectively bound to DOM. Experimentally produced and natural DOS from sediments were highly similar on a molecular and structural level. By combining our data with published benthic DOC fluxes we estimate that 30 – 200 Tg DOS are annually transported from anaerobic and sulfate reducing sediments to the oceans. Uncertainties in this first speculative assessment are large. However, this first attempt illustrates that benthic DOS flux is potentially one order of magnitude larger than that via rivers indicating that this could balance the estimated global net removal of refractory DOS.

Sulfate Reduction in Sediments Produces High Levels of Chromophoric Dissolved Organic Matter

Sulfate reduction plays an important role in altering dissolved organic matter (DOM) in estuarine and coastal sediments, although its role in the production of optically active chromophoric DOM (CDOM) and a subset of fluorescent DOM (FDOM) has not been previously investigated in detail. Freshwater sediment slurries were incubated anaerobically with added sulfate and acetate to promote sulfate-reducing bacteria. Ultraviolet visible (UV-Vis) absorbance and 3-dimensional excitation emission matrix (EEM) fluorescence spectra were measured over a five weeks anaerobic dark incubation period. Parallel Factor Analysis (PARAFAC) of FDOM determined components that increased significantly during dark and anaerobic incubation matching three components previously considered of terrestrially-derived or humic-like origin published in the OpenFluor database. The observed FDOM increase was strongly correlated (R2 = 0.96) with the reduction of sulfate. These results show a direct experimental link between sulfate reduction and FDOM production, which impacts our understanding of coastal FDOM sources and early sediment diagenesis. As 3D fluorescence techniques are commonly applied to diverse systems, these results provide increasing support that FDOM can have many diverse sources not consistently captured by common classifications such as “humic-like” fluorescence.

Impact of Aeolian Dry Deposition of Reactive Iron Minerals on Sulfur Cycling in Sediments of the Gulf of Aqaba.

The Gulf of Aqaba is an oligotrophic marine system with oxygen-rich water column and organic carbon-poor sediments (≤0.6% at sites that are not influenced by anthropogenic impact). Aeolian dust deposition from the Arabian, Sinai, and Sahara Deserts is an important source of sediment, especially at the deep-water sites of the Gulf, which are less affected by sediment transport from the Arava Desert during seasonal flash floods. Microbial sulfate reduction in sediments is inferred from the presence of pyrite (although at relatively low concentrations), the presence of sulfide oxidation intermediates, and by the sulfur isotopic composition of sulfate and solid-phase sulfides. Saharan dust is characterized by high amounts of iron minerals such as hematite and goethite. We demonstrated, that the resulting high sedimentary content of reactive iron(III) (hydr)oxides, originating from this aeolian dry deposition of desert dust, leads to fast re-oxidation of hydrogen sulfide produced during microbial sulfate reduction and limits preservation of reduced sulfur in the form of pyrite. We conclude that at these sites the sedimentary sulfur cycle may be defined as cryptic.

Carbon isotope biogeochemistry of acetate in sub-seafloor sediments in the Sea of Okhotsk near Sakhalin Island, Russia

Biogeochemical processes involving acetate in sub-seafl oor sediments were investigated by examining the stable carbon isotopic relationships of acetate and other relevant carbon-bearing materials (i.e., total organic carbon (TOC) and ΣCO2) in fi ve piston cores retrieved from the Sea of Okhotsk off Sakhalin Island, Russia. The cores were classifi ed into two types on the basis of SO42– content: (I) those with sulfate-reducing sediments in which the depletion of pore-water SO42– with increasing depth was slight, and (II) those with methanogenic sediments in which SO42– concentrations were less than 2 mM more than 3 m below the seafl oor. Acetate was detected in all cores. The acetate content in methanogenic sediments (2.6–23.0 μM) was relatively higher than in the sulfate-reducing sediment (2.8–8.8 μM). In the sulfate-reducing sediments, the depth profi les of δ13Cacetate approximately parallel those of δ13CTOC, with the δ13Cacetate values (–39‰ to –33‰) depleted by about 12‰ relative to δ13CTOC (–24.5‰ to –22.3‰). These approximately parallel depth profi les suggest that the principal acetate production process in the sulfate-reducing sediments is fermentation of dissolved organic compounds. The fermentation products, however, tend to be similar or slightly enriched in 13C compared to their substrates. Therefore, the 13C depletion of acetate relative to TOC in the sulfate reduction zone suggests that some portion of the acetate was synthesized by acetogenesis in which the synthesized acetate is depleted in 13C compared with its precursor. Given the large contribution of land-derived organic matter in the studied sediments, organoautotrophic acetogenesis using the lignin-derived syringate monomer, which originates from land plants, is likely. In the methanogenic sediments, the δ13Cacetate values in sediments throughout the cores (–39‰ to –25‰) were depleted compared to δ13CTOC (–25.5‰ to –21.4‰). This suggests some acetogenic contribution to the total acetate production. The depth profi les of δ13Cacetate in methanogenic sediments did not parallel those of either δ13CTOC or δ13CΣCO2, probably because of the mixed isotopic effect from some production and consumption processes; namely fermentation, acetogenesis, and acetoclastic methanogenesis. * Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan ** Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan *** Institute of Low Temperature Science, Hokkaido University, N19W8 Kita-ku, Sapporo, Hokkaido 060-0819, Japan **** Present address: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 4648601, Japan a Corresponding author. Tel: +81358414250. Fax: +81358418791. e-mail: ijiri@eps.s.u-tokyo.ac.jp Present address: Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

A model of phototrophic sulfide oxidation in a stratified estuary

AbstractSulfide accumulated as a result of microbial sulfate reduction in sediments and the bottom waters of stratified systems can be re‐oxidized via an array of biotic and abiotic reactions. A one‐dimensional diffusion‐reaction model was developed and applied to 4 yr of physical and chemical data from the stratified water column of the Chesapeake Bay in order to quantify the impact of different sulfide oxidation processes at the redox interface during slack tide. This dataset and the model simulations highlight short‐term and interannual variation in the physical structure and the distribution of key redox species (O2, H2S, S0, Fe2+, Mn2+, FeOOH, MnO2) throughout the water column. The model evaluates links between oxygen, sulfur, iron and manganese cycling, and includes the novel consideration of sulfide oxidation by phototrophic sulfide oxidizing bacteria (PSOB), which are a common component of the Bay. Model simulations demonstrate that phototrophic sulfide oxidation can account for up to 58% of total sulfide oxidation, depending upon the depth of the oxic/anoxic interface. The results of this study yield an approach to parameterizing PSOB activity, and quantify the variable role that PSOB play in sulfide oxidation in the Chesapeake Bay as redox conditions in the water column change, which has applications for both modern and ancient environments.

Seasonal Variation of Mercury Methylation Potential in Pristine Coastal Marine Sediment from the Gulf of Trieste (Northern Adriatic Sea)

ABSTRACTThe Gulf of Trieste (northern Adriatic Sea) is considered as an important “natural bioreactor” that can be stimulated to increase sedimentary methylmercury (MeHg) production by mobilizing its extensive mercury (Hg) pool. Sediment samples, collected in the southern pristine, less contaminated, part of the gulf, were amended with radioactive 197Hg in incubation experiments to determine the seasonal variation of Hg methylation potential at two depth layers (surface 0–2 cm and subsurface 2–4 cm). In parallel, Hg and MeHg were determined in the sediment solid phase and pore water. Total Hg and MeHg in the sediment solid phase ranged between 151 and 223 ng g−1 and between 0.17 and 0.68 ng g−1, and in pore water between 3.1 and 25.6 ng L−1 and between 0.03 and 6.16 ng L−1. Distribution coefficients (Kd) suggested that Hg species are strongly bound to the sediment (log Kd 3.86–4.87 and 1.67–3.39 for Hg and MeHg, respectively). Hg methylation potentials showed that MeHg is continuously produced, albeit not at a constant rate. The methylation potential in the sediment surface layer was higher and more variable in autumn, in parallel to higher bottom water temperature and O2 content and sediment sulfate reduction, than in late spring and summer except in July in the presence of higher labile sedimentary organic matter (OM) level. Surface sediment showed rapid methylation of added traced Hg (0–0.66% day−1 Hg methylated). However, a significant fraction of the radiolabeled Hg remained available. In the subsurface sediment, methylation was lower (0–0.22% day−1 Hg methylated). Hg methylation potentials suggest that MeHg is continuously produced, albeit not at a constant rate, in the studied sediment of the Gulf of Trieste.

Reaction time scales for sulphate reduction in sediments of acidic pit lakes and its relation to in-lake acidity neutralisation

Sulphate reduction is a key reaction to remove acidity from water bodies affected by acid mine drainage. In this study, 35SSO42− reduction rates determined in sediments from a variety of acidic lignite pit lakes have been compiled. The rates decreased with pH and are strongly dependent on carbon substrate. The rates were fitted to a Monod model adapted to the specific conditions of acidic pit lakes (APL) sediments: i) sulphate reduction rate is independent from sulphate concentration due to the high concentration typically observed in APL systems (10–30 mM), ii) the observed pH dependency of sulphate reduction was accounted for by an inhibition function Finihibt which considers the occurrence of low cell numbers of sulphate reducing bacteria at pH values < 4.75. Simulated steady-state sulphate reduction rates are predicting measured rates at carbon substrate concentrations of <10 μM. Estimated steady-state reaction time scales range between 2.4 h at pH 7 and 41 h at pH 3 at a carbon half-saturation constant of KC−S = 100 μM and are increasing with increasing KC−S values. Time scales at low pH are too long to allow for significant generation of alkalinity during the time of residence of groundwater passing through the top and hence most reactive zone of APL sediments which has important implications for the remediation of acidic pit lakes.

Relative contribution of iron reduction to sediments organic matter mineralization in contrasting habitats of a shallow eutrophic freshwater lake

Iron reduction is one of the important organic matter (OM) mineralization pathway in sediments. Here we investigated the rates and the relative contribution of iron reduction to OM mineralization in Zhushan bay (ZSB, cyanobacterial bloom biomass (CBB)-dominated habitats) and East Taihu Lake (ETL, submerged macrophypes (SM)-dominated habitats) of Lake Taihu, China. Anaerobic microcosm incubation revealed that the rate of iron reduction at ZSB (4.42 μmol cm−3 d−1) in summer was almost 1.5 times higher than at ETL (3.13 μmol cm−3 d−1). Iron reduction accounted for 66.5% (ZSB) and 31.8% (ETL) of total anaerobic carbon mineralization, respectively. No detectable methanogenesis was found at ZSB, while methanogenesis was responsible for 16.7% of total anaerobic respiration in sediments of ETL. Geochemical analysis of solid phase constituents indicated that ZSB surface sediments experienced highly oxidizing conditions with much higher amorphous Fe(III) (71 mmol m−2) than ETL (11 mmol m−2). Conversely, AVS inventories at ETL (38 mmol m−2) were up to 30 times higher than at ZSB (1.27 mmol m−2), indicating significant sulfate reduction in sediments of ETL. Overall results suggested that varying carbon sources and distinct geochemical characterizations of the sediments in contrasting habitats significantly influenced the rate of iron reduction and the pathway of C mineralization in a large freshwater lake.

Uncultured Desulfobacteraceae and Crenarchaeotal group C3 incorporate 13C-acetate in coastal marine sediment.

Stable isotope probing (SIP) of deoxyribonucleic acid (DNA) was used to identify microbes incorporating (13) C-labeled acetate in sulfate-reducing sediment from Aarhus Bay, Denmark. Sediment was incubated in medium containing 10 mM sulfate and different (13) C-acetate (10, 1, 0.1 mM) concentrations. The resultant changes in microbial community composition were monitored in total and SIP-fractionated DNA during long-term incubations. Chemical analyses demonstrated metabolic activity in all sediment slurries, with sulfate-reducing activity largely determined by initial acetate concentrations. Sequencing of 16S rRNA gene PCR amplicons showed that the incubations shifted the bacterial but not the archaeal community composition. After 3 months of incubation, only sediment slurries incubated with 10 mM (13) C-acetate showed detectable (13) C-DNA labeling. Based on 16S rRNA and dsrB gene PCR amplicon sequencing, the (13) C-labeled DNA pool was dominated by a single type of sulfate reducer representing a novel genus in the family Desulfobacteraceae. In addition, members of the uncultivated Crenarchaeotal group C3 were enriched in the (13) C-labeled DNA. Our results were reproducible across biological replicate experiments and provide new information about the identities of uncultured acetate-consuming bacteria and archaea in marine sediments.