Increasing Sulfide Concentration Research Articles

Laser remelting of high-velocity arc-sprayed Fe-based amorphous coating for improving corrosion resistance in 3.5 % NaCl solution with varying Na2S concentrations

In this study, high-velocity arc-spraying (HVAS) was employed to produce Fe-based amorphous coatings, and subsequent to that, laser remelting (LR) technology was applied on the as-sprayed coatings (ASC) to prepare laser remelted coatings (LRC). The corrosion resistance of both coatings was tested in 3.5 wt% NaCl solutions containing varying Na2S concentrations (0, 20, 50, 100, and 200 ppm) to accurately simulate the corrosion environment of offshore components. The corrosion behavior was investigated using electrochemical workstation, scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The experimental results revealed that LR resulted in the elimination of defects in Fe-based amorphous coatings, a substantial reduction in porosity, the disappearance of the laminar structure, and the achievement of a homogeneous composition. The corrosion tendency of ASC and LRC both exhibited a pattern of initially decreasing, then increasing and finally decreasing with increasing sulfide concentration. This was attributed to the deposition of corrosion products on the surface and the dissolution of the corrosion products. When the sulfide concentration exceeded 50 ppm, the corrosion current density of both coatings slowly and was lower than that at 20 ppm, indicating that both coatings possessed strong corrosion resistance at higher sulfide concentrations. Highly polarized anions in solution (e.g. HS−, S2−) occupied vacancies in the passivation film, destabilizing the coating's passivation film and consequently diminishing its corrosion resistance. From the electrochemical analysis, it was evident that LRC exhibited greater corrosion resistance than ASC at all sulfide-containing solution concentrations, which was attributed to its smoother surface, denser structure and more stable passivation film.

H2S-laden biogas triggers sulfur amino acids production in microbial protein synthesized during mixed culture fermentation by methane and sulfur oxidizing bacteria

Biogas can be converted into high-value bioproducts through fermentation processes. This study proposes a conversion platform where sulfide-rich biogas is utilized as the substrate for the growth of a mixed culture of methane- and sulfur-oxidizing bacteria for the production of high-value and protein-rich microbial biomass. The role of several process variables such as biomethane vs biogas, synthetic vs real biogas, biogas composition and H2S concentration during the fermentation process performed in bubble column reactors was evaluated in terms of biomass growth, yield and volumetric productivity, protein content and amino acids profile as well as microbial community composition and evolution. Despite the presence of 1500 ppm of equivalent H2S in the synthetic biomethane or biogas (CH4:CO2 ratio of 70:30), good performances in terms of biomass growth (559.9 ± 13.3–677.8 ± 46.8 mg of volatile suspended solids (VSS) per liter), biomass yield (0.042 ± 0.004–0.050 ± 0.08 mg VSS per mg of CH4-equivalent chemical oxygen demand (CH4-COD)) as well as protein content (51.9 ± 3.6% − 59.2 ± 3.6% g protein/g VSS) were achieved. The presence of increasing sulfide concentrations until 4000 ppm of equivalent H2S was well tolerated by the mixed culture and increased the production of valuable S-amino acids, i.e., cysteine and methionine (61.3 mg/g biomass and 58.3 mg/g biomass, respectively), which was up to 2.50 and 2.57 times higher than in the absence of sulfide, respectively.

The Impact of Sulfur-Containing Inorganic Compounds during the Depolymerization of Lignin by Hydrothermal Liquefaction of Black Liquor.

Lignin is a promising resource for the sustainable production of platform chemicals and biofuels. The paper industry produces large quantities of lignin every year, mostly dissolved in a black liquor. With the help of hydrothermal liquefaction, black liquor can be used directly as a feedstock to depolymerize the lignin to desired products. However, because various cooking chemicals (e.g., NaHS, NaOH) used in the Kraft process, dominant in the paper industry, are also dissolved in the black liquor, it is necessary to study in detail their influence on the process as well as their fate. In this work, the focus was on the fate of sulfur and the influence of sulfide (HS-). For this purpose, hydrothermal liquefaction experiments (250-400 °C) were carried out with black liquor and self-prepared model black liquor with different sulfide concentrations (0-3 g·L-1 HS-) in batch reactors (V = 25 mL), and the products were analyzed to understand the chemical pathways involving sulfur. It was found that the inorganic sulfur compounds react with organic matter to produce organic sulfur compounds. Dimethyl sulfide is the most abundant of these products. The HS- concentration correlates with the amount of dimethyl sulfide produced. Because methanethiol has also been qualitatively detected, the reaction mechanism of Karnofski et al. for the formation of dimethyl sulfide in the Kraft process also applies to the hydrothermal liquefaction of black liquor. Increased sulfide concentration in the feed leads to an accelerated depolymerization of lignin. In contrast, the yields of some aromatic monomers decrease slightly, possibly as a result of repolymerization reactions also occurring more quickly.

A sensitive voltammetric and density functional theory calculations analysis for the direct interaction of sulfide ion with the multiwalled carbon nanotube

A sensitive and simple cyclic voltametric method was investigated for the direct electrochemical determination of sulfide using multiwalled carbon nanotube paste electrode (MWCNTPE). The sulfide exhibited a distinct and remarkable direct oxidation peak at about 0.56 V in a solution containing sodium chloride as a suitable supporting electrolyte. The effect of voltametric behavior of sulfide was also studied in various supporting electrolyte. Among this sodium chloride containing solution showed a well-defined oxidation peak current as compared to the other supporting electrolyte. The direct oxidation behavior of sulfide in sodium chloride solution was mainly attributed to the less hindrance effect of the electrolyte anions on the surface of electrode. Under optimized condition the oxidation peak current values were found to increase in a linear way with increasing sulfide concentration in the range from 1 × 10−7 to 3 × 10−5 M (R2 = 0.9985). The detection limit (DL) was calculated to be 4 × 10−8 M for the sulfide concentrations. The developed method showed excellent selectivity towards the voltametric determination of sulfide as compared to other competing ions like Br−, NO3−, F−, PO43−, NO2−, I−, CH3COO− present in the solution. Moreover, the method demonstrated good and reproducible results with relative standard deviation (RSD) of 3.8% after 40 successive measurements. The method was successfully applied for determination of sulfide in local tap water samples without any matrix effect. Meanwhile to support the voltametric behavior of sulfide, density functional theory (DFT) calculations were performed at ωB97XD/6-31G(d,p) level of theory to get deep insight into the electronic properties of MWCNT and adsorbed analytes (sulfide) using Gaussian 09 package. The highest interaction energy and the decrease in energy gap of multiwall carbon nanotube with sulfide (MWCNT-S2-) complex are consistent with the highest sensing behavior of MWCNT towards sulfide ion in experimental results.

Control of reservoir souring by incomplete nitrate reduction in Indian oil fields

Reservoir souring is the increased sulfide concentration in oil production fluid, and it's the major problem faced during the secondary oil recovery process. Recently, nitrate injection emerged as a souring control strategy by inhibiting sulfate-reducing bacteria that cause souring. However, this strategy showed limited success due to the complete reduction of nitrate into nitrogen. The present study revealed that souring could be controlled by incomplete reduction of nitrate to nitrite at high temperatures (>55 °C) in Indian oil fields. Present study showed that, at 55 °C incomplete nitrate reduction led to the accumulation of nitrite and control of souring. This incomplete nitrate reduction was due to under-expression of the nitrite reductase gene at 55 °C. At 37 °C, nitrate and nitrite reductase genes were expressed consecutively, but at 55 °C, nitrite reductase gene expression decreased significantly and led to nitrite accumulation.

Sulfide perturbation of NOM-bearing oxic environments induces dark •OH production

The photo-irradiation of natural organic matter (NOM) is a well-documented pathway of •OH production in surface waters. Using humic acid (HA) as a model compound, we show that the perturbation of NOM-bearing oxic environments by naturally occurring reduced species, sulfide, contributes to •OH production under dark conditions. Sulfide concentration had a more prominent influence on •OH production than HA. When HA was controlled at 16 mM C under oxic and neutral pH conditions, the increase of sulfide concentration from 0 to 5 mM increased the cumulative •OH concentration from <2 to 72.2 μM within 120 min. When sulfide was maintained at 2.5 mM, the increase of HA concentration from 0 to 16 mM C increased the cumulative •OH concentration from <0.6 to 44.8 μM. •OH accumulation showed good consistency with sulfide removal and O2 consumption. O2 consumption increased with increasing HA concentration at a fixed sulfide concentration, and also increased with increasing sulfide concentration at a fixed HA concentration. UV–Vis and FTIR characterizations suggested that the reduced HA moieties resulting from sulfide interaction with HA acted as the redox-active moieties responsible for •OH production. Additionally, 3DEEM fluorescence suggested the reaction of sulfide with oxic HA as a partially reversible reduction-oxidation process with production of labile HA fluorophoric intermediates. Our findings implicate that the perturbation of NOM-bearing oxic environments by sulfide, i.e., from the discharge of anoxic groundwater, springs, seafloor black smokers, and hydrothermal vents, is another important source of •OH in natural environments in addition to photo-irradiation.

The critical roles of mitochondrial alternative chains in juvenile ark shells (Anadara broughtonii) exposed to acute hypoxia with or without sulfide.

Reduced oxygen levels and increased sulfide concentrations have become a concern for marine animals. This study examines respiratory and energetic adaption to acute (0-96h) hypoxia (0.5mg/L dissolved oxygen) with or without sulfide (0.2mM, 1mM, 3mM) in the hypoxia-resistant and sulfide-tolerant ark shell, Anadara broughtonii. The different states of aerobic respiration, energy-balance, and activity of the mitochondrial sulfide oxidation chain (MSOC) under these conditions were evaluated. The results indicated that the anaerobic pathway was activated by hypoxia at 24h without sulfide, but was activated in the presence of sulfide at only 2h. Exposure to sulfide resulted in significant accumulation of ATP, probably due to the activated MSOC and lowered metabolism via suppression of Na+-K+ ATPase activity and protein synthesis. During hypoxia, both enzyme activity and mRNA levels of alternative oxidase (AOX) increased while the key enzymes in MSOC, sulfide: quinone oxidoreductase (SQR) and sulfur dioxygenase (SDO), were not altered. With additional sulfide, the enzyme activity and mRNA levels of AOX, SQR, SDO significantly increased. Classical aerobic respiration was significantly inhibited, and induction of alternative respiration was detected. The corresponding alternative electron transport chain (AETC) accepted the electrons originating from both the tricarboxylic acid cycle and MSOC during the challenge, indicating that the capacity of aerobic respiration and sulfide-oxidation under a reduction state might greatly depend on AETC. The synergistically induced alternative chains (AETC and MSOC) and anaerobic pathway suggested energy-balance between respiration and sulfide-oxidation, which might contribute to the endurance of ark shells to acute sulfide exposure.

Adaptation dynamics and evolutionary rescue under sulfide selection in cyanobacteria: a comparative study between Microcystis aeruginosa and Oscillatoria sp. (cyanobacteria).

Experimental evolution studies using cyanobacteria as model organisms are scarce despite the role of cyanobacteria in the evolution of photosynthesis. Three different experimental evolution approaches have been applied to shed light on the sulfide adaptation process, which played a key role in the evolution of this group. We used a Microcystis aeruginosa sulfide-sensitive strain, unable to grow above ~0.1 mM, and an Oscillatoria sp. strain, isolated from a sulfureous spa (~0.2 mM total sulfide). First, performing a fluctuation analysis design using the spawaters as selective agent, we proved that M.aeruginosa was able to adapt to this sulfide level by rare spontaneous mutations. Second, applying a ratchet protocol, we tested if the limit of adaptation to sulfide of the two taxa was dependent on their initial sulfide tolerance, finding that M. aeruginosa adapted to 0.4 mM sulfide, and Oscillatoria sp. to ~2 mM sulfide, twice it highest tolerance level. Third, using an evolutionary rescue approach, we observed that both speed of exposure to increasing sulfide concentrations (deterioration rate) and populations' genetic variation determined the survival of M.aeruginosa at lethal sulfide levels, with a higher dependence on genetic diversity. In conclusion, sulfide adaptation of sensitive cyanobacterial strains is possible by rare spontaneous mutations and the adaptation limits depend on the sulfide level present in strain's original habitat. The high genetic diversity of a sulfide-sensitive strain, even at fast environmental deterioration rates, could increase its possibility of survival even to a severe sulfide stress.

Antimonite Complexation with Thiol and Carboxyl/Phenol Groups of Peat Organic Matter.

Peatlands and other wetlands with abundant natural organic matter (NOM) are important sinks for antimony (Sb). While formation of Sb(III) sulfide phases or Sb(III) binding to NOM are discussed to decrease Sb mobility, the exact binding mechanisms remain elusive. Here, we reacted increasing sulfide concentrations with purified model peat at pH 6, forming reduced organic sulfur species, and subsequently equilibrated the reaction products with 50 μM of antimonite under anoxic conditions. Sulfur solid-phase speciation and the local binding environment of Sb were analyzed using X-ray absorption spectroscopy. We found that 85% of antimonite was sorbed by untreated peat. Sulfide-reacted peat increased sorption to 98%. Shell-by-shell fitting of Sb K-edge X-ray absorption fine structure spectra revealed Sb in untreated peat bound to carboxyl or phenol groups with average Sb-carbon distances of ∼2.90 Å. With increasing content of reduced organic sulfur, Sb was progressively coordinated to S atoms at distances of ∼2.45 Å and Sb-carbon distances of ∼3.33 Å, suggesting increasing Sb-thiol binding. Iterative target factor analysis allowed exclusion of reduced inorganic Sb-sulfur phases with similar Sb-sulfur distances. In conclusion, even when free sulfide concentrations are too low for formation of Sb-sulfur precipitates, peat NOM can sequester Sb in anoxic, sulfur-enriched environments.

Elucidating the Role of Sulfide on the Stability of Ferrihydrite Colloids under Anoxic Conditions.

While the reaction mechanisms between ferrihydrite and sulfide are well-documented, the role of redox reactions on the particle-particle stability of ferrihydrite colloids is largely overlooked. Such reactions are critical for a number of (bio)geochemical processes governing ferrihydrite-based colloid processing and their associated role in nutrient and contaminant subsurface dynamics. Taking a fundamental colloid chemistry approach, along with a complementary suite of characterization techniques, here, we explore the stability mechanisms of ferrihydrite colloids over a wide range of environmentally relevant sulfide concentrations at pH 6.0. Results show that sulfide lowered the stability of both ferrihydrite colloids in a concentration-dependent fashion. At lower sulfide concentrations (15.6-62.5 μM), ferrihydrite colloids are apparently stable, but their critical coagulation concentration (CCC) in NaCl linearly decreases with increasing sulfide concentration. This is attributed to the formation of negatively charged elemental sulfur (S(0)) nanoparticles on the surfaces of positively charged ferrihydrite, intensifying the electrostatic attractions between oppositely charged regions on adjacent ferrihydrite surfaces. Further increasing sulfide concentration generates more S(0) attaching to the ferrihydrite surface. This results in effective surface charge neutralization and then subsequent charge reversal, leading to extensive aggregation of ferrihydrite (core) colloids. Interestingly, for the ferrihydrite colloids with higher hydrodynamic diameter, aggregation rates linearly decreases with increasing sulfide concentration from 156.3 to 312.5 μM, which is likely due to the formation of substantial amounts of negatively charged S(0) and FeS. Findings highlight the significance of sulfidation products in controlling the stability of ferrihydrite colloids in sulfidic environments.

The Microbial Communities of Leaves and Roots Associated with Turtle Grass (Thalassia testudinum) and Manatee Grass (Syringodium filliforme) are Distinct from Seawater and Sediment Communities, but Are Similar between Species and Sampling Sites.

Seagrasses are vital members of coastal systems, which provide several important ecosystem services such as improvement of water quality, shoreline protection, and serving as shelter, food, and nursery to many species, including economically important fish. They also act as a major carbon sink and supply copious amounts of oxygen to the ocean. A decline in seagrasses has been observed worldwide, partly due to climate change, direct and indirect human activities, diseases, and increased sulfide concentrations in the coastal porewaters. Several studies have shown a symbiotic relationship between seagrasses and their microbiome. For instance, the sulfur, nitrogen, and carbon cycles are important biochemical pathways that seem to be linked between the plant and its microbiome. The microbiome presumably also plays a key role in the health of the plant, for example in oxidizing phyto-toxic sulfide into non-toxic sulfate, or by providing protection for seagrasses from pathogens. Two of the most abundant seagrasses in Florida include Thalassia testudinum (turtle grass) and Syringodium filliforme (manatee grass), yet there is little data on the composition of the microbiome of these two genera. In this study, the microbial composition of the phyllosphere and rhizosphere of Thalassia testudinum and Syringodium filiforme were compared to water and sediment controls using amplicon sequencing of the V4 region of the 16S rRNA gene. The microbial composition of the leaves, roots, seawater, and sediment differ from one another, but are similar between the two species of seagrasses.

Bioreactor performance and microbial community analysis of autotrophic denitrification under micro-aerobic condition

Autotrophic denitrification process is an effective strategy for treating sulfide and nitrate-enriched wastewater with low organic carbon. This study determined the sulfide oxidation and nitrate reduction rates and characterized the dominant bacteria and microbial community structure stimulated by micro-aerobic conditions in autotrophic denitrification system. With gradually increased sulfide concentration, the sulfide removal rate decreased to 37.8% at S2− = 600 mg/L, while the peak sulfide and nitrate removal rates (100% and 53.8%) were achieved at S2− = 800 mg/L with the air aeration rate of 20 mL/min. The Illumina sequencing results indicated that Thiobacillus accounted for 63% of total operational taxonomic units at generic level with sulfide concentration of 200 mg/L under anaerobic condition. However, Azoarcus, Thauera and Aliidiomorina became the dominant genera under micro-aerobic condition and their abundance significantly and positively related to the sulfide concentration and aeration rate (p < 0.05). According to the 16S metaproteomics functional composition prediction, one potential mechanism for autotrophic denitrifying under micro-aerobic condition was deduced that the oxidation of sulfide to thiosulfate further to sulfite was reinforced by trace oxygen, while the sulfite reductase activity was restrained. The decreased sulfide concentration weakened the toxicity inhibition on denitrifiers and accordingly the performance of autotrophic denitrification process was enhanced by micro-aerobic condition.

The Seagrass Holobiont and Its Microbiome.

Seagrass meadows are ecologically and economically important components of many coastal areas worldwide. Ecosystem services provided by seagrasses include reducing the number of microbial pathogens in the water, providing food, shelter and nurseries for many species, and decreasing the impact of waves on the shorelines. A global assessment reported that 29% of the known areal extent of seagrasses has disappeared since seagrass areas were initially recorded in 1879. Several factors such as direct and indirect human activity contribute to the demise of seagrasses. One of the main reasons for seagrass die-offs all over the world is increased sulfide concentrations in the sediment that result from the activity of sulfate-reducing prokaryotes, which perform the last step of the anaerobic food chain in marine sediments and reduce sulfate to H2S. Recent seagrass die-offs, e.g., in the Florida and Biscayne Bays, were caused by an increase in pore-water sulfide concentrations in the sediment, which were the combined result of unfavorable environmental conditions and the activities of various groups of heterotrophic bacteria in the sulfate-rich water-column and sediment that are stimulated through increased nutrient concentrations. Under normal circumstances, seagrasses are able to withstand low levels of sulfide, probably partly due to microbial symbionts, which detoxify sulfide by oxidizing it to sulfur or sulfate. Novel studies are beginning to give greater insights into the interactions of microbes and seagrasses, not only in the sulfur cycle. Here, we review the literature on the basic ecology and biology of seagrasses and focus on studies describing their microbiome.

Self-accelerating sulfur reduction via polysulfide to realize a high-rate sulfidogenic reactor for wastewater treatment

Sulfur reduction is a promising alternative to sulfate reduction as it can generate sulfide at a low cost for the precipitation of heavy metals or autotrophic denitrification in wastewater treatment. However, the extremely low water solubility of elemental sulfur limits its bioavailability and results in a low sulfur-reduction rate. Polysulfide, which is naturally generated through reactions between sulfur and sulfide, can enhance the bioavailability of sulfur and thus contribute to high-rate sulfur reduction. Based on this principle, a laboratory-scale sulfur-reducing bioreactor was designed in this study for wastewater treatment. After 164 days of operation, the sulfide production rate (SPR) in the bioreactor reached 126 mg S/L-h, which is significantly higher than those of other sulfate-reducing systems. Moreover, dissolved zero-valent sulfur (referred to as polysulfide) was detected in the sulfur-reducing reactor when the organics were completely depleted, indicating that polysulfide can form naturally and be readily reduced to sulfide in the bioreactor. We found that the produced sulfide promoted the formation of more polysulfide, which enabled a self-accelerating chain reaction of sulfur reduction via polysulfide. This stimulation effect was further validated by the 7-h batch tests. In the batch test without sulfide addition initially, a continuous increase in the hourly SPR was observed with increasing sulfide concentration. Furthermore, in the batch tests with the addition of 50–200 mg S/L sulfide at the beginning, the average SPR in the first 3 h increased with elevating initial sulfide concentration due to more polysulfide formation and reduction. However, high sulfide concentration (>250 mg S/L) hindered the continuous increase in SPR. Additionally, when polysulfide formation was prevented through the addition of Fe2+, the SPR dropped by 97.6% compared to that in the presence of polysulfide. This validates the key role of polysulfide in the high-rate sulfur reduction process. Overall, the findings suggest that high-rate sulfur reduction can be achieved for autotrophic denitrification or heavy-metal removal in wastewater treatment.

Terbium (III) coordination polymer-copper (II) compound as fluorescent probe for time-resolved fluorescence 'turn-on' detection of hydrogen sulfide.

With recognition of the biological importance of hydrogen sulfide (H2 S), we present a simple and effective fluorescent probe for H2 S using a Tb3+ coordination polymer-Cu2+ compound (DPA/Tb/G-Cu2+ ). Dipicolinic acid (DPA) and guanosine (G) can coordinate with Tb3+ to form a macromolecular coordination polymer (DPA/Tb/G). DPA/Tb/G specifically binds to Cu2+ in the presence of coexisting cations, and obvious fluorescence quenching is observed. The quenched fluorescence can be exclusively recovered upon the addition of sulfide, which is measured in the mode of time-resolved fluorescence. The fluorescence intensities of the DPA/Tb/G-Cu2+ compound enhance linearly with increasing sulfide concentrations from 1 to 30μM. The detection limit for sulfide in aqueous solution is estimated to be 0.3μM (at 3σ). The DPA/Tb/G-Cu2+ compound was successfully applied to sense H2 S in human serum samples and exhibited a satisfactory result. It displays some desirable properties, such as fast detection procedure, high selectivity and excellent sensitivity. This method is very promising to be utilized for practical detection of H2 S in biological and environmental samples.

Odorous Volatile Organic Compounds, Escherichia coli, and Nutrient Concentrations when Kiln-Dried Pine Chips and Corn Stover Bedding Are Used in Beef Bedded Manure Packs.

Pine ( spp.) bedding has been shown to lower the concentration of odorous volatile organic compounds (VOCs) and pathogenic bacteria compared with corn ( L.) stover bedding, but availability and cost limit the use of pine bedding in cattle confinement facilities. The objectives of this study were to determine if the addition of pine wood chips to laboratory-scaled bedded packs containing corn stover (i) reduced odorous VOC emissions; (ii) reduced total ; and (iii) changed the nutrient composition of the resulting manure-bedded packs. Bedding treatments included 0, 10, 20, 30, 40, 60, 80, and 100% pine chips, with the balance being corn stover. Four bedded packs for each mixture were maintained for 42 d ( = 4 observations per bedding material). The production of total sulfur compounds increased significantly when 100% pine chips were used (44.72 ng L) compared with bedding mixture containing corn stover (18.0-24.56 ng L). The carbon-to-nitrogen ratio exceeded the ideal ratio of 24:1 for the optimum activity of soil microorganisms when ≥60% pine chips (25.3-27.5 ng L) were included in the mixture. The use of 100% pine chips as bedding increased sulfide concentration in the facility 1.8 to 2.4 times over the use of corn stover bedding. was not influenced by the addition of pine chips to the corn stover bedding material but did decrease as the bedded pack aged. Bedding material mixtures containing 30 to 60% pine and 40 to 70% corn stover may be the ideal combination to mitigate odors from livestock facilities using deep bedded systems.

Effect of Sulfide Concentration on Copper Corrosion in Anoxic Chloride-Containing Solutions

The structure and property of passive film on copper are strongly dependent on the sulfide concentration; based on this, a series of electrochemical methods were applied to investigate the effect of sulfide concentration on copper corrosion in anaerobic chloride-containing solutions. The cyclic voltammetry and x-ray photoelectron spectroscopy analysis demonstrated that the corrosion products formed on copper in anaerobic sulfide solutions comprise Cu2S and CuS. And the corrosion resistance of copper decreased with increasing sulfide concentration and faster sulfide addition, owing to the various structures of the passive films observed by the atomic force microscope and scanning electron microscope. A p-type semiconductor character was obtained under all experimental conditions, and the defect concentration, which had a magnitude of 1022–1023 cm−3, increased with increasing sulfide concentration, resulting in a higher rate of both film growth and dissolution.

Predation scar frequencies in chemosymbiotic bivalves at an Oligocene seep deposit and their potential relation to inferred sulfide tolerances

Three species of chemosymbiotic bivalves with different inferred sulfide tolerances and life habits from a lower Oligocene seep deposit in eastern Hokkaido, Japan, were investigated for drill holes and scars of durophagous predation. The thyasirid Conchocele bisecta had the lowest inferred sulfide tolerance and showed the highest frequencies of repair (0.4) and drilling predation (0.1), and specimens with drill holes were on average smaller than non-drilled specimens, indicating that large size is a refuge from drilling predation in this species. The bathymodiolin Bathymodiolus inouei and the vesicomyid Hubertschenckia ezoensis are inferred to have lived at higher sulfide concentrations than C. bisecta based on comparisons to their living relatives, and show lower repair frequencies (0.24 in B. inouei and 0.17 in H. ezoensis). Based on the assumption that increased sulfide concentrations are increasingly toxic to predators, we conclude that the frequency of shell injuries in the three investigated species is inversely related to the sulfide concentrations that these species are able to tolerate. Thus when applied carefully, the frequency of shell injuries among shelled invertebrates at fossil seeps may be used to infer their relative tolerances toward sulfide, and may thus represent a tool to assess the life habits of extinct invertebrates that inhabited methane seeps in the geologic past.

Sensitivity of Ruppia maritima and Zostera marina to sulfide exposure around roots

Ruppia maritima and Zostera marina often inhabit the same coastal areas and are often exposed to fairly high sulfide concentrations, with the former being most resilient. Our aim was to investigate differences in sulfide tolerance between the two species. Sensitivity towards sulfide in R. maritima and Z. marina was investigated in split chambers that allowed exposure of roots to a range of sulfide concentrations, while the leaves were kept under optimal light and oxygen conditions. Roots of R. maritima show a higher threshold towards sulfide (1.8–5.9mM H2S) than those of Z. marina (0–0.5mM H2S) after two weeks of incubation. The difference in sulfide sensitivity is attributed to different capacity of root oxygen leakage. Light and oxygen exposure enables oxygen transport from leaves down to the roots during both day and night. The oxic micro-shield around roots has a capacity to withstand sulfide intrusion until the sensitivity level is reached, when no other stressors are present. Leaves of exposed R. maritima plants are affected at similar sulfide concentrations as roots, while leaves of Z. marina are affected at higher sulfide concentrations (1–3.5mM H2S), reflecting different pathways of sulfide intrusion, root permeability and the ability to allocate energy away from exposed plant parts. Sensitivity towards sulfide occurs gradually and probably as a linear response to increasing sulfide concentration in R. maritima, but the exact pattern in Z. marina is still unclear. Other external stressors, such as anoxia, shading, or increased temperature may change the thresholds of the species by affecting radial oxygen loss from roots, and make especially R. maritima more prone to sulfide toxicity due to its highly permeable roots.

Species‐Specific Enzymatic Tolerance of Sulfide Toxicity in Plant Roots and Comparative Susceptibility between Plant and Catfish Tissue

Sulfide is a potent inhibitor of numerous enzymes, especially cytochrome c oxidase, which catalyzes the terminal step in aerobic respiration. However, some salt marsh plants live in sulfide-rich sediments. We hypothesized salt marsh plants might tolerate sulfide exposure better compared to upland plants at the metabolic level. Two enzymes were assayed to determine broader effects of sulfide toxicity on metabolism. Our objective was to compare roots of various plant species and catfish liver and muscle tissues. Cytochrome c oxidase and alcohol dehydrogenase activities were measured in root extracts and tissue homogenate exposed to 0, 5, 10, 15, and 20 μM sodium sulfide. Activities of cytochrome c oxidase were very sensitive to sulfide and were nearly undetectable at 5 to 10 μM sulfide. Alcohol dehydrogenase activities were less sensitive to sulfide, typically only reduced by about 50% at 20 μM sulfide. Cytochrome c oxidase activities in root extracts of flooding-sensitive plant species decreased to nearly zero when treated with 5 μM sulfide, whereas activities in some salt marsh plants did not decrease until 10 μM sulfide. Cytochrome c oxidase activities in catfish tissues were much higher than those measured in plant roots, but were very sensitive to increasing sulfide concentrations, with complete inhibition at 5 μM sulfide. Cytochrome c oxidase activities in some salt marsh plants were low even in the absence of sulfide, perhaps an adaptation to avoid sulfide vulnerability in their native habitat. This illustrates potent metabolic effects of sulfide, and variability in sulfide toxicity among plants. This work was supported by the Kansas Academy of Science and Kansas IDeA Network of Biomedical Research Excellence.