Sulfur production by obligately chemolithoautotrophic thiobacillus species

Influence of oxidation-reduction potential and pH on polysulfide concentrations and chain lengths in the biological desulfurization process under haloalkaline conditions

Biological desulfurization under haloalkaline conditions has been applied worldwide to remove hydrogen sulfide (H2S) from sour gas steams. The process relies on sulfide-oxidizing bacteria (SOB) to oxidize H2S to elemental sulfur (S8), which can then be recovered and reused. Recently, a dual-reactor biological desulfurization system was implemented where an anaerobic (sulfidic) bioreactor was incorporated as an addition to a micro-oxic bioreactor, allowing for higher S8 selectivity by limiting by-product formation. The highly sulfidic bioreactor environment enabled the SOB to remove (poly)sulfides (Sx2-) in the absence of oxygen, with Sx2- speculated as a main substrate in the removal pathway, thus making it vital to understand its role in the process. The SOB are influenced by the oxidation-reduction potential (ORP) set-point of the micro-oxic bioreactor as it is used to control the product of oxidation (S8 vs. SO42-), while the uptake of Sx2- by SOB has been qualitatively linked to pH. Therefore, to quantify these effects, this work determined the concentration and speciation of Sx2- in the biological desulfurization process under various pH values and ORP set-points. The total Sx2- concentrations in the sulfidic zone increased at elevated pH (8.9) compared to low pH (< 8.0), with on average 3.3 ± 1.0 mM-S more Sx2-. Chain lengths varied, with S72- only doubling in concentration while S52- increased 9 fold, which is in contrast with observations from abiotic systems. Changes to the ORP set-point of the micro-oxic reactor did not produce substantial changes in Sx2- concentration in the sulfidic zone. This illustrates that the reduction degree of the SOB in the micro-oxic bioreactor does not enhance their ability to interact with Sx2- in the sulfidic bioreactor. This increased understanding of how both pH and ORP affect changes in Sx2- concentration and chain length can lead to improved efficiency and design of the dual-reactor biological desulfurization process.

Microorganisms uptake zero-valent sulfur via membrane lipid dissolution of octasulfur and intracellular solubilization as persulfide

Zero-valent sulfur, commonly utilized as a fertilizer or fungicide, is prevalent in various environmental contexts. Its most stable and predominant form, octasulfur (S8), plays a crucial role in microbial sulfur metabolism, either through oxidation or reduction. However, the mechanism underlying its cellular uptake remains elusive. We presented evidence that zero-valent sulfur was adsorbed to the cell surface and then dissolved into the membrane lipid layer as lipid-soluble S8 molecules, which reacted with cellular low-molecular thiols to form persulfide, e.g., glutathione persulfide (GSSH), in the cytoplasm. The process brought extracellular zero-valent sulfur into the cells. When persulfide dioxygenase is present in the cells, GSSH will be oxidized. Otherwise, GSSH will react with another glutathione (GSH) to produce glutathione disulfide (GSSG) and hydrogen sulfide (H2S). The mechanism is different from simple diffusion, as insoluble S8 becomes soluble GSSH after crossing the cytoplasmic membrane. The uptake process is limited by physical contact of insoluble zero-valent sulfur with microbial cells and the regeneration of cellular thiols. Our findings elucidate the cellular uptake mechanism of zero-valent sulfur, which provides critical information for its application in agricultural practices and the bioremediation of sulfur contaminants and heavy metals.

Using the sulfide-oxidizing bacterium Geobacillus thermodenitrificans to restrict H2S release during chicken manure composting

Hydrogen sulfide (H2S) is a toxic gas massively released during chicken manure composting. Diminishing its release requires efficient and low cost methods. In recent years, heterotrophic bacteria capable of rapid H2S oxidation have been discovered but their applications in environmental improvement are rarely reported. Herein, we investigated H2S oxidation activity of a heterotrophic thermophilic bacterium Geobacillus thermodenitrificans DSM465, which contains a H2S oxidation pathway composed by sulfide:quinone oxidoreductase (SQR) and persulfide dioxygenase (PDO). This strain rapidly oxidized H2S to sulfane sulfur and thiosulfate. The oxidation rate reached 5.73 μmol min-1·g-1 of cell dry weight. We used G. thermodenitrificans DSM465 to restrict H2S release during chicken manure composting. The H2S emission during composting process reduced by 27.5% and sulfate content in the final compost increased by 34.4%. In addition, this strain prolonged the high temperature phase by 7 days. Thus, using G. thermodenitrificans DSM465 to control H2S release was an efficient and economic method. This study provided a new strategy for making waste composting environmental friendly and shed light on perspective applications of heterotrophic H2S oxidation bacteria in environmental improvements.

Polysulfide Concentration and Chain Length in the Biological Desulfurization Process: Effect of Biomass Concentration and the Sulfide Loading Rate

Removal of hydrogen sulfide (H2S) can be achieved using the sustainable biological desulfurization process, where H2S is converted to elemental sulfur using sulfide-oxidizing bacteria (SOB). A dual-bioreactor process was recently developed where an anaerobic (sulfidic) bioreactor was used between the absorber column and micro-oxic bioreactor. In the absorber column and sulfidic bioreactor, polysulfides (Sx2-) are formed due to the chemical equilibrium between H2S and sulfur (S8). Sx2- is thought to be the intermediate for SOB to produce sulfur via H2S oxidation. In this study, we quantify Sx2-, determine their chain-length distribution under high H2S loading rates, and elucidate the relationship between biomass and the observed biological removal of sulfides under anaerobic conditions. A linear relationship was observed between Sx2- concentration and H2S loading rates at a constant biomass concentration. Increasing biomass concentrations resulted in a lower measured Sx2- concentration at similar H2S loading rates in the sulfidic bioreactor. Sx2- of chain length 6 (S62-) showed a substantial decrease at higher biomass concentrations. Identifying Sx2- concentrations and their chain lengths as a function of biomass concentration and the sulfide loading rate is key in understanding and controlling sulfide uptake by the SOB. This knowledge will contribute to a better understanding of how to reach and maintain a high selectivity for S8 formation in the dual-reactor biological desulfurization process.

Enhanced biological S0 accumulation by using signal molecules during simultaneous desulfurization and denitrification

A high rate of elemental sulfur (S⁰) accumulation from sulfide-containing wastewater has great significance in terms of resource recovery and pollution control. This experimental study used Thiobacillus denitrificans and denitrifying bacteria incorporated with signal molecules (C6 and OHHL) for simultaneous sulfide (S^2-) and nitrate (NO₃^-) removal in synthetic wastewater. Also, the effects on S⁰ accumulation due to changes in organic matter composition and bacteria proportion through signal molecules were analyzed. The 99.0% of S^2- removal and 99.3% of NO₃^- was achieved with 66% of S⁰ accumulation under the active S^2- removal group. The S⁰ accumulation, S^2- and NO₃^- removal mainly occurred in 0-48 h. The S⁰ accumulation in the active S^2- removal group was 2.0-6.3 times higher than the inactive S^2- removal groups. In addition, S⁰/SO₄^2- ratio exhibited that S⁰ conversion almost linearly increased with reaction time under the active S^2- removal group. The proportion of Thiobacillus denitrificans and H⁺ consumption showed a positive correlation with S⁰ accumulation. However, a very high or low ratio of H⁺/S⁰ is not suitable for S⁰ accumulation. The signal molecules greatly increased the concentration of protein-I and protein-II, which resulted in the high proportion of Thiobacillus denitrificans. Therefore, high S⁰ accumulation was achieved as Thiobacillus denitrificans regulated the H⁺ consumption and electron transfer rate and provided suppressed oxygen environment. This technology is cost-effective and commercially applicable for recovering S⁰ from wastewater.

Trajectories for the evolution of bacterial CO2-concentrating mechanisms

Cyanobacteria rely on CO₂-concentrating mechanisms (CCMs) to grow in today's atmosphere (0.04% CO₂). These complex physiological adaptations require ≈15 genes to produce two types of protein complexes: inorganic carbon (Ci) transporters and 100+ nm carboxysome compartments that encapsulate rubisco with a carbonic anhydrase (CA) enzyme. Mutations disrupting any of these genes prohibit growth in ambient air. If any plausible ancestral form-i.e., lacking a single gene-cannot grow, how did the CCM evolve? Here, we test the hypothesis that evolution of the bacterial CCM was "catalyzed" by historically high CO₂ levels that decreased over geologic time. Using an E. coli reconstitution of a bacterial CCM, we constructed strains lacking one or more CCM components and evaluated their growth across CO₂ concentrations. We expected these experiments to demonstrate the importance of the carboxysome. Instead, we found that partial CCMs expressing CA or Ci uptake genes grew better than controls in intermediate CO₂ levels (≈1%) and observed similar phenotypes in two autotrophic bacteria, Halothiobacillus neapolitanus and Cupriavidus necator. To understand how CA and Ci uptake improve growth, we model autotrophy as colimited by CO₂ and HCO₃^-, as both are required to produce biomass. Our experiments and model delineated a viable trajectory for CCM evolution where decreasing atmospheric CO₂ induces an HCO₃^- deficiency that is alleviated by acquisition of CA or Ci uptake, thereby enabling the emergence of a modern CCM. This work underscores the importance of considering physiology and environmental context when studying the evolution of biological complexity.

Coupling sulfur-based denitrification with anammox for effective and stable nitrogen removal: A review

Anoxic ammonium oxidation (anammox) is an energy-efficient nitrogen removal process for wastewater treatment. However, the unstable nitrite supply and residual nitrate in the anammox process have limited its wide application. Recent studies have proven coupling of sulfur-based denitrification with anammox (SDA) can achieve an effective nitrogen removal, owing to stable provision of substrate nitrite from the sulfur-based denitrification, thus making its process control more efficient in comparison with that of partial nitrification and anammox process. Meanwhile, the anammox-produced nitrate can be eliminated through sulfur-based denitrification, thereby enhancing SDA's overall nitrogen removal efficiency. Nonetheless, this process is governed by a complex microbial system that involves both complicated sulfur and nitrogen metabolisms as well as multiple interactions among sulfur-oxidising bacteria and anammox bacteria. A comprehensive understanding of the principles of the SDA process is the key to facilitating the development and application of this novel process. Hence, this review is conducted to systematically summarise various findings on the SDA process, including its associated biochemistry, biokinetic reactions, reactor performance, and application. The dominant functional bacteria and microbial interactions in the SDA process are further discussed. Finally, the advantages, challenges, and future research perspectives of SDA are outlined. Overall, this work gives an in-depth insight into the coupling mechanism of SDA and its potential application in biological nitrogen removal.

Sulfane Sulfur Posttranslationally Modifies the Global Regulator AdpA to Influence Actinorhodin Production and Morphological Differentiation of Streptomyces coelicolor

The transcription factor AdpA is a key regulator controlling both secondary metabolism and morphological differentiation in Streptomyces. Due to its critical functions, its expression undergoes multilevel regulations at transcriptional, posttranscriptional, and translational levels, yet no posttranslational regulation has been reported. Sulfane sulfur, such as hydro polysulfide (HS_nH, n ≥ 2) and organic polysulfide (RS_nH, n ≥ 2), is common inside microorganisms, but its physiological functions are largely unclear. Here, we discovered that sulfane sulfur posttranslationally modifies AdpA in Streptomyces coelicolor via specifically reacting with Cys⁶² of AdpA to form a persulfide (Cys⁶²-SSH). This modification decreases the affinity of AdpA to its self-promoter P_adpA, allowing increased expression of adpA, further promoting the expression of its target genes actII-4 and wblA. ActII-4 activates actinorhodin biosynthesis, and WblA regulates morphological development. Bioinformatics analyses indicated that AdpA-Cys⁶² is highly conserved in Streptomyces, suggesting the prevalence of such modification in this genus. Thus, our study unveils a new type of regulation on the AdpA activity and sheds a light on how sulfane sulfur stimulates the production of antibiotics in Streptomyces.

Rapid start of high-concentration denitrification and desulfurization reactors by heterotrophic denitrification sulphur-oxidising bacteria

High sulphide concentrations can be toxic to denitrifying and desulphurising microorganisms. In this study, bioaugmentation was used to solve this problem. Pseudomonas sp. gs1 can tolerate 400 mg/L sulphide and converts most of the sulphide into elemental sulphur after 4 h. A solid inoculum of Pseudomonas sp. h1 was prepared. Two reactors, that is, one with and one without inoculum, were simultaneously run for 60 days. Bioreactor II to which bacterial inoculum was added reached a good treatment performance on day 3. The elemental sulphur concentration of the effluent was 342.6 mg/L. It was maintained at 245.3-333.8 mg/L during the subsequent operation. In contrast, reactor I without inoculants achieved the same performance on day 50. High-throughput sequencing shows that Pseudomonas and Azoarcus are the dominant genera. The abundance of the genus Pseudomonas and related denitrifying sulphur-oxidising bacteria in reactor I increases with the operation time. This phenomenon was confirmed by testing the sqr and gltA genes. The quantitative fluorescence PCR test also proves that the addition of bacteria leads to a rapid increase in the sulphur oxidation and carbon metabolism of the activated sludge in the reactor.

Regulating intermediates to realize the coupled hydrion with biology polysulfide in wastewater treatment

The purpose of this study is to clarify the mechanism of the coupled hydrion with biology polysulfide in the simultaneous denitrification and desulfurization process. The coupled hydrion with biology polysulfide, uncoupled hydrion with biology polysulfide and no polysulfide experiments were performed in wastewater with two kinds of sulfide loads (100 and 200 mg/L). When the concentration of thiosulfate was suitable, the free H⁺ concentration (74.2 and 91.0 mg/L) and the proportion of Thiobacillus denitrificans (85.4% and 59.7%) were both higher under the two kinds of sulfide loading conditions (100 and 200 mg/L), and coupled hydrion with biology polysulfide was realized (the production of elemental sulfur is as high as 33 and 101 mg/L). Further analysis shown that the way of coupled hydrion with biology polysulfide were both: 2.0S2-+6.4NO3-+30.1H++21.7e-→1.0S2-+1.0SO42-+3.2N2+15.0H2O. In addition, for the coupled hydrion with biology polysulfide, more nitrates could be utilized to produce elemental sulfur S⁰, and the lower ratio of H⁺/S⁰ and SO42-/S0 were observed (S^2- = 100 mg/L: 2.3 and 0.9; S^2- = 200 mg/L: 0.9 and 0.03), which could promote the growth of Thiobacillus denitrificans and increase the proportion of Thiobacillus denitrificans. This maybe one of the reasons why coupled hydrion with biology polysulfide could be achieved.

Microbiological Sulfide Removal-From Microorganism Isolation to Treatment of Industrial Effluent

Management of excessive aqueous sulfide is one of the most significant challenges of treating effluent after biological sulfate reduction for metal recovery from hydrometallurgical leachate. The main objective of this study was to characterize and verify the effectiveness of a sulfide-oxidizing bacterial (SOB) consortium isolated from post-mining wastes for sulfide removal from industrial leachate through elemental sulfur production. The isolated SOB has a complete sulfur-oxidizing metabolic system encoded by sox genes and is dominated by the Arcobacter genus. XRD analysis confirmed the presence of elemental sulfur in the collected sediment during cultivation of the SOB in synthetic medium under controlled physicochemical conditions. The growth yield after three days of cultivation reached ~2.34 g_protein/mol_sulfid, while approximately 84% of sulfide was transformed into elemental sulfur after 5 days of incubation. Verification of isolated SOB on the industrial effluent confirmed that it can be used for effective sulfide concentration reduction (~100% reduced from the initial 75.3 mg/L), but for complete leachate treatment (acceptable for discharged limits), bioaugmentation with other bacteria is required to ensure adequate reduction of chemical oxygen demand (COD).

Electron Storage in Electroactive Biofilms

Microbial electrochemical technologies (METs) are promising for sustainable applications. Recently, electron storage during intermittent operation of electroactive biofilms (EABs) has been shown to play an important role in power output and electron efficiencies. Insights into electron storage mechanisms, and the conditions under which these occur, are essential to improve microbial electrochemical conversions and to optimize biotechnological processes. Here, we discuss the two main mechanisms for electron storage in EABs: storage in the form of reduced redox active components in the electron transport chain and in the form of polymers. We review electron storage in EABs and in other microorganisms and will discuss how the mechanisms of electron storage can be influenced.

The identification of sulfide oxidation as a potential metabolism driving primary production on late Noachian Mars

The transition of the martian climate from the wet Noachian era to the dry Hesperian (4.1-3.0 Gya) likely resulted in saline surface waters that were rich in sulfur species. Terrestrial analogue environments that possess a similar chemistry to these proposed waters can be used to develop an understanding of the diversity of microorganisms that could have persisted on Mars under such conditions. Here, we report on the chemistry and microbial community of the highly reducing sediment of Colour Peak springs, a sulfidic and saline spring system located within the Canadian High Arctic. DNA and cDNA 16S rRNA gene profiling demonstrated that the microbial community was dominated by sulfur oxidising bacteria, suggesting that primary production in the sediment was driven by chemolithoautotrophic sulfur oxidation. It is possible that the sulfur oxidising bacteria also supported the persistence of the additional taxa. Gibbs energy values calculated for the brines, based on the chemistry of Gale crater, suggested that the oxidation of reduced sulfur species was an energetically viable metabolism for life on early Mars.

H2S removal from sour water in a combination system of trickling biofilter and biofilter

Desulfurization of sour water was investigated in a combination system of trickling biofilter (BTF) and biofilter (BF) filled with ceramic packing materials. A critical elimination capacity (EC) of 251.93 g S m^-3 h^-1 was obtained for the BTF/BF system during a stepwise increase of sulfide concentration from 10 to 60 g S m^-3. This stepwise increment of loading rate also led to critical ECs of 176.21 and 478.88 g S m^-3 h^-1 for BTF and BF, respectively. A dynamic model describing biological H₂S removal from sour water in the BTF/BF was developed and calibrated by a set of experimental data. The model includes the main processes occurring in the BTF/BF such as mass transfer between phases, diffusion and biological reaction inside the biofilm. The model also considers the intermediate (elemental sulfur) production/consumption and sulfate formation through the different oxidation pathways. The model validation was performed under a starvation period and a dynamic H₂S loading period. A sensitivity analysis was carried out to evaluate the relative importance of the key parameters on the performance of the BTF/BF system. Sensitivity analysis showed that the BTF performance is more affected by the parameters related to H₂S mass transfer.

Long term performance and dynamics of microbial biofilm communities performing sulfur-oxidizing autotrophic denitrification in a moving-bed biofilm reactor

Sulfide-oxidizing autotrophic denitrification (SOAD) implemented in a moving-bed biofilm reactor (MBBR) is a promising alternative to conventional heterotrophic denitrification in mainstream biological nitrogen removal. The sulfide-oxidation intermediate - elemental sulfur - is crucial for the kinetic and microbial properties of the sulfur-oxidizing bacterial communities, but its role is yet to be studied in depth. Hence, to investigate the performance and microbial communities of the aforementioned new biosystem, we operated for a long term a laboratory-scale (700 d) SOAD MBBR to treat synthetic saline domestic sewage, with an increase of the surface loading rate from 8 to 50 mg N/(m²·h) achieved by shortening the hydraulic retention time from 12 h to 2 h. The specific reaction rates of the reactor were eventually increased up to 0.37 kg N/(m³·d) and 0.73 kg S/(m³·d) for nitrate reduction and sulfide oxidation with no significant sulfur elemental accumulation. Two sulfur-oxidizing bacterial (SOB) clades, Sox-independent SOB (SOB_I) and Sox-dependent SOB (SOB_II), were responsible for indirect two-step sulfur oxidation (S^2-→S⁰→SO₄^2-) and direct one-step sulfur oxidation (S^2-→SO₄^2-), respectively. The SOB_II biomass-specific electron transfer capacity could be around 2.5 times greater than that of SOB_I (38 mmol e^-/(gSOB_II·d) versus 15 mmol e^-/(gSOB_I·d)), possibly resulting in the selection of SOB_II over SOB_I under stress conditions (such as a shorter HRT). Further studies on the methods and mechanism of selecting of SOB_II over SOB_I in biofilm reactors are recommended. Overall, the findings shed light on the design and operation of MBBR-based SOAD processes for mainstream biological denitrification.

Screening and characterization of mixotrophic sulfide oxidizing bacteria for odorous surface water bioremediation

Eight species of mixotrophic sulfide oxidizing bacteria (SOB) were isolated from activated sludge and identified using 16S rRNA sequence analysis. The effects of organic substances, dissolved oxygen (DO) and nitrate on sulfide oxidation and bacterial growth were studied in this work. The results showed that Paracoccus sp. (N1), Pseudomonas sp. (N2) and Pseudomonas sp. (S4) have strong adaptability to environments with low DO and high concentrations of organic substance. An SOB additive was optimized in artificial, odorous water. The optimized SOB additive is ablendof 80% N1 and 20% N2 bacteria solution with absorbance equal to 0.5 at a wavelength of 600 nm (OD₆₀₀), and the optimal dose of the additive is 20 ml/L. Oxidation-reduction potential (ORP), ammonia-nitrogen (NH₃-N) and released H₂S in an odorous river were measured with and without SOB additive, and the results indicated that the optimized SOB additive has excellent performance for odorous river bioremediation.

Development and validation of a physiologically based kinetic model for starting up and operation of the biological gas desulfurization process under haloalkaline conditions

Hydrogen sulfide is a toxic and corrosive gas that must be removed from gaseous hydrocarbon streams prior to combustion. This paper describes a gas biodesulfurization process where sulfur-oxidizing bacteria (SOB) facilitate sulfide conversion to both sulfur and sulfate. In order to optimize the formation of sulfur, it is crucial to understand the relations between the SOB microbial composition, kinetics of biological and abiotic sulfide oxidation and the effects on the biodesulfurization process efficiency. Hence, a physiologically based kinetic model was developed for four different inocula. The resulting model can be used as a tool to evaluate biodesulfurization process performance. The model relies on a ratio of two key enzymes involved in the sulfide oxidation process, i.e., flavocytochrome c and sulfide-quinone oxidoreductase (FCC and SQR). The model was calibrated by measuring biological sulfide oxidation rates for different inocula obtained from four full-scale biodesulfurization installations fed with gases from various industries. Experimentally obtained biological sulfide oxidation rates showed dissimilarities between the tested biomasses which could be explained by assuming distinctions in the key-enzyme ratios. Hence, we introduce a new model parameter α to whereby α describes the ratio between the relative expression levels of FCC and SQR enzymes. Our experiments show that sulfur production is the highest at low α values.

Increasing the Selectivity for Sulfur Formation in Biological Gas Desulfurization

In the biotechnological desulfurization process under haloalkaline conditions, dihydrogen sulfide (H₂S) is removed from sour gas and oxidized to elemental sulfur (S₈) by sulfide-oxidizing bacteria. Besides S₈, the byproducts sulfate (SO₄^2-) and thiosulfate (S₂O₃^2-) are formed, which consume caustic and form a waste stream. The aim of this study was to increase selectivity toward S₈ by a new process line-up for biological gas desulfurization, applying two bioreactors with different substrate conditions (i.e., sulfidic and microaerophilic), instead of one (i.e., microaerophilic). A 111-day continuous test, mimicking full scale operation, demonstrated that S₈ formation was 96.6% on a molar H₂S supply basis; selectivity for SO₄^2- and S₂O₃^2- were 1.4 and 2.0% respectively. The selectivity for S₈ formation in a control experiment with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up, the new process line-up immediately achieved lower SO₄^2- and S₂O₃^2- formations compared to the 1-bioreactor line-up. When the microbial community adapted over time, it was observed that SO₄^2- formation further decreased. In addition, chemical formation of S₂O₃^2- was reduced due to biologically mediated removal of sulfide from the process solution in the anaerobic bioreactor. The increased selectivity for S₈ formation will result in 90% reduction in caustic consumption and waste stream formation compared to the 1-bioreactor line-up.

Regulation of influent sulfide concentration on anaerobic denitrifying sulfide removal

The objective of this study is to find a comprehensive regulation for sulfide removal and elemental sulfur transformation based on the denitrifying sulfide removal process. The experiment was performed based on several influent sulfide concentrations (150-600 mg/L) and nitrate-to-sulfur (N/S) molar ratios (0.5-2.0) at reaction times of 24 and 48 h. Sulfide and nitrate removals were mainly dependent on the influent sulfide concentration at sulfide concentrations of 150-200 and 400-600 mg/L, but on the N/S ratio at sulfide concentrations of 250-350 mg/L. Up to 99.7% and 93.8% of sulfide and nitrate were removed, respectively, with 26.5% of elemental sulfur formed at sulfide concentrations of 250-350 mg/L (N/S of 1.0). Only 4-9.4% of elemental sulfur was formed, with sulfide and nitrate removals of 99.9% and 98.7%, respectively, at sulfide concentrations of 150-200 mg/L. Meanwhile, 46.9-94.7% of sulfate was formed with a nitrogen gas conversion rate of 18.2-57.1%. Fewer microorganisms were detected by fluorescence in situ hybridization (FISH) at high sulfide concentrations of 400-600 mg/L, suggesting that the processes of anaerobic denitrification and desulfurization were inhibited.

Biological sulfur oxidation in wastewater treatment: A review of emerging opportunities

Sulfide prevails in both industrial and municipal waste streams and is one of the most troublesome issues with waste handling. Various technologies and strategies have been developed and used to deal with sulfide for decades, among which biological means make up a considerable portion due to their low operation requirements and flexibility. Sulfur bacteria play a vital role in these biotechnologies. In this article, conventional biological approaches dealing with sulfide and functional microorganisms are systematically reviewed. Linking the sulfur cycle with other nutrient cycles such as nitrogen or phosphorous, and with continued focus of waste remediation by sulfur bacteria, has led to emerging biotechnologies. Furthermore, opportunities for energy harvest and resource recovery based on sulfur bacteria are also discussed. The electroactivity of sulfur bacteria indicates a broad perspective of sulfur-based bioelectrochemical systems in terms of bioelectricity production and bioelectrochemical synthesis. The considerable PHA accumulation, high yield and anoxygenic growth conditions in certain phototrophic sulfur bacteria could provide an interesting alternative for bioplastic production. In this review, new merits of biological sulfide oxidation from a traditional environmental management perspective as well as a waste to resource perspective are presented along with their potential applications.

Persulfide Dioxygenase From Acidithiobacillus caldus: Variable Roles of Cysteine Residues and Hydrogen Bond Networks of the Active Site

Persulfide dioxygenases (PDOs) are abundant in Bacteria and also crucial for H2S detoxification in mitochondria. One of the two pdo-genes of the acidophilic bacterium Acidithiobacillus caldus was expressed in Escherichia coli. The protein (AcPDO) had 0.77 ± 0.1 Fe/subunit and an average specific sulfite formation activity of 111.5 U/mg protein (Vmax) at 40°C and pH 7.5 with sulfur and GSH following Michaelis-Menten kinetics. KM for GSH and Kcat were 0.5 mM and 181 s-1, respectively. Glutathione persulfide (GSSH) as substrate gave a sigmoidal curve with a Vmax of 122.3 U/mg protein, a Kcat of 198 s-1 and a Hill coefficient of 2.3 ± 0.22 suggesting positive cooperativity. Gel permeation chromatography and non-denaturing gels showed mostly tetramers. The temperature optimum was 40-45°C, the melting point 63 ± 1.3°C in thermal unfolding experiments, whereas low activity was measurable up to 95°C. Site-directed mutagenesis showed that residues located in the predicted GSH/GSSH binding site and in the central hydrogen bond networks including the iron ligands are essential for activity. Among these, the R139A, D141A, and H171A variants were inactive concomitant to a decrease of their melting points by 3-8 K. Other variants were inactivated without significant melting point change. Two out of five cysteines are likewise essential, both of which lie presumably in close proximity at the surface of the protein (C87 and C224). MalPEG labeling experiments suggests that they form a disulfide bridge. The reducing agent Tris(2-carboxyethyl)phosphine was inhibitory besides N-ethylmaleimide and iodoacetamide suggesting an involvement of cysteines and the disulfide in catalysis and/or protein stabilization. Mass spectrometry revealed modification of C87, C137, and C224 by 305 mass units equivalent to GSH after incubation with GSSH and with GSH in case of the C87A and C224A variants. The results of this study suggest that disulfide formation between the two essential surface-exposed cysteines and Cys-S-glutathionylation serve as a protective mechanism against uncontrolled thiol oxidation and the associated loss of enzyme activity.

A comparative study on denitrifying sludge granulation with different electron donors: Sulfide, thiosulfate and organics

A comparative study on denitrifying sludge granulation with different electron donors (sulfide, thiosulfate and organics) was carried out. Longer time was spent on sulfide-denitrifying granular sludge (DGS) cultivation (88 days) than thiosulfate- and organics-DGS cultivations (57 days). All the three DGS were characterized in terms of particle size distribution, sludge settling ability (indicated by sludge volume index and settling velocity), permeability (indicated by fractal dimension) and extracellular polymeric substances (EPS, including polysaccharide and protein) secretion. Sludge productions in the three DGS-reactors were also monitored. The key functional microorganisms in three granular reactors were revealed via high through-put pyrosequencing analysis. Batch tests were performed to measure the denitrification activities of each DGS, including both denitratation (NO₃^- → NO₂^-) and denitritation (NO₂^- → N₂). We found that thiosulfate-driven denitrifying sludge granulation (TDDSG) should be the most efficient and compact technology for effective BNR in municipal wastewater treatment. The findings of this study suggests the TDDSG could further increase the nitrogen removal potential in an enhanced sulfur cycle-driven bioprocess for co-treatment of wet flue gas desulfurization wastes with fresh sewage depending on three short-cut biological reactions, including: 1) short-cut biological sulfur reduction (SO₄^2-/SO₃^2- → S₂O₃^2-); 2) thiosulfate-driven denitritation (S₂O₃^2- + NO₂^- → SO₄^2- + N₂↑); and 3) nitritation (NH₄⁺ + O₂ → NO₂^-).

Recovery of elemental sulphur from anaerobic effluents through the biological oxidation of sulphides

The aim of the present study was to evaluate the biological oxidation of sulphide in two different UASB reactors by assessing the occurrence of oxidized forms of sulphur in the effluents and the amount of S⁰ that could be recovered in the process. The bioreactors employed were an anaerobic hybrid (AH) reactor employing porous polyurethane foam as support media and a micro-aerated UASB reactor equipped with an aeration device above the digestion zone. The AH reactor produced a final effluent containing low concentrations of S^2- (3.87% of total sulphur load). It was achieved due to a complete oxidation of 56.1% of total sulphur. The partial biological oxidation that occurred in the AH reactor allowed the recovery of 30% of the sulphur load as S⁰. The effluent from the micro-aerated UASB reactor contained 5% of the sulphur load in the form of S^2-, while 20.9% was present as dissolved SO₄^2- and 46% was precipitated as S⁰. It is concluded that the AH reactor or micro-aeration carried out above the digestion zone of the UASB reactor favoured the biological oxidation of S^2- and the release of odourless effluents. Both technologies represent feasible and low-cost alternatives for the anaerobic treatment of domestic sewage.

Bioaugmentation of activated sludge with elemental sulfur producing strain Thiopseudomonas denitrificans X2 against nitrate shock load

The sulfide and nitrogen compounds in wastewaters are toxic and cause a serious environmental problem. Thiopseudomonas denitrificans X2, which is the type species of a novel genus Thiopseudomonas was used for bioaugmentation. It oxidized sulfide and acetate with nitrate, and generated elemental sulfur that could be recovered as resource. The generation rate of elemental sulfur was enhanced significantly by the bioaugmentation under the condition of excessive nitrate feeding. The inoculums survived and worked actively in the activated sludge system as the dominant population. Thiopseudomonas denitrificans X2 could be applied to wastewater treatment and resource recovery simultaneously.

Respirometric characterization of aerobic sulfide, thiosulfate and elemental sulfur oxidation by S-oxidizing biomass

Respirometry was used to reveal the mechanisms involved in aerobic biological sulfide oxidation and to characterize the kinetics and stoichiometry of a microbial culture obtained from a desulfurizing biotrickling filter. Physical-chemical processes such as stripping and chemical oxidation of hydrogen sulfide were characterized since they contributed significantly to the conversions observed in respirometric tests. Mass transfer coefficient for hydrogen sulfide and the kinetic parameters for chemical oxidation of sulfide with oxygen were estimated. The stoichiometry of the process was determined and the different steps in the sulfide oxidation process were identified. The conversion scheme proposed includes intermediate production of elemental sulfur and thiosulfate and the subsequent oxidation of both compounds to sulfate. A kinetic model describing each of the reactions observed during sulfide oxidation was calibrated and validated. The product selectivity was found to be independent of the dissolved oxygen to hydrogen sulfide concentration ratio in the medium at sulfide concentrations ranging from 3 to 30 mg S L(-1). Sulfide was preferentially consumed (SOURmax = 49.2 mg DO g(-1) VSS min(-1)) and oxidized to elemental sulfur at dissolved oxygen concentrations above 0.8 mg DO L(-1). Substrate inhibition of sulfide oxidation was observed (K(i,S(2-))= 42.4 mg S L(-1)). Intracellular sulfur accumulation also affected negatively the sulfide oxidation rate. The maximum fraction of elemental sulfur accumulated inside cells was estimated (25.6% w/w) and a shrinking particle equation was included in the kinetic model to describe elemental sulfur oxidation. The microbial diversity obtained through pyrosequencing analysis revealed that Thiothrix sp. was the main species present in the culture (>95%).

Influence of methanethiol on biological sulphide oxidation in gas treatment system

Inorganic and organic sulphur compounds such as hydrogen sulphide (H2S) and thiols (RSH) are unwanted components in sour gas streams (e.g. biogas and refinery gases) because of their toxicity, corrosivity and bad smell. Biological treatment processes are often used to remove H2S at small and medium scales (<50 tons per day of H2S). Preliminarily research by our group focused on achieving maximum sulphur production from biological H2S oxidation in the presence of methanethiol. In this paper the underlying principles have been further studied by assessing the effect of methanethiol on the biological conversion of H2S under a wide range of redox conditions covering not only sulphur but also sulphate-producing conditions. Furthermore, our experiments were performed in an integrated system consisting of a gas absorber and a bioreactor in order to assess the effect of methanethiol on the overall gas treatment efficiency. This study shows that methanethiol inhibits the biological oxidation of H2S to sulphate by way of direct suppression of the cytochrome c oxidase activity in biomass, whereas the oxidation of H2S to sulphur was hardly affected. We estimated the kinetic parameters of biological H2S oxidation that can be used to develop a mathematical model to quantitatively describe the biodesulphurization process. Finally, it was found that methanethiol acts as a competitive inhibitor; therefore, its negative effect can be minimized by increasing the enzyme (biomass) concentration and the substrate (sulphide) concentration, which in practice means operating the biodesulphurization systems under low redox conditions.

System evaluation and microbial analysis of a sulfur cycle-based wastewater treatment process for Co-treatment of simple wet flue gas desulfurization wastes with freshwater sewage

A sulfur cycle-based wastewater treatment process, namely the Sulfate reduction, Autotrophic denitrification and Nitrification Integrated process (SANI(®) process) has been recently developed for organics and nitrogen removal with 90% sludge minimization and 35% energy reduction in the biological treatment of saline sewage from seawater toilet flushing practice in Hong Kong. In this study, sulfate- and sulfite-rich wastes from simple wet flue gas desulfurization (WFGD) were considered as a potential low-cost sulfur source to achieve beneficial co-treatment with non-saline (freshwater) sewage in continental areas, through a Mixed Denitrification (MD)-SANI process trialed with synthetic mixture of simple WFGD wastes and freshwater sewage. The system showed 80% COD removal efficiency (specific COD removal rate of 0.26 kg COD/kg VSS/d) at an optimal pH of 7.5 and complete denitrification through MD (specific nitrogen removal rate of 0.33 kg N/kg VSS/d). Among the electron donors in MD, organics and thiosulfate could induce a much higher denitrifying activity than sulfide in terms of both NO3(-) reduction and NO2(-) reduction, suggesting a much higher nitrogen removal rate in organics-, thiosulfate- and sulfide-based MD in MD-SANI compared to sulfide alone-based autotrophic denitrification in conventional SANI(®). Diverse sulfate/sulfite-reducing bacteria (SRB) genera dominated in the bacterial community of sulfate/sulfite-reducing up-flow sludge bed (SRUSB) sludge without methane producing bacteria detected. Desulfomicrobium-like species possibly for sulfite reduction and Desulfobulbus-like species possibly for sulfate reduction are the two dominant groups with respective abundance of 24.03 and 14.91% in the SRB genera. Diverse denitrifying genera were identified in the bacterial community of anoxic up-flow sludge bed (AnUSB) sludge and the Thauera- and Thiobacillus-like species were the major taxa. These results well explained the successful operation of the lab-scale MD-SANI process.

Effect of Methanethiol Concentration on Sulfur Production in Biological Desulfurization Systems under Haloalkaline Conditions

Bioremoval of H2S from gas streams became popular in recent years because of high process efficiency and low operational costs. To expand the scope of these processes to gas streams containing volatile organic sulfur compounds, like thiols, it is necessary to provide new insights into their impact on overall biodesulfurization process. Published data on the effect of thiols on biodesulfurization processes are scarce. In this study, we investigated the effect of methanethiol on the selectivity for sulfur production in a bioreactor integrated with a gas absorber. This is the first time that the inhibition of biological sulfur formation by methanethiol is investigated. In our reactor system, inhibition of sulfur production started to occur at a methanethiol loading rate of 0.3 mmol L(-1) d(-1). The experimental results were also described by a mathematical model that includes recent findings on the mode of biomass inhibition by methanethiol. We also found that the negative effect of methanethiol can be mitigated by lowering the salinity of the bioreactor medium. Furthermore, we developed a novel approach to measure the biological activity by sulfide measurements using UV-spectrophotometry. On the basis of this measurement method, it is possible to accurately estimate the unknown kinetic parameters in the mathematical model.

Investigation on thiosulfate-involved organics and nitrogen removal by a sulfur cycle-based biological wastewater treatment process

Thiosulfate, as an intermediate of biological sulfate/sulfite reduction, can significantly improve nitrogen removal potential in a biological sulfur cycle-based process, namely the Sulfate reduction-Autotrophic denitrification-Nitrification Integrated (SANI(®)) process. However, the related thiosulfate bio-activities coupled with organics and nitrogen removal in wastewater treatment lacked detailed examinations and reports. In this study, S2O3(2-) transformation during biological SO4(2-)/SO3(2-) co-reduction coupled with organics removal as well as S2O3(2-) oxidation coupled with chemolithotrophic denitrification were extensively evaluated under different experimental conditions. Thiosulfate is produced from the co-reduction of sulfate and sulfite through biological pathway at an optimum pH of 7.5 for organics removal. And the produced S2O3(2-) may disproportionate to sulfide and sulfate during both biological S2O3(2-) reduction and oxidation most possibly carried out by Desulfovibrio-like species. Dosing the same amount of nitrate, pH was found to be the more direct factor influencing the denitritation activity than free nitrous acid (FNA) and the optimal pH for denitratation (7.0) and denitritation (8.0) activities were different. Spiking organics significantly improved both denitratation and denitritation activities while minimizing sulfide inhibition of NO3(-) reduction during thiosulfate-based denitrification. These findings in this study can improve the understanding of mechanisms of thiosulfate on organics and nitrogen removal in biological sulfur cycle-based wastewater treatment.

Biological removal of methanethiol from gas and water streams by using Thiobacillus thioparus: investigation of biodegradability and optimization of sulphur production

The present work mainly deals with biological oxidation, which was tested using the bacterium Thiobacillus thioparus in semi-batch bioreactor systems to evaluate the removal efficiencies and optimal conditions for the biodegradation of methanethiol (MT) in order to treat the natural gas and refinery output streams. The efficiency of this method is analysed by evaluating the concentration of MT in a bioreactor. The effect of operational parameters, such as initial concentration of MT, pH, temperature, dissolved oxygen (DO), initial concentration of bacteria and reaction time on the degradation of MT, were studied. In this process, MT is converted into elemental sulphur particles as an intermediate in the oxidation process of MT to sulphate. The obtained results showed that the highest degradation rate occurred during the first 300 minutes of reaction time. The optimal conditions of the different initial MT concentrations with 0.3-0.6 bacteria OD, DO of 0.5 ppm, acidic pH value of 6.2 and temperature of 300C are obtained. Acidic pH and oxygen-limiting conditions were applied to obtain 80-85% selectivity for elemental sulphur formation in products. Under the optimal conditions, and for the highest (8.51 mM) and the lowest (0.53 mM) concentration of MT, the biological removal was about 89% and 94%, respectively.

Simultaneous removal of sulfide, nitrate and acetate under denitrifying sulfide removal condition: modeling and experimental validation

Simultaneous removal of sulfide (S(2-)), nitrate (NO3(-)) and acetate (Ac(-)) under denitrifying sulfide removal process (DSR) is a novel biological wastewater treatment process. This work developed a mathematical model to describe the kinetic behavior of sulfur-nitrogen-carbon and interactions between autotrophic denitrifiers and heterotrophic denitrifiers. The kinetic parameters of the model were estimated via data fitting considering the effects of initial S(2-) concentration, S(2-)/NO3(-)-N ratio and Ac(-)-C/NO3(-)-N ratio. Simulation supported that the heterotrophic denitratation step (NO3(-) reduction to NO2(-)) was inhibited by S(2-) compared with the denitritation step (NO2(-) reduction to N2). Also, the S(2-) oxidation by autotrophic denitrifiers was shown two times lower in rate with NO2(-) as electron acceptor than that with NO3(-) as electron acceptor. NO3(-) reduction by autotrophic denitrifiers occurs 3-10 times slower when S(0) participates as final electron donor compared to the S(2-)-driven pathway. Model simulation on continuous-flow DSR reactor suggested that the adjustment of hydraulic retention time is an efficient way to make the reactor tolerating high S(2-) loadings. The proposed model properly described the kinetic behaviors of DSR processes over wide parametric ranges and which can offer engineers with basis to optimize bioreactor operation to improve the treatment capacity.

Distribution, diversity, and activities of sulfur dioxygenases in heterotrophic bacteria

Sulfur oxidation by chemolithotrophic bacteria is well known; however, sulfur oxidation by heterotrophic bacteria is often ignored. Sulfur dioxygenases (SDOs) (EC 1.13.11.18) were originally found in the cell extracts of some chemolithotrophic bacteria as glutathione (GSH)-dependent sulfur dioxygenases. GSH spontaneously reacts with elemental sulfur to generate glutathione persulfide (GSSH), and SDOs oxidize GSSH to sulfite and GSH. However, SDOs have not been characterized for bacteria, including chemolithotrophs. The gene coding for human SDO (human ETHE1 [hETHE1]) in mitochondria was discovered because its mutations lead to a hereditary human disease, ethylmalonic encephalopathy. Using sequence analysis and activity assays, we discovered three subgroups of bacterial SDOs in the proteobacteria and cyanobacteria. Ten selected SDO genes were cloned and expressed in Escherichia coli, and the recombinant proteins were purified. The SDOs used Fe(2+) for catalysis and displayed considerable variations in specific activities. The wide distribution of SDO genes reveals the likely source of the hETHE1 gene and highlights the potential of sulfur oxidation by heterotrophic bacteria.

Microbial community composition of a down-flow hanging sponge (DHS) reactor combined with an up-flow anaerobic sludge blanket (UASB) reactor for the treatment of municipal sewage

The microbial community composition of a down-flow hanging sponge (DHS) reactor in an up-flow anaerobic sludge blanket (UASB)-DHS system used for the treatment of municipal sewage was investigated. The clone libraries showed marked differences in microbial community composition at different reactor heights and in different seasons. The dominant phylotypes residing in the upper part of the reactor were likely responsible for removing organic matters because a significant reduction in organic matter in the upper part was observed. Quantification of the amoA genes revealed that the proportions of ammonia oxidizing bacteria (AOB) varied along the vertical length of the reactor, with more AOB colonizing the middle and lower parts of the reactor than the top of the reactor. The findings indicated that sewage treatment was achieved by a separation of microbial habitats responsible for organic matter removal and nitrification in the DHS reactor.

A physiologically based kinetic model for bacterial sulfide oxidation

In the biotechnological process for hydrogen sulfide removal from gas streams, a variety of oxidation products can be formed. Under natron-alkaline conditions, sulfide is oxidized by haloalkaliphilic sulfide oxidizing bacteria via flavocytochrome c oxidoreductase. From previous studies, it was concluded that the oxidation-reduction state of cytochrome c is a direct measure for the bacterial end-product formation. Given this physiological feature, incorporation of the oxidation state of cytochrome c in a mathematical model for the bacterial oxidation kinetics will yield a physiologically based model structure. This paper presents a physiologically based model, describing the dynamic formation of the various end-products in the biodesulfurization process. It consists of three elements: 1) Michaelis-Menten kinetics combined with 2) a cytochrome c driven mechanism describing 3) the rate determining enzymes of the respiratory system of haloalkaliphilic sulfide oxidizing bacteria. The proposed model is successfully validated against independent data obtained from biological respiration tests and bench scale gas-lift reactor experiments. The results demonstrate that the model is a powerful tool to describe product formation for haloalkaliphilic biomass under dynamic conditions. The model predicts a maximum S⁰ formation of about 98 mol%. A future challenge is the optimization of this bioprocess by improving the dissolved oxygen control strategy and reactor design.

Biomass accumulation modelling in a highly loaded biotrickling filter for hydrogen sulphide removal

A pilot scale test on a biotrickling filter packed with polyurethane foam cubes was carried out for 110 d at high volumetric mass load (up to 280 g m(bed)(-3) h(-1)) with the aim of studying the accumulation of solids in the treatment of H(2)S. Removal rate up to 245 g m(bed)(-3) h(-1) was obtained; however, an accumulation of gypsum, elemental sulphur and, above all, inert biomass was identified as the cause of an increased pressure drop over the long term. A mathematical model was applied and calibrated with the experimental results to describe the accumulation of biomass. The model was capable of describing the accumulation of solids and, corresponding to a solids retention time of 50 d, the observed yield resulted in 0.07 g of solids produced g(-1) H(2)S removed. Respirometric tests showed that heterotrophic activity is inhibited at low pH (pH < 2.3), and the contribution to biomass removal through decay was negligible.

Sulfur and Sulfur Compounds in Plant Defence

The multiplicity of chemical structures of sulfur containing compounds, influenced in part by the element's several oxidation states, directly results in diverse modes of action for sulfur-containing natural products synthesized as secondary metabolites in plants. Sulfur-containing natural products constitute a formidable wall of defence against a wide range of pathogens and pests. Steady progress in the development of new technologies have advanced research in this area, helping to uncover the role of such important plant defence molecules like endogenously-released elemental sulphur, but also deepening current understanding of other better-studied compounds like the glucosinolates. As studies continue in this area, it is becoming increasingly evident that sulfur and sulfur compounds play far more important roles in plant defence than perhaps previously suspected.

Sulfide oxidation in fluidized bed bioreactor using nylon support material

A continuous fluidized bed bioreactor (FBBR) with nylon support particles was used to treat synthetic sulfide wastewater at different hydraulic retention time of 25, 50 and 75 min and upflow velocity of 14, 17 and 20 m/hr. The effects of upflow velocity, hydraulic retention time and reactor operation time on sulfide oxidation rate were studied using statistical model. Mixed culture obtained from the activated sludge, taken from tannery effluent treatment plant, was used as a source for microorganisms. The diameter and density of the nylon particles were 2-3 mm and 1140 kg/m3, respectively. Experiments were carried out in the reactor at a temperature of (30 +/- 2) degrees C, at a fixed bed height of 16 cm after the formation of biofilm on the surface of support particles. Biofilm thickness reached (42 +/- 3) microm after 15 days from reactor start-up. The sulfide oxidation, sulfate and sulfur formation is examined at all hydraulic retention times and upflow velocities. The results indicated that almost 90%-92% sulfide oxidation was achieved at all hydraulic retention times. Statistical model could explain 94% of the variability and analysis of variance showed that upflow velocity and hydraulic retention time slightly affected the sulfide oxidation rate. The highest sulfide oxidation of 92% with 70% sulfur was obtained at hydraulic retention time of 75 min and upflow velocity of 14 m/hr.

Monitoring biological sulphide oxidation processes using combined respirometric and titrimetric techniques

The application of respirometric and titrimetric techniques to evaluate kinetic parameters and stoichiometry of the sulphide-oxidising biomass is a new promising approach for biotechnological sulphide oxidation process monitoring. It was possible to estimate the yield coefficients of each oxidation step of sulphide to elemental sulphur and to sulphate using respirometric tests, while evaluating the behaviour of the biomass in endogenous conditions. Furthermore, it was demonstrated how the combined application of titrimetric and respirometric techniques enabled the monitoring of sulphur and sulphate formation as a function of the environmental conditions. This approach provided valuable information of the biological sulphide oxidation processes and preliminary results may be used as a starting point for the formulation and use of a mathematical model.

Biochemistry and molecular biology of lithotrophic sulfur oxidation by taxonomically and ecologically diverse bacteria and archaea

Lithotrophic sulfur oxidation is an ancient metabolic process. Ecologically and taxonomically diverged prokaryotes have differential abilities to utilize different reduced sulfur compounds as lithotrophic substrates. Different phototrophic or chemotrophic species use different enzymes, pathways and mechanisms of electron transport and energy conservation for the oxidation of any given substrate. While the mechanisms of sulfur oxidation in obligately chemolithotrophic bacteria, predominantly belonging to Beta- (e.g. Thiobacillus) and Gammaproteobacteria (e.g. Thiomicrospira), are not well established, the Sox system is the central pathway in the facultative bacteria from Alphaproteobacteria (e.g. Paracoccus). Interestingly, photolithotrophs such as Rhodovulum belonging to Alphaproteobacteria also use the Sox system, whereas those from Chromatiaceae and Chlorobi use a truncated Sox complex alongside reverse-acting sulfate-reducing systems. Certain chemotrophic magnetotactic Alphaproteobacteria allegedly utilize such a combined mechanism. Sulfur-chemolithotrophic metabolism in Archaea, largely restricted to Sulfolobales, is distinct from those in Bacteria. Phylogenetic and biomolecular fossil data suggest that the ubiquity of sox genes could be due to horizontal transfer, and coupled sulfate reduction/sulfide oxidation pathways, originating in planktonic ancestors of Chromatiaceae or Chlorobi, could be ancestral to all sulfur-lithotrophic processes. However, the possibility that chemolithotrophy, originating in deep sea, is the actual ancestral form of sulfur oxidation cannot be ruled out.

Effects of methanethiol on the biological oxidation of sulfide at natron-alkaline conditions

The effects of methanethiol (MT) on biological sulfide oxidation were studied in a continuously operated bioreactor, in which chemolithoautotrophic bacteria belonging to the genus Thioalkalivibrio convert hydrogen sulfide (H2S) at natron-alkaline conditions. Previous bioreactor experiments have shown that always a fraction of the H2S is oxidized to sulfate and thiosulfate. This is unwanted, as it leads to caustic requirements for pH control and the formation of a bleed stream to discharge these compounds from the process. The current research shows that due to the addition of MT, sulfate formation is prevented. As a result, all supplied H2S is completely converted into elemental sulfur. Treatment of a continuous supply of 51.0 mM day(-1) H2S and 79 microM day(-1) MT was feasible for a prolonged period, with 99 mol% selectivity for sulfur formation. A part of the MT reacts with the freshly produced sulfur particles to form dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS). Results indicate that MT, DMDS, and DMTS partly adsorb onto the biosulfur particles. At concentrations above 10 microM, these volatile organic sulfur compounds induce biomass decay.

Investigation on laboratory and pilot-scale airlift sulfide oxidation reactor under varying sulfide loading rate

Airlift bioreactor was established for recovering sulfur from synthetic sulfide wastewater under controlled dissolved oxygen condition. The maximum recovered sulfur was 14.49 g/day when sulfide loading rate, dissolved oxygen (DO) and pH values were 2.97 kgHS(-)/m(3)-day, 0.2-1.0 mg/L and 7.2-7.8, respectively. On the other hand, the increase in recovered sulfur reduced the contact surface of sulfide oxidizing bacteria which affects the recovery process. This effect caused to reduce the conversion of sulfide to sulfur. More recovered sulfur was produced at high sulfide loading rate due to the change of metabolic pathway of sulfide-oxidizing bacteria which prevented the toxicity of sulfide in the culture. The maximum activity in this system was recorded to be about 3.28 kgS/kgVSS-day. The recovered sulfur contained organic compounds which were confirmed by the results from XRD and CHN analyzer. Afterwards, by annealing the recovered sulfur at 120 degrees C for 24 hrs under ambient Argon, the percentage of carbon reduced from 4.44% to 0.30%. Furthermore, the percentage of nitrogen and hydrogen decreased from 0.79% and 0.48% to 0.00% and 0.14%, respectively. This result showed the success in increasing the purity of recovered sulfur by using the annealing technique. The pilot-scale biological sulfide oxidation process was carried out using real wastewater from Thai Rayon Industry in Thailand. The airlift reactor successfully removed sulfide more than 90% of the influent sulfide at DO concentration of less than 0.1 mg/L, whereas the elementary sulfur production was 2.37 kgS/m(3)-day at sulfide loading rate of 2.14 kgHS(-)/m(3)-day. The sulfur production was still increasing as the reactor had not yet reached its maximum sulfide loading rate.