Desulfurization of light gas oil in immobilized-cell systems of Gordona sp. CYKS1 and Nocardia sp. CYKS2

Nanocatalysts for Oxidative Desulfurization of Liquid Fuel: Modern Solutions and the Perspectives of Application in Hybrid Chemical-Biocatalytic Processes

In this paper, the current advantages and disadvantages of using metal-containing nanocatalysts (NCs) for deep chemical oxidative desulfurization (ODS) of liquid fuels are reviewed. A similar analysis is performed for the oxidative biodesulfurization of oil along the 4S-pathway, catalyzed by various aerobic bacterial cells of microorganisms. The preferences of using NCs for the oxidation of organic sulfur-containing compounds in various oil fractions seem obvious. The text discusses the development of new chemical and biocatalytic approaches to ODS, including the use of both heterogeneous NCs and anaerobic microbial biocatalysts that catalyze the reduction of chemically oxidized sulfur-containing compounds in the framework of methanogenesis. The addition of anaerobic biocatalytic stages to the ODS of liquid fuel based on NCs leads to the emergence of hybrid technologies that improve both the environmental characteristics and the economic efficiency of the overall process. The bioconversion of sulfur-containing extracts from fuels with accompanying hydrocarbon residues into biogas containing valuable components for the implementation of C-1 green chemistry processes, such as CH4, CO2, or H2, looks attractive for the implementation of such a hybrid process.

Efficient biotransformation of sulfide in anaerobic sequencing batch reactor by composite microbial agent: performance optimization and microbial community analysis

Sulfur-containing wastewater is very common as an industrial waste, yet a high-efficiency composite microbial agent for sulfur-containing wastewater treatment is still lacking. In this work, three novel and efficient desulfurizing bacteria were isolated from the sewage treatment tank of Zhejiang Satellite Energy Co., Ltd. They were identified as Brucella melitensis (S1), Ochrobactrum oryzae (S8), and Achromobacter xylosoxidans (S9). These three strains of bacteria were responsible for the oxidative metabolism of sodium sulfide via a similar polythionate pathway, which could be expressed as follows: S^2-→S₂O₃^2-/S⁰→SO₃^2-→SO₄^2-. Activated carbon, wheat bran, and diatomite at 1:1:1 ratio are used as carriers to construct a composite microbial agent containing the three bacteria. The desulfurization efficiency of 95% was predicted by response surface methodology under the following optimum conditions: the dosage of the inoculum was 3 g/L, pH 7.86, and temperature of 39 °C. Additionally, the impact resistance was studied in the anaerobic sequencing batch reactor. The removal capacity of microbial agent reached 98%. High-throughput analysis showed that composite microbial agent increased bacterial evenness and diversity, and the relative abundance of Brucellaceae increased from 5.04 to 8.79% in the reactor. In the process of industrial wastewater transformation, the transformation rate of sulfide by composite microbial agent was maintained between 70 and 81%. The composite microbial agent had potential for the treatment of sulfur-containing wastewater.

Uptake and detoxification of diesel oil by a tropical soil Actinomycete Gordonia amicalis HS-11: Cellular responses and degradation perspectives

A tropical soil Actinomycete, Gordonia amicalis HS-11, has been previously demonstrated to degrade unsaturated and saturated hydrocarbons (squalene and n-hexadecane, respectively) in an effective manner. In present study, G. amicalis HS-11 degraded 92.85 ± 3.42% of the provided diesel oil [1% (v/v)] after 16 days of aerobic incubation. The effect of different culture conditions such as carbon source, nitrogen source, pH, temperature, and aeration on degradation was studied. During degradation, this Actinomycete synthesized surface active compounds (SACs) in an extracellular manner that brought about a reduction in surface tension from 69 ± 2.1 to 30 ± 1.1 mN m^-1 after 16 days. The morphology of cells grown on diesel was monitored by using a Field Emission Scanning Electron Microscope. Diesel-grown cells were longer and clumped with smooth surfaces, possibly due to the secretion of SACs. The interaction between the cells and diesel oil was studied by Confocal Laser Scanning Microscope. Some cells were adherent on small diesel droplets and others were present in the non-attached form thus confirming the emulsification ability of this organism. The fatty acid profiles of the organism grown on diesel oil for 48 h were different from those on Luria Bertani Broth. The genotoxicity and cytotoxicity of diesel oil before and after degradation were determined. Cytogenetic parameters such as mitotic index (MI); mitosis distribution and chromosomal aberration (type and frequency) were assessed. Oxidative stress was evaluated by measuring levels of catalase, superoxide dismutase and concentration of malondialdehyde. On the basis of these studies it was deduced that the degradation metabolites were relatively non-toxic.

An overview of conventional and alternative technologies for the production of ultra-low-sulfur fuels

Environmental concerns have given a great deal of attention for the production of ultra-low-sulfur fuels. The conventional hydrodesulfurization (HDS) process has high operating cost and also encounters difficulty in removing sulfur compound with steric hindrance. Consequently, various research efforts have been made to overcome the limitation of conventional HDS process and exploring the alternative technologies for deep desulfurization. The alternative processes being explored for the production of ultra-low-sulfur content fuel are adsorptive desulfurization (ADS), biodesulfurization (BDS), oxidative desulfurization (ODS), and extractive desulfurization (EDS). The present article provided the comprehensive information on the basic principle, reaction mechanism, workability, advantages, and disadvantages of conventional and alternative technologies. This review article aims to provide valuable insight into the recent advances made in conventional HDS process and alternative techniques. For deep desulfurization of liquid fuels, integration of conventional HDS with an alternative technique is also proposed.

Whole Cell Actinobacteria as Biocatalysts

Production of fuels, therapeutic drugs, chemicals, and biomaterials using sustainable biological processes have received renewed attention due to increasing environmental concerns. Despite having high industrial output, most of the current chemical processes are associated with environmentally undesirable by-products which escalate the cost of downstream processing. Compared to chemical processes, whole cell biocatalysts offer several advantages including high selectivity, catalytic efficiency, milder operational conditions and low impact on the environment, making this approach the current choice for synthesis and manufacturing of different industrial products. In this review, we present the application of whole cell actinobacteria for the synthesis of biologically active compounds, biofuel production and conversion of harmful compounds to less toxic by-products. Actinobacteria alone are responsible for the production of nearly half of the documented biologically active metabolites and many enzymes; with the involvement of various species of whole cell actinobacteria such as Rhodococcus, Streptomyces, Nocardia and Corynebacterium for the production of useful industrial commodities.

A new functional biofilm biocatalyst for the simultaneous removal of dibenzothiophene and quinoline using Rhodococcus rhodochrous and curli amyloid overproducer mutants derived from Cobetia sp. strain MM1IDA2H-1

Biocatalyst systems based on biofilms were developed to remove nitrogen and sulfur-containing heterocyclic hydrocarbons using Cobetia sp. strain MM1IDA2H-1 and Rhodococcus rhodochrous. The curli overproducers mutants CM₁ and CM₄ were derived from Cobetia sp. strain and used to build monostrain biofilms to remove quinoline; and together with R. rhodochrous to simultaneously remove quinoline and dibenzothiophene using mixed biofilms. The quinoline removal using biofilms were 96% and 97% using CM₁ or CM₄ curli overproducers respectively, whereas bacterial suspensions assays yielded 19% and 24% with the same strains. At the other hand, the simultaneous removal of quinoline and dibenzothiophene using mixed biofilms were respectively 50% and 58% using strains R. rhodochrous with CM₁ and 75% and 50% using R. rhodochrous with CM₄. Results show that biofilms were more efficient than bacterial suspension assays and that in mixed biofilms the shared surface area by two or more bacteria could affect the final yield.

Enhancement of bio-desulfurization capability of a newly isolated thermophilic bacterium using starch/iron nanoparticles in a controlled system

Due to the increasing application of oil and petroleum products, increased environmental contamination has become a matter of concern. Bio-desulfurization process may be used to eliminate sulfur from fossil fuels in the moderate condition. In this study, a thermophilic bacterium was isolated that was able to desulfurize dibenzothiophene. 16S rRNA sequencing indicated that this strain is related closely to Bacillus thermoamylovorans (97%). This strain grew in Basal salt medium containing DBT (100 mgl^-1) as the only sulfur source, at 55°C and showed maximum growth (OD₆₆₀ = 0.850) following 72 h incubation time. 2‑hydroxybiphenyl was produced at the maximal concentration (26.13 ± 0.12 mgl^-1) at 72 h. Bio-desulfurization and growth rate factors were optimized using response surface methodology. Starch/Fe₃O₄ and starch/Fe nanoparticles were used for enhancement of BDS efficiency. The size of starch/Fe₃O₄ and starch/Fe nanoparticles were 20 and 30-40 nm, respectively, as described by using scanning electron microscope and transmission electron microscope. The results showed that the immobilized cells by starch/Fe₃O₄ and starch/Fe nanoparticles had higher desulfurization capacity, about 10% and 22% more, respectively. Also, BDS in a bioreactor in the presence of nanoparticles was increased 25% with respect of the process occurred in the flask.

Desulfurization of JP-8 jet fuel: challenges and adsorptive materials

This review describes ongoing efforts to remove the bulky organosulfur compounds from Jet Propellant 8 (JP-8) that cannot be removed by hydrodesulfurization.

Biodesulphurization of gasoline by Rhodococcus erythropolis supported on polyvinyl alcohol

A new biodesulphurization (BDS) method has been considered using Rhodococcus erythropolis supported on polyvinyl alcohol (PVA) for BDS of thiophene as a gasoline sulphur model compound in n-hexane as the solvent, subsequently this biocatalyst has been applied to BDS of gasoline samples. The obtained results according to UV-Spectrophotometer analysis at 240 nm showed that 97·41% of thiophene at the optimum condition of primary concentration 80 mg l^-1 , pH = 7, by 0·1 g of biocatalyst in 30°C and after 20 h of contact time has been degraded. These optimum conditions have been applied to gasoline BDS and the biodegradation of gasoline thiophenic compounds have been investigated by gas chromatography-mass spectrometry (GC-MS). According to GC-MS, thiophene and its 2-methyl, 3-methyl and 2- ethyl derivatives had acceptable biodegradation efficiencies of about 26·67, 21·03, 23·62% respectively. Also, benzothiophene that has been detected in a gasoline sample had 38·89% biodegradation efficiency at optimum conditions, so biomodification of PVA by R. erythropolis produces biocatalysts with an active metabolism that facilitates the interaction of bacterial strain with gasoline thiophenic compounds. The morphology and surface functional groups of supported R. erythropolis on PVA have been investigated by scanning electron microscope (SEM) and FT-IR spectroscopy respectively. SEM images suggest some regular layered shape for the supported bacteria. FT-IR spectra indicate a desirable interaction between bacterial cells and polymer supports. Also, the recovery of biocatalyst has been investigated and after three times of using in BDS activity, its biocatalytic ability had no significant decreases.

SIGNIFICANCE AND IMPACT OF THE STUDY

The biomodification of polyvinyl alcohol by Rhodococcus erythropolis described herein produces a new biocatalyst which can be used for significantly reducing the thiophenic compounds of gasoline and other fossil fuels. The immobilization process is to increase the biodegradation efficiency of cells and accelerating the biodesulphurization process.

Advances in the Reduction of the Costs Inherent to Fossil Fuels' Biodesulfurization towards Its Potential Industrial Application

Biodesulfurization (BDS) process consists on the use of microorganisms for the removal of sulfur from fossil fuels. Through BDS it is possible to treat most of the organosulfur compounds recalcitrant to the conventional hydrodesulfurization (HDS), the petroleum industry's solution, at mild operating conditions, without the need for molecular hydrogen or metal catalysts. This technique results in lower emissions, smaller residue production and less energy consumption, which makes BDS an eco-friendly process that can complement HDS making it more efficient. BDS has been extensively studied and much is already known about the process. Clearly, BDS presents advantages as a complementary technique to HDS; however its commercial use has been delayed by several limitations both upstream and downstream the process. This study will comprehensively review and discuss key issues, like reduction of the BDS costs, advances and/or challenges for a competitive BDS towards its potential industrial application aiming ultra low sulfur fuels.

Fructophilic behaviour of Gordonia alkanivorans strain 1B during dibenzothiophene desulfurization process

Biodesulfurization (BDS) aims at the removal of recalcitrant sulfur from fossil fuels at mild operating conditions with the aid of microorganisms. These microorganisms can remove sulfur from dibenzothiphene (DBT), a model compound, or other polycyclic aromatic used as sulfur source, making BDS an easy and environmental friendly process. Gordonia alkanivorans strain 1B has been described as a desulfurizing bacterium, able to desulfurize DBT to 2-hydroxybiphenyl (2-HBP), the final product of the 4S pathway, using d-glucose as carbon source. However, both cell growth and desulfurization can be largely affected by the nutrient composition of the growth medium, due to cofactor requirements of many enzymes involved in the BDS biochemical pathway. In this study, the main goal was to investigate the influence of several sugars, as carbon source, on the growth and DBT desulfurization ability of G. alkanivorans strain 1B. The results of desulfurization tests showed that the lowest values for the growth rate (0.025 hour(-1)) and for the overall 2-HBP production rate (1.80 μm/hour) by the strain 1B were obtained in glucose grown cultures. When using sucrose, the growth rate increase exhibited by strain 1B led to a higher biomass productivity, which induced a slightly increase in the 2-HBP production rate (1.91 μm/hour), conversely in terms of 2-HBP specific production rate (q2-HBP) the value obtained was markedly lower (0.718 μmol/g/hour in sucrose versus 1.22 μmol/g/hour in glucose). When a mixture of glucose and fructose was used as carbon source, strain 1B reached a value of q2-HBP=1.90 μmol/g/hour, close to that in fructose (q2-HBP=2.12 μmol/g/hour). The highest values for both cell growth (μ=0.091 hour(-1)) and 2-HPB production (9.29μm/hour) were obtained when strain 1B was desulfurizing DBT in the presence of fructose as the only carbon source, indicating a fructophilic behaviour by this bacterium. This fact is in agreement with the highest value of biomass productivity by strain 1B be in fructose, which resulted in a higher amount cells fulfilling the DBT-desulfurization. The greater number of functional cells conducted to a more effectiveness BDS process by strain 1B, as they attained a q2-HBP about 74% higher than in glucose grown cultures. Moreover, this significant BDS enhancement can better be observed in terms of the overall 2-HBP production rate, which increased over 5-fold, from 1.80 μm/hour (in glucose) to 9.29 μm/hour (in fructose).

Thermophilic desulfurization of dibenzothiophene and different petroleum oils by Klebsiella sp. 13T

PURPOSE

Biodesulfurization (BDS) has the potential to desulfurize dibenzothiophene (DBT) and its alkylated derivatives, the compounds that are otherwise refractory to hydrodesulfurization (HDS). Thermophilic microorganisms are more appropriate to be used for BDS applications following HDS. The aim of the present study was to isolate a thermophilic microorganism and to explore its commercial relevance for BDS process.

METHODS

The desulfurizing thermophilic strain was isolated and enriched from various soil and water samples using sulfur free medium (SFM) supplemented with DBT. Microbiological and genomic approach was used to characterize the strain. Desulfurization reactions were carried out using DBT and petroleum oils at 45°C followed by different analytical procedures.

RESULTS

We report the isolation of a thermophilic bacterium Klebsiella sp. 13T from contaminated soils collected from petroleum refinery. HPLC analysis revealed that Klebsiella sp. 13T could desulfurize DBT to 2-hydroxybiphenyl (2-HBP) at 45°C through 4S pathway. In addition, adapted cells of Klebsiella sp. 13T were found to remove 22-53% of sulfur from different petroleum oils with highest sulfur removal from light crude oil.

CONCLUSION

Klebsiella sp. 13T is a potential candidate for BDS because of its thermophilic nature and capability to desulfurize petroleum oils.

Oxidation of dibenzothiophene (DBT) by Serratia marcescens UCP 1549 formed biphenyl as final product

BACKGROUND

The desulphurization of dibenzothiophene (DBT), a recalcitrant thiophenic fossil fuel component by Serratia marcescens (UCP 1549) in order for reducing the Sulphur content was investigated. The Study was carried out establishing the growth profile using Luria Bertani medium to different concentrations of DBT during 120 hours at 28°C, and orbital Shaker at 150 rpm.

RESULTS

The results indicated that concentrations of DBT 0.5, 1.0 and 2.0 mM do not affected the growth of the bacterium. The DBT showed similar Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MCB) (3.68 mM). The desulphurization of DBT by S. marcescens was used with 96 hours of growth on 2 mM of DBT, and was determined by gas chromatography (GC) and GC-mass spectrometry. In order to study the desulphurization process by S. marcescens was observed the presence of a sulfur-free product at 16 hours of cultivation.

CONCLUSIONS

The data suggests the use of metabolic pathway "4S" by S. marcescens (UCP 1549) and formed biphenyl. The microbial desulphurization process by Serratia can be suggest significant reducing sulphur content in DBT, and showed promising potential for reduction of the sulfur content in diesel oil.

Toxicity evaluation of 2-hydroxybiphenyl and other compounds involved in studies of fossil fuels biodesulphurisation

The acute toxicity of some compounds used in fossil fuels biodesulphurisation studies, on the respiration activity, was evaluated by Gordonia alkanivorans and Rhodococcus erythropolis. Moreover, the effect of 2-hydroxybiphenyl on cell growth of both strains was also determined, using batch (chronic bioassays) and continuous cultures. The IC₅₀ values obtained showed the toxicity of all the compounds tested to both strains, specially the high toxicity of 2-HBP. These results were confirmed by the chronic toxicity data. The toxicity data sets highlight for a higher sensitivity to the toxicant by the strain presenting a lower growth rate, due to a lower cells number in contact with the toxicant. Thus, microorganisms exhibiting faster generation times could be more resistant to 2-HBP accumulation during a BDS process. The physiological response of both strains to 2-HBP pulse in a steady-state continuous culture shows their potential to be used in a future fossil fuel BDS process.

Desulfurization of dibenzothiophene (DBT) by a novel strain Lysinibacillus sphaericus DMT-7 isolated from diesel contaminated soil

A new bacterial strain DMT-7 capable of selectively desulfurizing dibenzothiophene (DBT) was isolated from diesel contaminated soil. The DMT-7 was characterized and identified as Lysinibacillus sphaericus DMT-7 (NCBI GenBank Accession No. GQ496620) using 16S rDNA gene sequence analysis. The desulfurized product of DBT, 2-hydroxybiphenyl (2HBP), was identified and confirmed by high performance liquid chromatography analysis and gas chromatography-mass spectroscopy analysis respectively. The desulfurization kinetics revealed that DMT-7 started desulfurization of DBT into 2HBP after the lag phase of 24 hr, exponentially increasing the accumulation of 2HBP up to 15 days leading to approximately 60% desulfurization of the DBT. However, further growth resulted into DBT degradation. The induced culture of DMT-7 showed shorter lag phase of 6 hr and early onset of stationary phase within 10 days for desulfurization as compared to that of non-induced culture clearly indicating the inducibility of the desulfurization pathway of DMT-7. In addition, Lysinibacillus sphaericus DMT-7 also possess the ability to utilize broad range of substrates as sole source of sulfur such as benzothiophene, 3,4-benzo DBT, 4,6-dimethyl DBT, and 4,6-dibutyl DBT. Therefore, Lysinibacillus sphaericus DMT-7 could serve as model system for efficient biodesulfurization of diesel and petrol.

Biocatalytic Desulfurization of Diesel Oil in an Air-Lift Reactor with Immobilized Gordonia nitida CYKS1 Cells

A new type of air-lift reactor with immobilized Gordonia nitida CYKS1 cells on a fibrous support was designed and used for the biocatalytic desulfurization (BDS) of diesel oil. Its performance was evaluated at different phase ratios of the oil to the aqueous medium (or oil phase fractions) and different sucrose concentrations. When the reaction mixture contained 10% diesel oil (v/v), 61-67% of sulfur was removed as the sulfur content decreased from 202-250 to 76-90 mg L(-1) in 72 h. The sulfur content did not decrease any further because the remaining sulfur compounds were recalcitrant to BDS. During the desulfurization, the strain CYKS1 consumed hydrocarbons in the diesel oil, mainly n-alkanes with 10-26 carbons, as carbon source even though an easily available carbon source, sucrose, was supplied.

Degradation of carbazole by microbial cells immobilized in magnetic gellan gum gel beads

Polycyclic aromatic heterocycles, such as carbazole, are environmental contaminants suspected of posing human health risks. In this study, we investigated the degradation of carbazole by immobilized Sphingomonas sp. strain XLDN2-5 cells. Four kinds of polymers were evaluated as immobilization supports for Sphingomonas sp. strain XLDN2-5. After comparison with agar, alginate, and kappa-carrageenan, gellan gum was selected as the optimal immobilization support. Furthermore, Fe(3)O(4) nanoparticles were prepared by a coprecipitation method, and the average particle size was about 20 nm with 49.65-electromagnetic-unit (emu) g(-1) saturation magnetization. When the mixture of gellan gel and the Fe(3)O(4) nanoparticles served as an immobilization support, the magnetically immobilized cells were prepared by an ionotropic method. The biodegradation experiments were carried out by employing free cells, nonmagnetically immobilized cells, and magnetically immobilized cells in aqueous phase. The results showed that the magnetically immobilized cells presented higher carbazole biodegradation activity than nonmagnetically immobilized cells and free cells. The highest biodegradation activity was obtained when the concentration of Fe(3)O(4) nanoparticles was 9 mg ml(-1) and the saturation magnetization of magnetically immobilized cells was 11.08 emu g(-1). Additionally, the recycling experiments demonstrated that the degradation activity of magnetically immobilized cells increased gradually during the eight recycles. These results support developing efficient biocatalysts using magnetically immobilized cells and provide a promising technique for improving biocatalysts used in the biodegradation of not only carbazole, but also other hazardous organic compounds.

Biodesulfurization of dibenzothiophene by microbial cells coated with magnetite nanoparticles

Microbial cells of Pseudomonas delafieldii were coated with magnetic Fe3O4 nanoparticles and then immobilized by external application of a magnetic field. Magnetic Fe3O4 nanoparticles were synthesized by a coprecipitation method followed by modification with ammonium oleate. The surface-modified Fe3O4 nanoparticles were monodispersed in an aqueous solution and did not precipitate in over 18 months. Using transmission electron microscopy (TEM), the average size of the magnetic particles was found to be in the range from 10 to 15 nm. TEM cross section analysis of the cells showed further that the Fe3O4 nanoparticles were for the most part strongly absorbed by the surfaces of the cells and coated the cells. The coated cells had distinct superparamagnetic properties. The magnetization (delta(s)) was 8.39 emu.g(-1). The coated cells not only had the same desulfurizing activity as free cells but could also be reused more than five times. Compared to cells immobilized on Celite, the cells coated with Fe3O4 nanoparticles had greater desulfurizing activity and operational stability.

Microbial desulfurization of gasoline in a Mycobacterium goodii X7B immobilized-cell system

Mycobacterium goodii X7B, which had been primarily isolated as a bacterial strain capable of desulfurizing dibenzothiophene to produce 2-hydroxybiphenyl via the 4S pathway, was also found to desulfurize benzothiophene. The desulfurization product was identified as o-hydroxystyrene by gas chromatography (GC)-mass spectrometry analysis. This strain appeared to have the ability to remove organic sulfur from a broad range of sulfur species in gasoline. When Dushanzi straight-run gasoline (DSRG227) containing various organic sulfur compounds was treated with immobilized cells of strain X7B for 24 h, the total sulfur content significantly decreased, from 227 to 71 ppm at 40 degrees C. GC flame ionization detection and GC atomic emission detection analysis were used to qualitatively evaluate the effects of M. goodii X7B treatment on the contents of gasoline. In addition, when immobilized cells were incubated at 40 degrees C with DSRG275, the sulfur content decreased from 275 to 54 ppm in two consecutive reactions. With this excellent efficiency, strain X7B is considered a good potential candidate for industrial applications for the biodesulfurization of gasoline.

Biotechnology of desulfurization of diesel: prospects and challenges

To meet stringent emission standards stipulated by regulatory agencies, the oil industry is required to make a huge investment to bring down the sulfur content in diesel to the desired level, using conventional hydrodesulfurization (HDS) technology, by which sulfur is catalytically converted to hydrogen sulfide in the presence of hydrogen. These reactions proceed rapidly only at high temperature and pressure and therefore the capital cost as well as the operating cost associated with HDS very high. Biological desulfurization has the potential of being developed as a viable technology downstream of classical HDS. Various attempts have been made to develop biotechnological processes based on microbiological desulfurization employing aerobic and anaerobic bacteria. However, there are several bottlenecks limiting commercialization of the process. This review discusses various aspects of microbial desulfurization and the progress made towards its commercialization.

Biodesulfurization of fossil fuels

Biotechnological techniques enabling the specific removal of sulfur from fossil fuels have been developed. In the past three years there have been important advances in the elucidation of the mechanisms of biodesulfurization; some of the most significant relate to the role of a flavin reductase, DszD, in the enzymology of desulfurization, and to the use of new tools that enable enzyme enhancement via DNA manipulation to influence both the rate and the substrate range of Dsz. Also, a clearer understanding of the unique desulfinase step in the pathway has begun to emerge.