Simultaneously saccharification and fermentation approach as a tool for enhanced fossil fuels biodesulfurization

Strawberry Tree Fruit Residue as Carbon Source Towards Sustainable Fuel Biodesulfurization by Gordonia alkanivorans Strain 1B

Biodesulfurization (BDS) is a clean technology that uses microorganisms to efficiently remove sulfur from recalcitrant organosulfur compounds present in fuels (fossil fuels or new-generation fuels resulting from pyrolysis and hydrothermal liquefaction). One of the limitations of this technology is the low desulfurization rates. These result in the need for greater amounts of biocatalyst and lead to increased production costs. To mitigate this issue, several approaches have been pursued, such as the use of alternative carbon sources (C-sources) from agro-industrial waste streams or the co-production of high-added-value products by microorganisms. The main goal of this work is to assess the potential of strawberry tree fruit residue (STFr) as an alternative C-source for a BDS biorefinery using Gordonia alkanivorans strain 1B, a well-known desulfurizing bacterium with high biotechnological potential. Hence, the first step was to produce sugar-rich liquor from the STFr and employ it in shake-flask assays to evaluate the influence of different pretreatments (treatments with 1-4% activated charcoal for prior phenolics removal) on metabolic parameters and BDS rates. Afterwards, the liquor was used as the C-source in chemostat assays, compared to commercial sugars, to develop and optimize the use of STFr-liquor as a viable C-source towards cost-effective biocatalyst production. Moreover, the high-market-value bioproducts simultaneously produced during microbial growth were also evaluated. In this context, the best results, considering both the production of biocatalysts with BDS activity and simultaneous bioproduct production (carotenoids and gordofactin biosurfactant/bioemulsifier) were achieved when strain 1B was cultivated in a chemostat with untreated STFr-liquor (5.4 g/L fructose + glucose, 6:4 ratio) as the C-source and in a sulfur-free mineral-minimized culture medium at a dilution rate of 0.04 h-1. Cells from this steady-state culture (STFr L1) achieved the highest desulfurization with 250 mM of dibenzothiophene as a reference organosulfur compound, producing a maximum of ≈213 mM of 2-hydroxibyphenil (2-HBP) with a corresponding specific rate (q2-HBP) of 6.50 µmol/g(DCW)/h (where DCW = dry cell weight). This demonstrates the potential of STFr as a sustainable alternative C-source for the production of cost-effective biocatalysts without compromising BDS ability. Additionally, cells grown in STFr L1 also presented the highest production of added-value products (338 ± 15 µg/g(DCW) of carotenoids and 8 U/mL of gordofactin). These results open prospects for a future G. alkanivorans strain 1B biorefinery that integrates BDS, waste valorization, and the production of added-value products, contributing to the global economic viability of a BDS process and making BDS scale-up a reality in the near future.

New Insights on Gordonia alkanivorans Strain 1B Surface-Active Biomolecules: Gordofactin Properties

Biosurfactants/bioemulsifiers (BSs/BEs) can be defined as surface-active biomolecules produced by microorganisms with a broad range of applications. In recent years, due to their unique properties like biodegradability, specificity, low toxicity, and relative ease of preparation, these biomolecules have attracted wide interest as an eco-friendly alternative for several industrial sectors, escalating global microbial BS/BE market growth. Recently, Gordonia alkanivorans strain 1B, a bacterium with significant biotechnological potential, well known for its biodesulfurizing properties, carotenoid production, and broad catabolic range, was described as a BS/BE producer. This study focuses on the characterization of the properties of the lipoglycopeptide BSs/BEs produced by strain 1B, henceforth referred to as gordofactin, to better understand its potential and future applications. Strain 1B was cultivated in a chemostat using fructose as a carbon source to stimulate gordofactin production, and different purification methods were tested. The most purified sample, designated as extracted gordofactin, after lyophilization, presented a specific emulsifying activity of 9.5 U/mg and a critical micelle concentration of 13.5 mg/L. FT-IR analysis revealed the presence of basic hydroxyl, carboxyl, ether, amine/amide functional groups, and alkyl aliphatic chains, which is consistent with its lipoglycopeptide nature (60% lipids, 19.6% carbohydrates, and 9% proteins). Gordofactin displayed remarkable stability and retained emulsifying activity across a broad range of temperatures (30 °C to 80 °C) and pH (pH 3-12). Moreover, a significant tolerance of gordofactin emulsifying activity (EA) to a wide range of NaCl concentrations (1 to 100 g/L) was demonstrated. Although with a great loss of EA in the presence of NaCl concentrations above 2.5%, gordofactin could still tolerate up to 100 g/L NaCl, maintaining about 16% of its initial EA for up to 7 days. Furthermore, gordofactin exhibited growth inhibition against both Gram-positive and Gram-negative bacteria, and it demonstrated concentration-dependent free radical scavenging activity for 2,2-diphenyl-1-picrylhydrazyl (IC50 ≈ 1471 mg/L). These promising features emphasize the robustness and potential of gordofactin as an eco-friendly BS/BE alternative to conventional surfactants/emulsifiers for different industrial applications.

Paixão S, S. Fernandes A, C. Roseiro J, Alves L. New Insights on Carotenoid Production by Gordonia alkanivorans Strain 1B

Gordonia alkanivorans strain 1B is a desulfurizing bacterium and a hyper-pigment producer. Most carotenoid optimization studies have been performed with light, but little is still known on how carbon/sulfur-source concentrations influence carotenoid production under darkness. In this work, a surface response methodology based on a two-factor Doehlert distribution (% glucose in a glucose/fructose 10 g/L mixture; sulfate concentration) was used to study carotenoid and biomass production without light. These responses were then compared to those previously obtained under light. Moreover, carbon consumption was also monitored, and different metabolic parameters were further calculated. The results indicate that both light and glucose promote slower growth rates, but stimulate carotenoid production and carbon conversion to carotenoids and biomass. Fructose induces higher growth rates, and greater biomass production at 72 h; however, its presence seems to inhibit carotenoid production. Moreover, although at a much lower yield than under light, results demonstrate that under darkness the highest carotenoid production can be achieved with 100% glucose (10 g/L), ≥27 mg/L sulfate, and high growth time (>216 h). These results give a novel insight into the metabolism of strain 1B, highlighting the importance of culture conditions optimization to increase the process efficiency for carotenoid and/or biomass production.

Mechanistic Understanding of Gordonia sp. in Biodesulfurization of Organosulfur Compounds

Although conventional oil refining process like hydrodesulfurization (HDS) is capable of removing sulfur compounds present in crude oil, it cannot desulfurize recalcitrant organosulfur compounds such as dibenzothiophenes (DBTs), benzothiophenes (BTs), etc. Biodesulfurization (BDS) is a process of selective removal of sulfur moieties from DBT or BT by desulfurizing microbes. Therefore, BDS can be used as a complementary and economically feasible technology to achieve deep desulfurization of crude oil without affecting the calorific value. In the recent past, members of biodesulfurizing actinomycete genus Gordonia, isolated from versatile environments like soil, activated sludge, human beings etc. have been greatly exploited in the field of petroleum refining technology. The bacterium Gordonia sp. is slightly acid-fast and has been used for unconventional but potential oil refining processes like BDS in petroleum refineries. Gordonia sp. is unique in a way, that it can desulfurize both aliphatic and aromatic organosulfurs without affecting the calorific value of hydrocarbon molecules. Till date, approximately six different species and nineteen strains of the genus Gordonia have been recognized for BDS activity. Various factors such as enzyme specificity, availability of essential cofactors, feedback inhibition, toxicity of organic pollutants and the oil-water separations limit the desulfurization rate of microbial biocatalyst and influence its commercial applications. The current review selectively highlights the role of this versatile genus in removing sulfur from fossil fuels, mechanisms and future prospects on sustainable environment friendly technologies for crude oil refining.

Adsorption desulfurization performance of PdO/SiO2@graphene oxide hybrid aerogel: Influence of graphene oxide

Preparation of PdO/SiO₂@graphene oxide (GO) hybrid aerogels were carried out sol-gel method combined with atmospheric drying technology to study their adsorption performance for thiophenics and compared with PdO/SiO₂. Scanning electron microscope (SEM), N₂ adsorption-desorption isotherms, X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), and fourier transformation infrared spectroscopy (FT-IR) for samples were performed. The adsorption performance of PdO/SiO₂@GO for thiophene were better than that of PdO/SiO₂, attributed to that incorporation of GO increased the specific surface area and the Pd incorporation rate, where Pd²⁺ ions acted as the π-complexation and sulfur-metal (SM) bond adsorption active centers, as well as GO adsorbed thiophene by the π-π stacking effect. The adsorption capacities of PdO/SiO₂@GO-1.0 for thiophene (TH), benzothiophene (BT) and dibenzothiophene (DBT) were 8.89, 9.3 and 12.6 mg-S/g_ads, respectively. The addition of GO in aerogels could improve the inhibition effect of toluene, cyclohexene and pyridine while decreased the inhibition effect of MTBE and H₂O for the adsorption of thiophene, due to the π-π stacking effect and the hydrophobicity of GO, respectively. The adsorption process was spontaneous and exothermic, be well fitted by the apparent second-order kinetic model and dominated by chemical interaction. Pd/SiO₂@GO-1.0 had a good solvent elution regeneration performance.

Effect of dibenzothiophene and its alkylated derivatives on coupled desulfurization and carotenoid production by Gordonia alkanivorans strain 1B

Nowadays, the production of green transportation fuels is essential for a healthy life and environment. Effective and complete removal of organosulfur recalcitrant compounds from fuel oils is crucial to meet the stringent requirements of sulfur standards. However, the industry's solution (Hydrodesulfurization, HDS) is not effective in the removal of complex sulfur heterocyclic hydrocarbons. Thus, the development of more efficient and ecofriendly/sustainable desulfurization methods is critical, as either an alternative or a complement to HDS, foreseeing the production of ultra-low sulfur fuels (ULSF). Among the desulfurization techniques available, microbial desulfurization of organosulfur hydrocarbons (biodesulfurization, BDS) is attracting great attention. BDS is carried out at mild operation conditions, making it energetically cheaper and more ecofriendly, since it does not require hydrogen and produces far less greenhouse gases emission than HDS. In this context, the behavior of Gordonia alkanivorans strain 1B, a desulfurizing bacterium and hyper-pigment producer, was evaluated in the presence of four sulfur sources common in fuel oils: dibenzothiophene (DBT); 4-mDBT; 4,6-dmDBT and 4,6-deDBT (single/mixed), in terms of both desulfurization rate and overall carotenoid production. Simultaneously, the influence of the carbon source used (fructose vs glucose) on the overall effectiveness of the coupled bioprocesses was also assessed. The results obtained highlight the potential of strain 1B to desulfurize all the tested recalcitrant compounds and simultaneously produce carotenoids. However, the highest BDS values were observed for 4,6-deDBT (5.75 μmol/g (DCW)/h) and for the mix of DBTs (5.20 μmol/g (DCW)/h), when fructose was used as carbon source. Indeed, when the mixture of DBTs ("model oil surrogate") was desulfurized by cells growing in fructose both desulfurization rate and total pigments amount were higher than those observed for glucose growing cells. Moreover, under these conditions, the strain 1B was able to produce high added-value carotenoids, namely astaxanthin, lutein and canthaxanthin. Hence, these results are promising when aiming to achieve a scale-up scenario. In fact, the inclusion of the production of high added-value products within a BDS process targeting ULSF may be a sustainable way to turn its scale-up economically viable.

Investigations in enhancement biodesulfurization of model compounds by ultrasound pre-oxidation

In this study, complicated model sulfur compounds in crude oil were biodesulfurized in a batch process by microbial consortium enriched from oil contaminated soil. Dibenzothiophene (DBT) was selected as model sulfur compounds. Ultrasonic radiation was used to pre-oxidize the model sulfur compounds before the biodesulfurization (BDS) process. The enhancement mechanism of ultrasound pre-oxidation (UPO) on the biodesulfurization of DBT was investigated. The effects of initial conditions on the biodesulfurization of DBT in UPO/BDS system such as solution initial pH, DBT initial concentration, sulfur source, biocatalyst initial concentration, and incubation temperature were discussed. The results show that the application of UPO before BDS procedure significantly improved the efficiency of the biodesulfurization and allowed sulfur removal in shorter time through oxidizing DBT to DBT sulfone, resulting in shortening the "4S" pathway for biodesulfurization from 4 steps to 2 steps, enhancement in reaction velocity and enzyme-substrate affinity as well as reduction in substrate inhibition. The concentration of 2-HBP increased fast with the use of ultrasound pre-oxidation, which was dependent on solution initial pH, DBT initial concentration, sulfur source, biocatalyst initial concentration, and incubation temperature.

On the road to cost-effective fossil fuel desulfurization byGordonia alkanivoransstrain 1B

Biodesulfurization (BDS) is an ecofriendly process that uses microorganisms to efficiently remove sulfur from fossil fuels. To make the BDS process economically competitive with the deep hydrodesulfurization process, which is currently used in the oil industry, it is necessary to improve several factors. One crucial limitation to be overcome, common within many other biotechnological processes, is the cost of the culture medium. Therefore, an important line of work to make BDS scale-up less costly is the optimization of the culture medium composition aiming to reduce operating expenses and maximize biocatalyst production. In this context, the main goal of this study was on the minimization of inorganic key components of sulfur-free mineral (SFM) medium in order to get the maximal production of efficient desulfurizing biocatalysts. Hence, a set of assays was carried out to develop an optimal culture medium containing minimal amounts of nitrogen (N) and magnesium (Mg) sources and trace elements solution (TES). These assays allowed the design of a SFMM (SFM minimum) medium containing 85% N-source, 25% Mg-source and 25% TES. Further validation consisted of testing this minimized medium using two carbon sources: the commercial C-source (glucose + fructose) versus Jerusalem artichoke juice (JAJ) as a cheaper alternative. SFMM medium allowed microbial cells to almost duplicate their specific desulfurization rate (q_2-HBP) for both tested C-sources, namely from 2.15 to 3.39 μmoL g⁻¹ (DCW) h⁻¹ for Fru + Glu and from 1.91 to 3.58 μmoL g⁻¹ (DCW) h⁻¹ for JAJ, achieving a similar net 2-hydroxybiphenyl produced per g of consumed sugar (∼17 μmoL g⁻¹). These results point out the great advantage of using cheaper culture medium that in addition enhances the bioprocess effectiveness, paving the way to a sustainable scale-up for fossil fuel BDS.

The utilization of desulfurizing microorganisms that can grow in low nutrient culture media without vitamins and other growth promoters (e.g. yeast extract, peptone) is an advantage for BDS upgrade since it may reduce the biocatalyst production costs significantly

Ionic liquid-supported 3DOM silica for efficient heterogeneous oxidative desulfurization

DBT is absorbed by IL-3DOM SiO₂ and then oxidized to DBTO₂ in the presence of H₂O₂.