Coal is an abundant and rich energy source of fossil fuels origin, that has been exploited in several industries. Though its primary use is in electricity generation, fulfilling about 39% of global electricity demand (Vega et al., 2019), it is also used in steel production, as syngas and synfuels, for household heating, and in the synthesis of different materials and chemicals such as plastics, solvent, soap, dyes, phenol, naphthalene, etc. (Mark & Callaghan, 2018). However, the combustion of an excessive volume of sulfur-containing coal is responsible for an increased discharge of harmful or toxic gases, especially SO2 (Xu et al., 2019). Such sulfur-containing toxic gases after release can cause health issues, acid rain, and other environmental problem including air pollution (Khan et al., 2022a).
To achieve sustainable growth in a civilized society, it is not a wise decision to burn coal and get industrial development while causing harm to the environment (Ghosh et al., 2015). Therefore, a level of sulfur lesser than 15 ppm has been demanded by environmental regulations. To achieve such a goal, researchers all over the world are trying to reduce the fraction of sulfur in coal before it is utilized for different industrial purposes (Maass et al., 2015).
The most common method of eliminating sulfur from fossil fuels is hydrodesulfurization (HDS). During hydrodesulfurization, the sulfur in fossil fuels is reduced to hydrogen sulfide (H2S) in the presence of hydrogen gas, a metal catalyst like NiMo/Al2O3 or CoMo/Al2O3 under an elevated level of temperature (ranging from 200–425°C) and pressure (150–250 psi) (Soleimani et al., 2007). Hydrodesulfurization remains successful in removing a large amount of inorganic sulfur and a small amount of organic sulfur from fossil fuels. However, this process finds it difficult to remove sulfur from recalcitrant heterocyclic sulfur-containing organic compounds such as dibenzothiophene (DBT) and its alkylated derivatives which constitute about 70% of the organic sulfur in fossil fuels (Martinez et al., 2015). Some other disadvantages of the process include the involvement of costly catalysts, the requirement of an extensive amount of hydrogen gas, and taking place at an elevated level of temperature and pressure conditions, ultimately making the process energy-intensive (Sadare et al., 2017). To achieve full-scale fossil fuels desulfurization, it is essential to have a method that must be cost-effective and capable of eliminating sulfur from recalcitrant compounds under ambient operating conditions (Etemadifar et al., 2014). Biodesulfurization (BDS) is a complementary and alternative technique to hydrodesulfurization. This process has the capability of removing sulfur from recalcitrant heterocyclic sulfur-containing organic compounds without damaging the carbon skeleton of the parent compound. The advantages of the BDS process are lower energy consumption, a lesser amount of sulfur emission, and the minimum generation of unwanted byproducts, mainly attributed to the capability of the biocatalyst (microbes or enzymes) (Khan et al., 2022b). DBT is used as a model sulfur-containing compound due to which the search for an efficient microbial strain has been continued (Al-Jailawi et al., 2015).
To obtain sulfur from DBT and its alkylated derivatives for energy purposes as well as for maintaining growth, bacteria have followed different types of biochemical pathways. The two most important pathways identified in different bacteria for the desulfurization of DBT are the Kodama and the “4S” pathways (Mohebali et al., 2007, Bordoloi et al., 2016). The Kodama pathway is also called a degradative or ring-destructive pathway because in this pathway the bond between two carbon atoms is broken (Silva et al., 2018). The “4S” pathway is also called a sulfur-specific pathway in the sense, that the sulfur atom in DBT is released as sulfite while the carbon skeleton of DBT remains unchanged, and results in retaining the calorific value of fossil fuels.
Most of the earlier work on the desulfurization of polyaromatic sulfur heterocycles (PASH) was performed with either pure microbial culture or microbial consortium constructed artificially (Ghazali et al., 2004). However, pure culture or artificial microbial consortia are not the actual representers of the ongoing activities of microbes in the environment because the relationship between the new combined species is changed in an artificial microbial consortium (Gonzalez et al., 2011). Similarly, pure culture is also assumed to be incapable of metabolizing different compounds in a mixture. In comparison, mixed microbial consortia with a broader substrate range, have the benefits of co-metabolism and synergic effect performing biodegradation in a way of commensalism and cooxidation (Gojgic-Cvijovic et al., 2012).
Further, mixed microbial consortia have the additional advantage of containing different types of metabolic capabilities that can enhance the rate of BDS. Such bacterial consortia frequently grow in highly contaminated regions, where they are facing intense environmental conditions. This can further enhance their capability of degrading and tolerating different types of recalcitrant substances (Krishna and Philip, 2008). Thus, such characteristics of combined and cooperative interaction of natural microbial consortia must be examined for an enhanced rate of desulfurization of DBT and fossil fuels (Ismael et al., 2016).
Based on the above-mentioned facts, the purpose of the study explained herein was to isolate and characterize an efficient DBT desulfurizing microbial consortium from hydrocarbon-contaminated soil samples by the traditional enrichment technique. In addition, the impact of the product of DBT desulfurization (2-HBP and sulfate) on the sulfur metabolizing activities of the isolated consortium was also investigated.