Bio-solubilization of the untreated low rank coal by alkali-producing bacteria isolated from soil

Coal is a hydrocarbon-rich fossil fuel considered as a possible replacement for petroleum as a feedstock for the production of fuel and valuable chemicals. In this study, bacteria capable of solubilizing untreated low rank coal were isolated from soil. A total of 19 microorganisms were isolated from soil enriched in MR medium with coal and were identified based on 16S rRNA sequencing. The identified soil isolates belonging to the genera Citricoccus, Comamonas, Cupriavidus, Sphingomonas, and Sphingopyxis were screened based on their growth in the chemically defined MR medium containing different concentrations of coal. Among the identified microbial strains, Cupriavidus necator S2A2, Sphingopyxis ginsengisoli S2B14 and Sphingomonas sp. S2B18 were further characterized for their ability to degrade low-rank coal. Cupriavidus necator S2A2, Sphingopyxis ginsengisoli S2B14 and Sphingomonas sp. S2B18 were found to solubilize untreated low-rank coal as indicated by the release of solubilized coal products detected at OD450 when they were grown in LB medium containing 1% coal. Sphingomonas sp. S2B18 showed the highest coal solubilization activity, based on the high absorbance of its culture supernatant (0.190). Although laccase-like activity was not detected in these strains when tested for RBBR dye degradation, increase in the pH of the culture medium up to 8.25- 8.34 was observed. This may be attributed to the excretion of alkaline substances in the culture medium. Since biosolubilization of coal by microorganisms is a good alternative for the chemical conversion of coal, microorganisms screened in this study can be potentially used as biological catalysts for the conversion of coal into valuable chemicals.