Biosolubilization of Polish brown coal by Gordonia alkanivorans S7 and Bacillus mycoides NS1020

•Crude oil hydrocarbon-degrading bacteria Gordonia alkanivorans S7 and Bacillus mycoides efficiently solubilized brown coal.•The efficiency of biosolubilization was increased by almost 89% when the brown coal was pretreated using 8N nitric acid.•Mechanisms of biosolubilization of the nitric acid pretreated brown coal by the two tested microbial strains were different.

Two of five tested crude oil hydrocarbon-degrading bacteria, Gordonia alkanivorans S7 and Bacillus mycoides NS1020, solubilized crude and nitric acid-pretreated brown coal. Around 30–36% crude coal dry weight was solubilized within 14 days. The nitric acid pretreatment of the coal increased process efficiency by around 89% for G. alkanivorans S7. The extracellular coal-solubilizing activities of both the strains decreased with a decrease in pH, suggesting the contribution of some alkaline substances to lignite solubilization. Furthermore, this process was considerably intensified by ammonium oxalate. Mechanisms of lignite biosolubilization by the two strains were different because the active extracellular substances synthesized by B. mycoides NS1020 were thermostable while in the case of G. alkanivorans S7 some enzymes might be involved. Elemental analysis and FTIR spectra of 8N HNO3 pretreated coal and biosolubilization products generated by G. alkanivorans S7 and B. mycoides NS1020 showed significant differences between the lignite and these products. This is the first report presenting brown coal biosolubilization by crude oil hydrocarbon-degrading bacteria.