Characterization of cyclohexane and hexane degradation by Rhodococcus sp. EC1

Cyclohexane is a recalcitrant compound that is more difficult to degrade than even n-alkanes or monoaromatic hydrocarbons. In this study, a cyclohexane-degrading consortium was obtained from oil-contaminated soil by an enrichment culture method. Based on a 16S rDNA polymerase chain reaction-denaturing gradient gel electrophoresis method, this consortium was identified as comprising Alpha-proteobacteria, Actinobacteria, and Gamma-proteobacteria. One of these organisms, Rhodococcus sp. EC1, was isolated and shown to have excellent cyclohexane-degrading ability. The maximum specific cyclohexane degradation rate (V_max) for EC1 was 246 μmol g-DCW^-1 (dry cell weight) h^-1. The optimum conditions of cyclohexane degradation were 25-35 °C and pH 6-8. In addition to its cyclohexane degradation abilities, EC1 was also able to strongly degrade hexane, with a maximum specific hexane degradation rate of 361 μmol g-DCW^-1 h^-1. Experiments using ¹⁴ C-hexane revealed that EC1 mineralized 40% of hexane into CO₂ and converted 53% into biomass. Moreover, EC1 could use other hydrocarbons, including methanol, ethanol, acetone, methyl tert-butyl ether, pyrene, diesel, lubricant oil, benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene. These findings collectively suggest that EC1 may be a useful biological resource for removal of cyclohexane, hexane, and other recalcitrant hydrocarbons.