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• Paracoccus sp. MJ9 produces a promising biosurfactant. • The surface tension was reduced from 65.56 mN/m to 38.33 ± 2.33 mN/m by MJ9. • Results indicate that the surface-active substance is rhamnolipid. • The bacterias enhance petroleum oil degradation by producing biosurfactants. Biosurfactant is known as an efficient accelerator for hydrocarbon biodegradation by enhancing their bioavailability and facilitating their degradation by microorganisms. The present work aimed to characterize the structure of the biosurfactant produced by Paracoccus sp. MJ9 and to evaluate its potential use in diesel degradation in marine environment. In the present study, an efficient biosurfactant-producing strain, namely, Paracoccus sp. MJ9, was selected using an oil spreading test, drop collapse method and surface tension test in four petroleum-degrading strains isolated from Jiaozhou Bay in Qingdao, Shandong Province. Results showed that the surface-active substance produced by the strain MJ9 reduced the surface tension of water from 65.56 mN/m to 38.33 mN/m. Results of thin-layer chromatography analysis (TLC), fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy analysis indicated that the surface-active substance was rhamnolipid. Furthermore, diesel oil with MJ9 or crude biosurfactant produced by MJ9 was qualitatively analyzed by gas chromatography and mass spectrometry before and after biodegradation. Results showed that the removal efficiency of 81 % was achieved within 5 days. This work has screened and identified a bacterium with superior biosurfactant production capabilities, characterized the biosurfactant as rhamnolipid and indicated the potential applications in bioremediation processes to enhance the solubility of hydrophobic compounds. [ABSTRACT FROM AUTHOR]