Effect of rhamnolipids on enhanced anaerobic degradation of petroleum hydrocarbons in nitrate and sulfate sediments.

Anaerobic degradation of petroleum hydrocarbons (PH) is an important process in contaminated environment. The application of rhamnolipids in anaerobic degradation of PH was not extensively studied and inconclusive. This study explored the combined effect of rhamnolipids and electron acceptors on the anaerobic degradation process of total petroleum hydrocarbons (TPH) in sediment from an oil field. The results indicated that rhamnolipids decreased the surface tension of the medium and increased the desorption of TPH from the sediment. After 10-wk culture, the maximum degradation rate of TPH in nitrate and sulfate condition was found to be 32.2% and 24.0%, respectively, with rhamnolipids concentration of 150 mg/L. The addition of 45 and 150 mg/L rhamnolipids increased the degradation rate of TPH but the promotion effect was weakened in the treatment with 450 mg/L rhamnolipids. The copy number of two degradation genes (1-methylalkyl) succinate synthase gene (masD) and 6-oxocyclohex-1-ene-1-carbonyl-CoA hydrolase gene (bamA) increased with incubation time and showed higher copy numbers in treatments with 45 and 150 mg/L rhamnolipids. In the first week, with the increase of rhamnolipids concentration, the copy number of 16S rDNA increased rapidly and the concentration of electron receptors decreased correspondingly. Moreover, no nitrate was detected in treatments of nitrate with 450 mg/L rhamnolipids after the first week. Microbial community structure analysis result showed that Thiobacillus was the dominant bacteria in all treatments with nitrate as electron acceptor and its proportion gradually decreased with the increase of rhamnolipids concentration. The addition of rhamnolipids changed the subdominant bacteria in the treatments with nitrate as electron acceptor. Methanothrix was the dominant archaea in all treatments with rhamnolipids content of lower than 45 mg/L. When the rhamnolipids concentration increased, the dominant archaea changed to Methanogenium or Methanobacterium. In conclusion, suitable concentrations of rhamnolipids could promote the anaerobic degradation of PH in the sediment.