Interactions of earthworms with indigenous and bioaugmented PCB-degrading bacteria

Partial bioremediation of polychlorinated biphenyl (PCB)-contaminated soils has been achieved using bioaugmentation with PCB-degrading bacteria and earthworms. To further study the contribution of earthworms to bioremediation, an experiment was conducted in which the changes in indigenous and bioaugmented PCB-degrading bacteria were analyzed during treatment of contaminated soil using earthworms (Pheretima hawayana) alone or in combination with the PCB-degrading bacteria, Ralstonia eutrophus and Rhodococcus sp. ACS. Bacteria used for bioaugmentation were induced with carvone and salicylic acid in culture and were repeatedly applied every 3-4 days to the surface of unmixed, 20-cm long soil columns containing 100 ppm Aroclor 1242. After 9 weeks of treatment, the soil bacterial communities were analyzed using PCR primers for the bph genes. Results showed that approximately 50% of the PCBs were removed in the top 9 cm using a combination of earthworms and bioaugmentation, whereas bioaugmentation or earthworms applied alone were effective only for removing PCBs from the top 3 cm of the soil columns. Enhanced removal of PCBs caused by earthworms was associated with an increase in the population size of culturable, indigenous biphenyl-degrading bacteria, and an increase in the level of the bphA and bphC genes. The results suggest that earthworms facilitate PCB bioremediation by enhancing the dispersal of PCB-degrading bacteria in bioaugmented columns, as well as providing environmental conditions that favor the growth and activity of indigenous PCB-degrading bacteria.