Biodiversity and Dynamics (Rate of Change) of Bacterial Communities Involved in the Biodegradation of Petroleum Refinery Sludge in Contaminated Soils

Aim: The study assessed the bio treatability of the petroleum refinery sludge in contaminated soils by indigenous bacterial communities and the effects of the sludge contamination and bio stimulants on the biodiversity and dynamics (rate of change) of the bacterial communities involved in the biodegradation of the sludge, using the molecular biology technique, Denaturing Gradient Gel Electrophoresis (DGGE).

Study Design: The randomnized block design was used for the study.

Place and duration of the Study: The research was conducted in the biology laboratory of Flinders University, Adelaide, South Australia.

Methodology: The percentage of total petroleum hydrocarbons (TPH) degraded and the bacterial load in the test microcosms was assessed tri-weekly for 12 weeks. The percentage TPH was assessed using Gas chromatography, while the bacterial count was determined as gene copies using the culture independent molecular tool, quantitative real-time PCR (qRt-PCR) analysis. The effects of the experimental treatments on the biodiversity and dynamics (rate of change) of the bacterial communities involved in the biodegradation of the sludge in the soils was determined by the culture-independent molecular biology technique, DGGE. Moving Windows Analysis (MWA) and Shannon Weaver diversity index were used to determine the dynamics (rate of change) and biodiversity of the bacterial communities respectively.

Results: Results obtained for the Moving Window Analysis (MWA) which is used to determine the dynamics (Dy), or rate of change of the bacterial communities, showed that, the 1% and 5% sludge contaminated soils biostimulated with compost, recorded the highest Dy of 86.0 ± 1.90% and 87.0 ± 2.20% respectively.NPK biostimilated soil microcosms however recorded a lower Dy of 33.75± 3.20 and 32.50 ± 4.68% for 1% and 5% sludge contamination respectively. The biodiversity of the bacterial communities expressed as Shannon -Weaver index (H1), recorded the highest value of 2.76 ±0.02 for the compost biostimulated microcosm in the 1% sludge treatment, while for the 5% sludge contamination, the treatment with NPK and surfactant enhanced the bacterial biodiversity most with a value of 2.76 ±0.07%. In the test soils with 1% sludge contamination, bio stimulation with NPK gave the highest % TPH degradation (78.25%) while the treatment with NPK and Triton-X 100 had the highest TPH degradation (46.55%) for the 5% sludge contaminated soils. There was insignificant difference in the % sludge degradation between the control and other treatments at P > 0.05 and F = 4.07 for the 1% sludge treated soils, while for the soils treated with 5% sludge there was significant difference between the control and other treatments at P < 0.05 and F= 4.07.

Conclusion: Bacteria species identified in the sludge by molecular biology techniques included; Pseudomonas sp. ITRI77, Uncultured Thauera sp., Uncultured Pseudomonas sp., Flavobacterium sp., Bacillaceae bacterium, Uncultured soil bacterium, Clostridium sp., most of which are Gram negative. Biostimulation with compost enhanced a higher biodiversity (H i) and dynamics (Dy) of the bacterial communities involved in the biodegradation of the sludge. Though the NPK treated soils enhanced the biodegradation of the sludge most, degradation started declining by the 9th week while that of compost continued to rise steadily till the 12th week. Results obtained indicate that compost is as good as NPK in the biodegradation of petroleum sludge especially at 1% sludge contamination, since there was no statistical difference between the % TPH degraded and the use of compost is environmentally friendly and economically sustainable.