Isolation of Oil Degrading Bacteria from Oil Contaminated Soil Around the Oil Refinery and Petrochemical Plants of Tabriz and Identification of the Efficient Bacteria

Petroleum hydrocarbons are the most common organic contaminants in water and soil ecosystems around the world. Application of soil microorganisms is one of the cost-effective and environmentally friendly approaches to remove or reduce the oil pollution. Isolation and assessment of oil-degrading bacteria is the first step in bioremediation. Accordingly, the aim of this study was isolation of crude oil degrading (COD) bacteria from contaminated soil samples around the refinery and petrochemical plants of Tabriz and assessment of their ability in oil degradation.

In order to isolate the oil degrading bacteria from oil contaminated sludge collected from evaporation ponds of Tabriz Refinery, the carbon free minimal medium (CFMM) by enrichment methods were used. Based on bacterial growth indicators (e.g. optical density (OD) and microbial biomass, percent of crude oil degradation and production of biosurfactant), 20 efficient isolates were selected among the 60 isolates using CFMM supplemented with crude oil. All of these 20 isolates were evaluated by biosurfactant production tests. Then, 20 isolates were selected based on their growth rate, crude oil degradation potential, and their ability to degrade recalcitrant compounds (light naphta, heavy naphta, styrene and anthracene).

The most efficient strains that had the highest OD, microbial biomass and percentage of oil biodegradation with biosurfactant production ability were the isolates COD2-1,  COD1-5, COD4-3, COD4-2, COD9-3, COD6-3, COD7-1, COD3-1, COD1-1 and COD8-1. While other efficient isolates (COD1-4, COD5-6, COD4-5, COD7-3, COD4-6, COD6-1, COD6-4, COD2-3, COD8-2 and COD3-3) did not produced biosurfactant.  Among these bacteria, 11 efficient isolates were identified by molecular techniques. The results of molecular identification of bacteria showed that these isolates belong to the genus and species Stenotrophomonas sp.,  Achromobacter sp.,  Psedochrobactrum sp., Arthrobacter sp.,  Shewanella sp., Alcaligenes sp., Pseudomonas sp . and Acinetobacter baumannii. 

Among the isolates, the genus Stenotrophomonas sp., Psedochrobactrum sp., Arthrobacter sp. and Shewanella sp. with high quantitative and qualitative indices in terms of bacterial growth, microbial biomass, crude oil degradation and production of biosurfactant were the best candidates for bioremediation experiments in contaminated soil.