Screening oil tank bottom sludge microbial community for identification of native efficient hydrocarbon-degrading bacteria for bioremediation purposes

Due to high concentrations of toxic organic compounds and heavy metals, oil sludge is considered as a pollution source, so its disposal without suitable treatment will be hazardous for environment. Bioremediation as an ecofriendly and profitable treatment, can convert oil sludge to low-toxicity compounds. The aim was isolation and identification of oil sludge hydrocarbon-degrading bacteria and evaluation of their potential in oil sludge treatment. Sludge samples were taken from an oil tank reservoir located in Nezamieh, Ahvaz, Iran. Bacterial screening was done based on biosurfactant production tests including hemolysis, oil spreading assay, oil drop assay, tilting slide assay, hydrocarbon overlay assay, extracellular biosurfactant production, anionic biosurfactant production, emulsification index 24, foaming, surface tension reduction, demulsification, and microbial adhesion to hydrocarbons, as well as their oil hydrocarbon degradation potential and resistance to salt and heavy metals. From 19 isolates, six isolates with the best results in mentioned experiments and high salt and heavy metal tolerance were selected and identified according to 16S rRNA sequencing. All six isolates showed remarkable biosurfactant production and oil degradation activities. From them Acinetobacter lactucae strain Ib-30 was most notable with anionic biosurfactant production, foaming (67%), surface tension (29.4%), and emulsification of hydrophobic compounds (58.8%) and high biosurfactant production potential. These results suggest that oil tank bottom sludge have unique bacterial inhabitants that are well adapted to oil hydrocarbons and hence can be good candidates for oil pollution bioremediation practices. Using such bacteria as microbial consortium can significantly increase success in bioremediation processes.