Background and Aim: In this research we isolated the bacterial species degrading polycyclic aromatic hydrocarbons (PAHs) and determined optimal salt concentration for PAH degradation.

Materials and Methods: We used naphthalene and anthracene as the only sources of carbon for isolating PAH-degrading bacteria; the main culture medium was ONR7a. The bacteria isolated by this method were used for the study of the effect of salt concentration on PAH degradation.

Results: Pseudomonas is the only PAH-degrading bacterium isolated from the lake's water and sediment. The greatest numbers of naphthalene- and anthracene-degrading bacteria where found in the water and sediment from the Khoshk River (mean counts: 227 and 167 per ml water and 290 and 193 per gram of sediment). The mean counts of naphthalene- and anthracene degrading bacteria in Pol- Fasa River were 107 and 73 per ml water and 157 and 127 per gram sediment, respectively. Values from Nazar Abad River were 27 and 17 /cc water and 43 and 20 / g sediment. In the center of the lake counts were 63 and 33 per ml water and 123 and 73 per gram sediment and in the western part of the lake counts were 30 and 13 /ccl water and 43 and 17 /g sediment.Maharloo lake is located in south-east of Shiraz, 23 km from the city and contains saline water with a mean salt content of 220 g/L. Among the most important pollutants entering the lake are polycyclic aromatic hydrocarbons (PAHs). Our research showed that overall, naphthalene-degrading bacteria are present in greater numbers compared to anthracene-degrading bacteria. The optimal salt concentration for the growth of Pseudomonas spp growth and PAH-degradation is around 6% the rate of degradation increases with time.
Conclusion: Regarding isolation of PAHs degrading bacteria from the lake, it is obvious that part of the degradation is done by bacteria. The main bacterium here is pseudomonas and it is optimally active at a salt concentration of 6%.