The treated petrochemical industry waste water (TW), ground water (GW) and the soil characteristics have been monitored for several years. A higher value of heavy metal content was observed in the soil repeatedly irrigated with TW as compared to soil receiving the GW. However, the level of these heavy metals both in waste water and soil were well within the permissible limits. The analysis of various groups of microorganism and their interaction with metals revealed that the viable count of aerobic heterotrophs, asymbiotic nitrogen fixers, actinomycetes and fungi were in the range of 2.4 x 106 - 7.2 x 106, 2.8 x 104 - 1.3 x 105, 2.7 x 104 - 6.8 x 105 and 4.4 x 104 - 1.3 x 105 respectively. The presence of Rhizobium spp. in the soil was shown by the nodulation in the leguminous crops cultivated in the experimental field. The significant viable counts of Rhizobium spp on each heavy metal supplemented plate indicated the tolerance of higher levels (200 µg/ml) of metal. Among the microbial population, nitrogen fixers and aerobic heterotrophs had a higher degree of resistance to heavy metal than actinomycetes. The strains of Rhizobium, isolated from the experimental field, showed a higher degree of resistance under in vitro conditions and the minimum inhibitory concentration (MIC) was in the range of 50 - 800 µg/ml, determined. It may therefore, concluded that continuous application of treated refinery wastewater had not significantly changed the microbial population of the soil and possibly the bacteria had adapted to the various levels of metal resistance.
Rhizobium, petrochemical industry, wastewater, antibiotic resistance, heavy metals
