Isolation and characterization of symbiotic N 2-fixing Rhizobium meliloti from soils contaminated with aromatic and chloroaromatic hydrocarbons: PAHs and PCBs

Abstract

Using plant modulation tests on Medicago sativa (alfalfa), the presence of a variety of strains of Rhizobium meliloti was demonstrated in soils contaminated with aromatic/chloroaromatic hydrocarbons and with no history of indigenous alfalfa presence. The contaminants were PAHs and PCBs which occurred as a result of long term (> 15 years) industrial use and/or spillage. We purified some of these strains and characterized their colony morphologies, antibiotic sensitivities, plasmid profiles, species-specific insertion element, insertion sequence IS Rm5, genomic RFL (restriction fragment length) hybridization patterns and random primer generated DNA fragment amplification patterns with PCR (polymerase chain reaction). From these results it was concluded that the population of R. meliloti in each of these soils, is not homogeneous and is composed of several phenotypically and genetically distinct strains. Strains obtained from all three soils were all effective in symbiotic N 2-fixation, irrespective of the nature or level of contamination, and their genomic DNAs hybridized strongly with a DNA fragment bearing bphABC genes from Comamonas testosteroni B-356. These findings indicate that at least some strains of R. meliloti are capable of inhabiting contaminated soils that have not been previously planted with a compatible host plant in the past, and suggest a possible role of rhizobia in decontamination and recycling of organic compounds; a role which could potentially use these agronomically important microbes for environmental clean up. To the best of our knowledge, this is first report on isolation of rhizobia from soils contaminated with aromatic and chloroaromatic compounds.