Effectiveness and genetic diversity of Rhizobium leguminosarum bv. trifolii isolates in Portuguese soils polluted by industrial effluents

The effects of heavy metals from industrial effluents on symbiotic nitrogen fixation were studied using soils from an industrial area polluted for nearly 40 y. The work reported here indicated a decrease in effectiveness and genetic diversity of Rhizobium leguminosarum bv. trifolii population present in contaminated soils, although the size of this population was not affected by the presence of heavy metals. Twenty-seven Rhizobium isolates from polluted soil and 32 isolates from unpolluted soil were evaluated for effectiveness by acetylene reduction assay (ARA) determination using Trifolium subterraneum cv. Clare as the host plant and for genetic diversity by examination of the banding patterns of DNA on gel electrophoresis. Only 15% of the isolates from polluted soil were effective. Almost all the isolates (94%) from unpolluted soil were effective showing an ARA activity similar to the standard Rhizobium strain (recommended as inoculant for Trifolium subterraneum). The isolates from unpolluted soil had a greater diversity in terms of the numbers of plasmid types than the isolates from the polluted soil. The plasmids groups dominant in the unpolluted soil were often absent from the isolates from the polluted soil.