Cadmium uptake and nitrogen fixing ability in heavy-metal-resistant laboratory and field strains of Rhizobium leguminosarum biovar trifolii
Rhizobium leguminosarum bv. trifolii produced mucoid colonies on yeast mannitol agar, but non-mucoid colonies appeared with a frequency of 10−7 on agar additionally containing 4 mM ZnCl2. Cells producing non-mucoid colonies were also obtained from root nodules of white clover from experimental field plots treated with heavy-metal-contaminated sewage sludge. Compared to mucoid strains, non-mucoid strains produced approximately half of the extracellular polysaccharide, two to three fold higher levels of intracellular carbohydrate and possessed larger intracellular inclusions. They also showed up to two-fold times greater resistance to Cd2+, Cu2+, Ni2+ and Zn2+. The total uptakes of Cd2+ were similar between the laboratory strains and amongst the field strains. In non-mucoid strains, Cd2+ was not removed by washing in 0.1 M HNO3, suggesting Cd2+ was bound intracellularly. All strains were effective in fixing nitrogen in white clover.