Screening and identification of cadmium-tolerant bacteriafrom rhizosphere soils under rice

To identify the bacteria for remediation of cadmium contaminated paddy soils, a plate method (with cadmium in culture medium) was used to screen cadmium tolerant bacterial strains in rhizosphere soils of rice. Three cadmium-tolerant strains were screened and classified as Pseudomonas sp. by using 16S rDNA sequence analysis. While microbial cell morphology observations (using electronic microscope) and growth curve determinations indicated no significant inhibition effects on growth rates of the three strains under 100 mg?L^?1 cadmium treatment, some adaptive morphological changes were noted. For over 300 mg?L^?1cadmium treatments, growth rates of the strains were obviously inhibited. Nevertheless, the three strains were tolerant to cadmium concentra-tions as higher as 700 mg?L^?1 for long periods up to stationary phase. However, growth rates lowered after logarithmic growing phase. Chemical analysis showed that cadmium contents in the bacterial strains under 100 mg?L^?1 cadmium concentration were 9.04 mg?g^?1, 4.96 mg?g^?1 and 28.58 mg?g^?1 by dry-weight. The corresponding enrichment coefficients were 90.4, 49.6 and 285.8, respectively. It then suggested that all the three strains had stronger tolerance and absorption abilities of cadmium. This was an important potential microbial resource for remediation of paddy soils contaminated with cadmium.