Effects of Cd-resistant bacteria and calcium carbonate + sepiolite on Cd availability in contaminated paddy soil and on Cd accumulation in brown rice grains.

A pot experiment was conducted to evaluate the effects of combined application of cadmium (Cd)-resistant bacteria (J) and calcium carbonate + sepiolite (G) on both Cd bioavailability in contaminated paddy soil and on Cd accumulation in rice plants. Adding the mixture (J + G) to the soils significantly increased soil pH, decreased extractable Cd contents, and increased Fe/Mn-oxide Cd and organic-bound Cd contents. The applying of J + G, J and G decreased Cd contents in various rice tissues (roots, stems and leaves, husks, and brown rice grains) to different degrees. Compared with those of the CK, Cd contents decreased by 17.8%-53.3% in the roots, 12.3%-27.4% in the stems and leaves, 25.4%-44.6% in the husks, and 28.8%-55.7% in the brown rice grains for the application of J + G; Cd contents decreased by 8.2%-28.5% in the roots, 11.5%-32.0% in the husks, and 27.8%-45.9% in the brown rice grains for the application of J; Cd contents decreased by 12.9%-26.5% in the roots, in the stems and leaves decreased by 4.6%-34.1% in the stems and leaves, 60.2%-79.7% in the husks, and 35.7%-47.6% in the brown rice grains for the application of G. The alone application of bacteria (J) could reduce the bioavailability of Cd in soil and the contents of Cd in brown rice grains to some extent. Moreover, when the bacteria were applied in combination with mineral (J + G), it was a more effective method than the alone application of J or G to reduce the soil Cd bioavailability. Under all the tested conditions, applications of J4+G4 (320 mL kg-1 of J + 8 g kg-1 of G) resulted in the greatest reduction in Cd contents in brown rice grains. Overall, the results indicated that the combination of Cd-resistant bacteria and mineral material could effectively reduce Cd bioavailability in paddy soils and inhibit Cd accumulation in brown rice grains.

Cadmium Pollution Impact on the Bacterial Community Structure of Arable Soil and the Isolation of the Cadmium Resistant Bacteria.

Microorganisms play an important role in the remediation of cadmium pollution in the soil and their diversity can be affected by cadmium. In this study, the bacterial community in arable soil samples collected from two near geographical sites, with different degrees of cadmium pollution at three different seasons, were characterized using Illumina MiSeq sequencing. The result showed that cadmium is an important factor to affect the bacterial diversity and the microbial communities in the high cadmium polluted area (the site H) had significant differences compared with low cadmium polluted area (the site L). Especially, higher concentrations of Cd significantly increased the abundance of Proteobacteria and Gemmatimonas whereas decreased the abundance of Nitrospirae. Moreover, 42 Cd-resistant bacteria were isolated from six soil samples and evaluated for potential application in Cd bioremediation. Based on their Cd-MIC [minimum inhibitory concentration (MIC) of Cd2+], Cd2+ removal rate and 16S rDNA gene sequence analyses, three Burkholderia sp. strains (ha-1, hj-2, and ho-3) showed very high tolerance to Cd (5, 5, and 6 mM) and exhibited high Cd2+ removal rate (81.78, 79.37, and 63.05%), six Bacillus sp. strains (151-5,151-6,151-13, 151-20, and 151-21) showed moderate tolerance to Cd (0.8, 0.4, 0.8, 0.4, 0.6, and 0.4 mM) but high Cd2+ removal rate (84.78, 90.14, 82.82, 82.39, 81.79, and 84.17%). Those results indicated that Burkholderia sp. belonging to the phylum Proteobacteria and Bacillus sp. belonging to the phylum Firmicutes have developed a resistance for cadmium and may play an important role in Cd-contaminated soils. Our study provided baseline data for bacterial communities in cadmium polluted soils and concluded that Cd-resistant bacteria have potential for bioremediation of Cd-contaminated soils.