Isolation and Characterization of Thallium-tolerant Bacteria from Heavy Metal-polluted River Sediment and Non-polluted Soils

Thallium (Tl) is a heavy metal found in trace amounts in the earth's crust and has been studied to a much lesser degree than other heavy metals. Since the discovery of high-temperature superconducting components in the Tl-Ca-Ba-Cu-O system, Tl has attracted greater attention as a potential major pollution source of the future. In this study, the response of soil culturable bacterial communities to the addition of Tl was examined with fresh samples of non-polluted garden and arable soils. A preserved air-dried sample from heavy metal-polluted river sediment was also used as a reference. Soil suspension experiments were performed to homogenize the soils to evenly distribute both the microbial populations and Tl and reduce spatial variability. From soil suspensions loaded with Tl at 0.24 to 0.98 mM, bacteria tolerant to 0.49 mM Tl (100 mg Tl l-1) were isolated and characterized phylogenetically. The background level of culturable Tl-tolerant bacterial populations was 0.0003-0.013% for the non-polluted soils. The Tl-tolerant fraction increased rapidly in response to Tl loading and dominated the bacterial community, which might be attributed mainly to an immediate effect due to the death of Tl-sensitive microorganisms and leaking of nutrients from cell lysis of sensitive species supporting the growth of primarily Tl-tolerant heterotrophs. Such Tl-tolerant culturable bacteria were isolated and classified into 9 groups of Alphaproteobacteria, those from the arable soil were identified as Pseudomonas spp., and those from the contaminated river sediment were identified as Bacillus niacini.