Abstract
The capacity to solubilize inorganic phosphates by phosphate-solubilizing bacterial strains isolated from soil samples collected from different agricultural fields in Matara District was assessed. The isolated phosphate solubilizing bacterial strains were then tested for tolerance against four heavy metals such as cadmium (Cd), lead (Pb), zinc (Zn) and copper (Cu) each with three concentrations (100 mg/L, 200 mg/L, and 400 mg/L). The bacterial strains with the highest phosphate solubilization capacity and resistance to heavy metal were selected and the phosphate solubilization potential under different heavy metals was then assessed. The results showed that most of the tested isolates proved to be tolerant to the heavy metals at low concentrations. However, a subsequently significant reduction in tolerance was observed when heavy metal concentration increased. Except three isolates, all the other isolates were proved to be vulnerable to the heavy metals of Cd and Pb at the 400 mg/L concentration. Among the tested 15 isolates, PSB-14 showed the highest tolerance to 100 mg/L, 200 mg/L and 400 mg/L of Cd, Pb, Cu and Zn. The strain was identified as Enterobacter cancerogenous according to the 16 rRNA analysis. The bacterial strain showed very high degree of reductions in phosphate solubilization in the presence of heavy metals Cd and Pb. The order of the toxicity of the metals to strain was found to be Pb > Cd > Cu > Zn. According to the results, it could be concluded that heavy metals Cd and Pb was shown to display a major impact on phosphate solubilization while Cu and Zn had a mild effect.
Highlights
Heavy metal toxicity to various environmental niches has created environmental problems due to their potential adverse ecological effects and is a great matter of concern for environmentalists (Jiang et al, 2008)
Many soil bacteria are tolerant to heavy metals it is reported that bacterial strains isolated from heavy metals polluted environments were shown to be tolerant to higher concentrations of metals than those isolated from unpolluted areas (Rajkumar et al, 2010)
Assay of inorganic phosphate solubilization The isolated bacterial strains were grown in sterilized liquid NBRIP medium (20 ml) at 300C for 2 days with continuous shaking at 150 rpm
Summary
Heavy metal toxicity to various environmental niches has created environmental problems due to their potential adverse ecological effects and is a great matter of concern for environmentalists (Jiang et al, 2008). Toxic effects of heavy metals such as Cd, Cu, Zn, Ni, Co, Cr, Pb and As last longer and they cannot be degraded chemically or biologically (Ahemad, 2012). Continuous and excess use of agrochemicals such as fertilizers and pesticides could directly or indirectly have negative impacts as they could release toxic materials such as heavy metals to the environment. Many soil bacteria are tolerant to heavy metals it is reported that bacterial strains isolated from heavy metals polluted environments were shown to be tolerant to higher concentrations of metals than those isolated from unpolluted areas (Rajkumar et al, 2010). Phosphate solubilizing bacterial strains with genetic potential for increasing tolerance to elevated levels of heavy metals are substantially improved the growth of plants implanted in heavy metal contaminated polluted soils by lowering the metal toxicity (Wani and Khan, 2010). The present study was undertaken to isolate phosphate solubilizing bacterial strains from different agricultural fields and to assess their heavy metal resistance in the presence of different heavy metals
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