Proteobacteria response to heavy metal pollution stress and their bioremediation potential

Heavy metal pollution is a serious global environmental problem as it adversely affects biotic and abiotic components of the ecosystem. Heavy metals are elements with an atomic number higher than 20, an atomic mass greater than 40 g, and a specific weight of more than 5 g/cm 3 . Proteobacteria make up one of the largest phyla and most versatile in the bacteria domain. Proteobacteria include phototrophs, chemolithotrophs, and heterotrophs. The study was undertaken to examine the response of Proteobacteria to different heavy metal salt in an elevated concentration. Surface soil samples at depth of 0–15 cm were collected at random from five different sites. The response of heavy metal salt on Proteobacteria isolated from the soil samples was investigated by supplementing different heavy metal salts namely copper (Cu), nickel (Ni), lead (Pb), and cadmium (Cd) with the bacteria isolate in simulated environment. The cultures were incubated for 7 days. Growth of the isolate was measured by withdrawing samples from the medium every 24 h and absorbance of the turbidity measured at 600 nm using spectrophotometer. All Proteobacteria bacteria showed high tendency to decrease optical density while increasing metal concentration in the medium. Toxicity of the heavy metal salts was in the order of Cd > Cu > Ni > Pb. There was a significant difference ( P < 0.05) in the proteobacteria response rates to heavy metal salts. Tolerance for the metal was dependent on concentration, time, and the isolate tested. The highest tolerance capacity was observed for Achromobacter xylosoxidans (A2) and Achromobacter insolitus (A5) which were further used for biosorption study. At 28 days of incubation, Achromobacter xylosoxidans bioaccumulated 90.1% of copper, 96.51% of nickel, 92.0% of lead, and 84.82% of cadmium and Achromobacter insolitus bioaccumulated 90.04% of copper, 94.67% of nickel, 90.25% of lead, and 89.21% of cadmium.