Response of the carotenoidless mutant Rhodobacter sphaeroides growing cells to cobalt and nickel exposure

The interaction of cobalt (Co2+) and nickel (Ni2+) ions with whole cells of the photosynthetic purple bacterium Rhodobacter sphaeroides strain R26 was investigated. Active and passive uptakes were examined in cells grown in the presence of increasing amounts of Co2+ and Ni2+. Inductively coupled plasma atomic emission spectroscopy (ICP-AES), pH titration, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy were used to assess the role of cell envelope and metabolism in accumulating the two heavy metals. The chosen microorganism was able to uptake cobalt and nickel up to 2.2 and 0.25 mg per gram of dried cells respectively, with the largest part found bound to the cell surface. Carboxylate groups lying on the cell wall of this Gram-negative bacterium proved to be the major candidates for binding protons and metal cations. Co2+ was found to interfere with Mg2+ extracellular immobilization and transport across the membrane, indicating that these ions share binding sites on the cell envelope and ion transport systems. According to the presence of a competition mechanism, bacterial growth experiments showed that high Mg2+ concentrations are able to rescue R. sphaeroides from Co2+ toxicity.