Nickel ion extracellular uptake by the phototrophic bacterium Rhodobacter sphaeroides: new insights from Langmuir modelling and X-ray photoelectron spectroscopic analysis

The presence of anionic functional groups on the surface of microbial cells makes the bacterial biomass very attractive as metal cation removal tool for environmental bioremediation. To go insight to the heavy metal sorption abilities of Gram-negative bacteria, we characterized the extracellular uptake of Ni2+ ions by the phototrophic bacterium Rhodobacter sphaeroides, whose ability to sequester toxic metal ions has been widely documented. The sorption process proved to be well described by the Langmuir model, suggesting that the cell envelope behaves as an ideal adsorbent surface, showing a layer of chemically equivalent binding sites with high affinity for Ni2+. X-ray photoelectron spectroscopic data reveal that, at binding site saturation, Ni2+ ions fully displace alkali metal ions from the bacterial surface, while they are unable to displace Ca2+ and Mg2+ from their tight binding sites. The analysis of carbon, nitrogen and oxygen speciation, as arisen from high resolution spectra fitting, highlights that the Ni2+/K+ cation exchange affects the surface distribution of main chemical components, with increased contribution by peptides and phosphatidylcholines. A role of Ni2+ ions in favoring the stacking and/or packing of lipopolysaccharide sugar chains by interaction with carboxylate groups is proposed and discussed.