Solubilization of minerals by bacteria: Electrophoretic mobility of Thiobacillus ferrooxidans in the presence of iron, pyrite, and sulfur

Abstract

Thiobacillus ferrooxidans is an obligate acidophile that respires aerobically on pyrite, elemental sulfur, or soluble ferrous ions. The electrophoretic mobility of the bacterium was determined by laser Doppler velocimetry under physiological conditions. When grown on pyrite or ferrous ions, washed cells were negatively charged at pH 2.0. The density of the negative charge depended on whether the conjugate base was sulfate, perchlorate, chloride, or nitrate. The addition of ferric ions shifted the net charge on the surface asymptotically to a positive value. When grown on elemental sulfur, washed cells were close to their isoelectric point at pH 2.0. Both pyrite and colloidal sulfur were negatively charged under the same conditions. The electrical double layer around the bacterial cells under physiological conditions exerted minimal electrostatic repulsion in possible interactions between the cell and either of its charged insoluble substrates. When Thiobacillus ferrooxidans was mixed with either pyrite or colloidal sulfur at pH 2.0, the mobility spectra of the free components disappeared with time to be replaced with a new colloidal particle whose electrophoretic properties were intermediate between those of the starting components. This new particle had the charge and size properties anticipated for a complex between the bacterium and its insoluble substrates. The utility of such measurements for the study of the interactions of chemolithotrophic bacteria with their insoluble substrates is discussed.