Mechanism of elemental sulfur oxidation by acidophilic bacteria

We observed sulfur oxidation by both free and adsorbed bacteria. A dominant oxidation role for free bacteria was suggested based on kinetic observations in both growing and non-growing cultures. The linear phase of sulfur oxidation was related to sulfur limitation in agreement with the Monod equation. The active growth phase always followed adsorption of bacteria on sulfur; however, the special metabolic role of adsorbed bacteria was unclear. The presence of the pigment iodinin, an A. ferrooxidans culture metabolite, was demonstrated after growth of bacteria on elemental sulfur. Although its physiological function is unknown, it is possible that sites on the cell surface are covered with this metabolite, thereby facilitating hydrophobic interactions between cell surfaces and sulfur particles. Such an interaction would be important for the adsorption of A. ferrooxidans onto sulfur surfaces, in addition to other possible adsorption mechanisms. The role of adsorbed bacteria may be to solubilize sulfur. But sulfur limitation and oxidation activity of free cells were fundamental to explain kinetic data of sulfur biooxidation.