Cell population behavior of the unicellular red alga Galdieria sulphuraria during precious metal biosorption

This study investigates the biosorption mechanism, including cell population behavior, of trace amounts of precious metals (gold, palladium, and platinum) in a unicellular red alga, Galdieria sulphuraria. Single-cell inductively coupled plasma mass spectrometry showed that the number of adsorbing cells and the concentration of adsorbed metal per cell varied depending on solution acidity and metal species. The X-ray absorption fine structure in 5 mM HCl solution indicated that the adsorbed Au formed inner-sphere complexes with S, whereas the adsorbed Pd and Pt formed an inner-sphere complexes with N and/or S. In 500 mM HCl solution, the adsorbed Au and Pd formed inner-sphere complexes only with S, and the Au formed a structure similar to Au2S. At higher acidity, Au and Pd were recovered by interacting with residues that formed more stable complexes, which was accompanied by changes in the behavior of cell populations adsorbing the metals. This is the first study to demonstrate the relationship between changes in the behavior of cell populations and chemical interactions that occur between substrate elements and biomaterial residues during biosorption. The findings of this study provide deeper insights into the biosorption mechanism and a background for the design of an environmentally friendly biosorbent.