Arsenic Biomineral Formation Leads to Partial Encrustation of Thermoacidophilic Archaeon

Acidophilic iron oxidizing Sulfolobales spp. can mediate the formation of jarosite nanoprecipitates and precursors of the arsenic biomineral scorodite [1]. In batch experiments, scorodite formation by Sulfolobales spp. was induced at a pH of 1 and 75°C [2]. At these conditions, we observed incomplete encrustation of Sulfolobales spp. cells with precursors (nuclei) of scorodite, formed on the cell surface. The low pH in these experiments is expected to prevent encrustation of the archaeal cells by ferric iron precipitates, which stays in solution below pH 2. Therefore, we suggest that the mechanism of scorodite formation begins with the sorption of ferric iron onto charged groups on the cell surface, followed by the formation of nuclei from the adsorbed ferric iron and dissolved arsenate. As the encrustation of cells was incomplete, the archaea continued oxidizing ferrous iron. Growth and ageing of the precipitates into crystals was favoured under the applied conditions. Our results show that incomplete encrustation only occurs when arsenate is added. Encrustation does not occur at all in absence of arsenic, i.e., jarosite precipitates induced by the Sulfolobales were not attached to the cell surface. Possibly, the formation of ferric arsenate precipitates is induced by the archaea as a strategy against arsenic inhibition.