Extracellular Production of Tellurium Nanoprecipitates by the Photosynthetic Bacterium Rhodobacter capsulatus

Tellurium belongs to the group 16 of the periodic table. Remarkably, the high toxicity of the Te oxyanions (mainly tellurite, TeIV) causes environmental problems in contaminated soils and water bodies. The facultative photosynthetic bacterium Rhodobacter capsulatus is featured by a significant level of resistance to tellurite that is dependent on the growth mode. Recently, we have reported that the redox mediator lawsone (2-hydroxy-1,4-naphtoquinone), known as "henna leaf extract" (Lawsonia inermis), allows anaerobic light-grown cultures of R. capsulatus to generate Te0 nano-precipitate outside the cells, i.e. in the growth medium. Here we have optimized the cultural conditions to generate Te0 nano-precipitates by R. capsulatus as a function of the carbon source, lawsone concentration and nano-particles generation kinetics. Pyruvate resulted to be the best electron donor for Te0 generation while lawsone, when used at <10 ?M, affected both the kinetic and amount of nano-particles production. Further, growing cultures over a 10 days period with daily additions of 1 mM tellurite, led to the accumulation of progressively larger tellurite nano-precipitates showing a wide size-range up to 600 nm in length. This finding suggests that nucleation of new particles takes place over the entire cell growth period although the addition of new material to pre-formed particles is the main strategy used by R. capsulatus to accumulate Te0 outside the cells. Notably, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses of Te0 particles indicate the presence of an external organic coating that keeps the particles in solution in aqueous solvents. The abundance of carbonyl groups (C=O) points to proteins as one of the main components of the particle-external coating