The facultative photosynthetic bacterium Rhodobacter capsulatus is characterized in its interaction with the toxic oxyanions tellurite (Tel(IV)) and selenite (Se-IV) by a highly variable level of resistance that is dependent on the growth mode making this bacterium an ideal organism for the study of the microbial interaction with chalcogens. As we have reported in the past, while the oxyanion tellurite is taken up by R. capsulatus cells via acetate permease and it is reduced to Te degrees in the cytoplasm in the form of splinter-like black intracellular deposits no clear mechanism was described for Se degrees precipitation. Here, we present the first report on the biotransformation of tellurium and selenium oxyanions into extracellular Te degrees and Se degrees nanoprecipitates (NPs) by anaerobic photosynthetically growing cultures of R. capsulatus as a function of exogenously added redox-mediator lawsone, i.e. 2-hydroxy-1,4-naphthoquinone. The NPs formation was dependent on the carbon source used for the bacterial growth and the rate of chalcogen reduction was constant at different lawsone concentrations, in line with a catalytic role for the redox mediator. X-ray diffraction (XRD) analysis demonstrated the Te degrees and Se degrees nature of the nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.