Biomineralization of Nickel Struvite Linked to Metal Resistance in Streptomyces mirabilis

Abstract

Biomineral formation is a common trait and prominent for soil Actinobacteria, including the genus Streptomyces. We investigated the formation of nickel-containing biominerals in the presence of a heavy-metal-resistant Streptomyces mirabilis P16B-1. Biomineralization was found to occur both in solid and liquid media. Minerals were identified with Raman spectroscopy and TEM-EDX to be either Mg-containing struvite produced in media containing no nickel, or Ni-struvite where Ni replaces the Mg when nickel was present in sufficient concentrations in the media. The precipitation of Ni-struvite reduced the concentration of nickel available in the medium. Therefore, Ni-struvite precipitation is an efficient mechanism for tolerance to nickel. We discuss the contribution of a plasmid-encoded nickel efflux transporter in aiding biomineralization. In the elevated local concentrations of Ni surrounding the cells carrying this plasmid, more biominerals occurred supporting this point of view. The biominerals formed have been quantified, showing that the conditions of growth do influence mineralization. This control is also visible in differences observed to biosynthetically synthesized Ni-struvites, including the use of sterile-filtered culture supernatant. The use of the wildtype S. mirabilis P16B-1 and its plasmid-free derivative, as well as a metal-sensitive recipient, S. lividans, and the same transformed with the plasmid, allowed us to access genetic factors involved in this partial control of biomineral formation.