Accumulation and Release of Rare Earth Ions by Spores of Bacillus Species and the Location of These Ions in Spores.

Two rare earth ions, Tb3+ and Dy3+, were incorporated into spores of Bacillus species in ≤5 min at neutral pH to 100 to 200 nmol per mg of dry spores, which is equivalent to 2 to 3% of the spore dry weight. The uptake of these ions had, at most, minimal effects on spore wet heat resistance or germination, and the ions were all released upon germination, probably by complex formation with the huge depot of dipicolinic acid (DPA) released when spores germinate. Adsorbed Tb3+/Dy3+ were also released by exogenous DPA within a few minutes and faster than in spore germination. The accumulation of Tb3+/Dy3+ was not reduced in Bacillus subtilis spores by several types of coat defects, significant modification of the spore cortex peptidoglycan structure, specific loss of components of the outer spore crust layer, or the absence of DPA in the spore core. All of these findings are consistent with Tb3+/Dy3+ being accumulated in spores' outer layers, and this was confirmed by transmission electron microscopy. However, the identity of the outer spore components binding the Tb3+/Dy3+ is not clear. These findings provide new information on the adsorption of rare earth ions by Bacillus spores and suggest this adsorption might have applications in capturing rare earth ions from the environment.IMPORTANCE Biosorption of rare earth ions by growing cells of Bacillus species has been well studied and has attracted attention for possible hydrometallurgy applications. However, the interaction of spores from Bacillus species with rare earth ions has not been well studied. We investigated here the adsorption and/or desorption of two rare earth ions, Tb3+ and Dy3+, by Bacillus spores, the location of the adsorbed ions, and the spore properties after ion accumulation. The significant adsorption of rare earth ions on the surfaces of Bacillus spores and the ions' rapid release by a chelator could allow the development of these spores as a biosorbent to recover rare earth ions from the environment.