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

Abstract

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.

Bioadsorption of Terbium(III) by Spores of Bacillus subtilis

“…The PS533 strain was inserted with the Kar gene and is therefore resistant to kanamycin. The PS4150 strain was inserted with the tetR and spnR genes, replacing the cotE and gerE genes; therefore, it is resistant to spectinomycin and has a highly defective spore surface that lacks most spore coat proteins [5,21].…”

“…Rare earth ions are difficult to separate from each other due to their similar chemical properties, ionic radii, and trivalent positive charge [4]. The complexity of this task, coupled with the demand for environmentally friendly technologies, is driving the development of biohydrometallurgy [5]. Compared with chemical precipitation, dissolution extraction, ion exchange, and membrane separation, biosorption is a more environmentally friendly, Minerals 2022, 12, 866 2 of 14 efficient, and economical technique for the treatment and recovery of REEs [6].…”

“…Bacillus spores are characterized by high-pressure resistance and rapid reproduction; they can survive at high temperatures, and under aerobic and even anaerobic conditions. Bacillus spores are highly resistant to harmful external factors and can also be used as effective biosorbents of REEs [5].…”

Properties of spores of Bacillus subtilis with or without a transposon that decreases spore germination and increases spore wet heat resistance

“…The core water content of spores was determined on both intact spores and spores that were chemically decoated as described (Dong et al . 2019), by layering ˜100 μl with ˜10 7 spores on a step gradient of Histodenz from 51 to 70% with fifteen 200 μl steps of 2%. The samples were then centrifuged at 5000 g min −1 for 45 min at 20°C in a swinging bucket rotor in a TLS‐55 rotor (Beckman Coulter Life Sciences, Brea, CA), the location at which the spores banded determined by inspection and the core wet density and the water content were calculated as described previously (Lindsay et al .…”

Applications of Bacillus subtilis Spores in Biotechnology and Advanced Materials

“…While most previous studies utilized growing bacteria to absorb REEs, a few reports have examined REE absorption by bacterial spores (86). Notably, Dong et al recently reported that B. subtilis spores could be a candidate for the adsorption of the REEs Tb 3ϩ and Dy 3ϩ (87). They found that Tb 3ϩ and Dy 3ϩ are incorporated into spores' outer layers (Fig.…”