Phosphate-Suppressed Selenite Biotransformation by Escherichia coli

Abstract

Biotransformation of selenite to valuable elemental selenium nanoparticles (Se0) is a promising avenue to remediate seleniferous environments and simultaneously recover selenium (Se). However, the underlying oxyanion competition and selenite transformation mechanism in prokaryotes are poorly understood. In this work, the impacts of phosphate on selenite uptake and transformation were elucidated with Escherichia coli and its mutant deficient in phosphate transport as model microbial strains. Selenite uptake was inhibited by phosphate in E. coli. Moreover, the transformation of internalized Se was shifted from Se0 to toxic organo-Se with elevated phosphate levels, as evidenced by the linear combination fit analysis of the Se K-edge X-ray absorption near-edge structure. Such a phosphate-regulated selenite biotransformation process was mainly assigned to the competitive uptake of phosphate and selenite, which was primarily mediated by a low affinity phosphate transporter (PitA). Under phosphate-deficient conditions, the cells not only produced abundant Se0 nanoparticles but also maintained good cell viability. These findings provide new insights into the phosphate-regulated selenite biotransformation by prokaryotes and contribute to the development of new processes for bioremediating Se-contaminated environments, as well as bioassembly of Se0.

Tellurite and Selenite: how can these two oxyanions be chemically different yet so similar in the way they are transformed to their metal forms by bacteria?

“…Janine: Considering the various reports about selenite transporters we already mentioned, I’m tempted to conclude that tellurite and selenite should be transported by the same carriers, contrary to what is believed, and that many different transporters are involved in the transport of these oxyanions. In a recent work with E. coli [ 92 ], for example, it is reported that in cultures amended with 0.1 mM selenite, addition of increasing phosphate concentration up to 30 mM, progressively decreased the incorporation of Se in the biomass, thus indicating that phosphate ions prevented selenite uptake. It must be noted, however, that none of the phosphate concentrations tested, completely suppressed selenite uptake (see also Table 1 ).…”

“…Davide: Janine, let me remind you that the latter competition experiments you mentioned [ 92 ], were carried out under experimental conditions very far from the expected 1 to 1 ratio between two reactants that should compete specifically for a common carrier, as vice versa we observed between tellurite and acetate in R. capsulatus [ 64 , 65 ].…”

“…Janine: Davide, you are right, but in any case TEM images have shown that the amounts of cytosolic Se 0 nanoparticles largely decreased with increasing phosphate concentration outside the cells, and that these particles were barely produced in the presence of 30 mM phosphate [ 92 ]. Zhu T-T et al [ 92 ] also studied selenite uptake and reduction using mutants of the low-affinity phosphate transport system ( Δpit A mutant), as well as of the high-affinity phosphate transport system ( Δpst A mutant) that was investigated by Claudio Vásquez for tellurite transport, some time ago [ 66 ]. Interestingly, although both mutants grew well in the presence of 1 mM selenite, its reduction to Se 0 was slowed down in cultures of the Δpit A mutant demonstrating the crucial role of the low affinity phosphate transporter on selenite uptake in E. coli.…”

Selenium Nanoparticles as an Innovative Selenium Fertilizer Exert Less Disturbance to Soil Microorganisms

“…The results indicated that SeNPs were less bioavailable than SeO 3 2– . It has been reported that SeO 3 2– can be easily transported across the membrane via binding to the sulfhydryl sites on the cell membrane or by phosphate transfer protein ( Yu et al, 2018 ; Zhu et al, 2020 ). In contrast, SeNPs are in a reduced state.…”

Artificially regulated synthesis of nanocrystals in live cells

“…In E. coli , Na 2 SeO 3 uptake is inhibited by phosphate, and intracellular Se is transformed from Se 0 to organoselenium compounds when the phosphate level is high. This is mainly attributed to the competitive uptake of phosphate and Na 2 SeO 3 mediated by the low-affinity phosphate transporter PitA [ 16 ].…”