## Results and Discussion  
### Uranium and Nickel Depletion Potential of Arthrobacter sp. SRS-1-W-2016  
The ability of bacterial strains to resist U toxicity has been previously demonstrated. In fact, 90% of bacteria representative of taxa Firmicutes, Gammaproteobacteria, Actinobacteria, Bacteroidetes, and Betaproteobacteria identified from the Uranium mining site of Domiostat, India, were found to be resistant to a high U concentration of 4 mM [21]. [PERSON] et al. [22] isolated two phosphatase producing bacteria- _Escherichia coli_ and _Deinococcus radiodurans_, which were found to grow in as high as 20 mM U. _Burkholderia fungorum_ isolated from Rifle, Colorado, was found to grow on 10 \(\upmu\)M U [23]. Similarly, several strains resistant to particularly high concentrations of Ni have been isolated from various contaminated sources. [PERSON] et al. [24] isolated several strains, namely _Methylobacterium_ sp. ATHA7 and _Klebsiella oxytoca_ ATHA6 resistant to 24 mM and 16 mM Ni, respectively, from industrial sewage. _Cupiravidus taiwanensis_ isolated from the root nodules of _Mimosa pudica_ was found to be resistant to 1.5 mM Ni, along with some other heavy metals [25]. To determine the resistance potential of strain SRS-1-W-2016 against U and Ni, different concentrations of these metals were supplemented in 4M media and the growth results are presented in Figure 1a. Strain SRS-1-W-2016 could resist up to 4.2 mM of U and 8.5 mM of Ni, which are the typical concentrations of these metals in the sampling location [13], albeit, rapid growth was only observed at much lower concentrations of 500 \(\upmu\)M U and 1000 \(\upmu\)M Ni, respectively; a stationary phase was reached in 48 h. A similar trend was shown in the presence of 1000 \(\upmu\)M Ni. When grown in the combination of 1000 \(\upmu\)M Ni and 500 \(\upmu\)M U, the stationary phase was only achieved after 60 h of incubation, potentially due to the combined stress of both U and Ni. In the native soil environment from where the strain was isolated, both U and Ni are the major co-contaminants and therefore the growth profile obtained for these co-contaminants provides a more accurate representation of the strain's metal resistance ability.  
To further evaluate the bioremediation potential of strain SRS-1-W-2016 for U and Ni separately, as well as in combination, microcosms were established for quantification of the amended metals by inductively coupled plasma-mass spectrometry (ICP-MS) and these results are shown in Figure 1b. It is clear that the initial concentration of U in the microcosms declined rapidly in merely 24 h and most of the spiked U declined completely in the supernatant by 72 h, strongly suggesting that strain SRS-1-W-2016 has the ability to quickly biotransform and/or bioremediate U. Several mechanisms are recognized by which microbes are able to transform and resist toxicity posed by extracellular U. Some of these mechanisms include reductive precipitation by outer membrane cytochromes, conductive pili, or spores, surface adsorption by exopolysaccharideacharide (EPS) or S-layers, or precipitation with phosphate compounds, respectively. To evaluate the mechanism utilized by strain SRS-1-W-2016 to grow in the presence of U, we used the histochemical growth assay on trytpose phosphate methyl green (TPMG) media. We observed an intense green coloration formed by strain SRS-W-1-2016 during growth on an Luria Bertani (LB) plate supplemented with U (Figure 1c), indicating the activity of phosphatase enzyme-based bioremediation of U. Surprisingly, no decline in the concentrations of the amended Ni was observed under the tested conditions (Figure 1b). It is highly likely that efflux mechanisms were employed by the strain to prevent the cellular uptake of Ni and hence no bioremediation activity against Ni was documented.  
Figure 1: _Cont._