Overview
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- effectively removed Cr(VI) under anaerobic conditions, with total removal efficiencies of 98.3 ± 0.6% and 97.2 ± 0.9%,
- analyses showed Cr deposition on the surface of Bacillus sp. S9 (13.3 ± 0.5%) and Enterobacter sp. Z11 (33.9 ± 2%), with
- enzymatic reduction and biosorption as key mechanisms in sampling site ranged between an average of 25 °C in winter,
- have examined these processes in bacteria from unique eco- site water was 2%, pH = 9.05 and temperature = 25 ± 0.1 °C
- triplicate, and growth was assessed by OD600 measurements 11), and salinities (1.5%, 3%, and 5%) to optimize growth
- after 48 h of incubation at 30 °C. Two facultative anaerobes, conditions. The range of parameters was selected to mimic
- These results confirmed their facultative anaerobic nature tubes, each containing 1% inoculum (i.e., 50 µL contain-
- day for three days until the OD values reached 1.50 and 1.65 ments was 1% (i.e., 300 µL containing ca. 27.0 ± 0.6 mg of
- 2021). The medium was inoculated with 1% inoculum (i.e., brated using the C 1 s peak at 284.6 eV.
- lular Cr, the cells were subjected to digestion in 1% HNO3. milligram of total protein present in the CFE (µmol/min/mg
- The buffer was removed and washed using 100%, 90%, obic vs. anaerobic and between the two different strains).
- 70%, and 50% ethanol. The cells were dried at room tem- One-way analysis of variance (ANOVA) was applied to
- 2 h, and weighed. Protein and carbohydrate contents of EPS with 3% salinity at 35 °C (Table S1). Both strains demon-
- Cr(VI) bioaccumulation as anaerobic conditions (Table S1, Fig. 1). As expected,
- preparation for TEM and STEM followed previously growth model (Table S2), which revealed that lower Cr(VI)
- were fixed with osmium tetroxide (2%) and potassium fer- toxicity effects. Model parameters further demonstrated
- rocyanide (1%) in 0.1 M carbonate buffer for 1 h, and then that Cr(VI) reduced maximum growth rates (µmax) and
- show mean values with error bars 1.5 B D
- mean (n = 3). Different lowercase a
- 52 ± 0.5 mg L− 1 for Enterobacter sp. Z11 (Table 1), which Bacillus sp. S9 (7 ± 0.5 mg L− 1, p < 0.001), accounting for
- could be attributed to technical and analytical reasons. The 33.9 ± 2% and 13.3 ± 0.5% of total added Cr, respectively
- recovered concentrations after 7 days incubations were (Table 1; Fig. 2C, D). SEM-EDX analysis confirmed the
- tively, which corresponded to ca. 98 ± 0.01% of the initially well as Enterobacter sp. Z11 (Fig. 3A, B). The SEM-EDX
- tant, on the cell surface, and intracellularly (Table 1; Fig. 2A, ther validated this absorption by confirming the identity of
- sp. Z11 (p < 0.001, Table 1; Fig. 2A, B). In the case of Bacil- process (Fig. 4). FTIR analysis revealed distinct spectral dif-
- C). This corresponds to the reduction of 77.6 ± 1% of the ini- O–H and N–H stretching vibrations, corresponding to con-
- mg L− 1 of Cr(VI) to Cr(III) at a removal rate of 3.8 ± 0.06 groups, indicated in the 2800–3000 cm⁻¹ range, originated
- remained as Cr(VI) in the supernatant (Table 1). the methyl (–CH₃) and methylene (–CH₂) groups from lip-
- Table 1 Cr(VI) removal by Bacillus sp. S9 and Enterobacter sp. Z11 grown anaerobically in the presence of Cr(VI) through reduction to Cr(III),
- Total bioreduction of Cr(VI)$ % 77.6±1 51.0±0.4
- % biosorption of Cr(VI)$ % 13.3±0.5 33.9±2.0
- % reduction of Cr(VI)* % 85 88
- Total bioreduction$ % 11.3±0.7p<0.001 30.0±2.0
- % bioaccumulation of Cr(VI) % 07.4±0.3 12.3±0.3
- % reduction of Cr(VI)* % 74 32
- Total bioreduction$ % 05.5±0.38 p<0.001
- C–N stretching, characteristic of the amide II region of pro- Cr(III), accounting for 88% (15.6 ± 2 out of 17.6 ± 2 mg L− 1)
- teins. Peaks identified at lower wave lengths, such as those and 85% (6 ± 0.7 out of 7 ± 0.5 mg L− 1) of the total detected
- attributed to C–N, C–O, and PO₄³⁻ stretching, confirmed the Cr on the surface, respectively (Table 1; Fig. 2C and D). The
- (Fig. 5A). For Cr 2P3/2, Cr(0) accounted for 5.6% of total Cr
- while Cr(III) and Cr(VI) accounted for 42.8% and 51.6% of
- accounted for 6.0, 75.9 and 18.1%, respectively. In case of
Methods (brief)
- (Narayani and Shetty 2013; Padma et al. 2023; Upadhyay facultative anaerobes
- obes have been reported from genera such as Shewanella, The original microbial mats were collected from a Cr(VI)-
- determined after drying the samples at 60 °C for 24 h to in triplicate.
- For mechanistic experiments, minimal medium was used state of Cr on the cell surface. The collected cell pellet was
- MgSO4 stock and 0.1 ml of 1 M CaCl2 stock were added. samples were then centrifuged and filtered (0.22 μm) to
- lular Cr, the cells were subjected to digestion in 1% HNO3. milligram of total protein present in the CFE (µmol/min/mg
- lent lot code AA7A0727 (Trace2O, UK) at 540 nm with a more active enzyme sample.
- (ICP-OES) (Optima 8000DV, Perkin Elmer, USA). The To investigate the adsorption of Cr on the surface of the
- by centrifuging at 5000 rpm for 5 min and sodium cacodyl- dent-sample student t-test to compare the means between
- was applied consistently to all samples for accurate com-
- (JEOL, JEM 2100 F, Japan) were employed. The sample centrations (Fig. 1C, D). This was supported by the logistic
- measured amounts. At the end of the Cr(VI) removal experi- of the control samples exhibited no Cr on the surface of the
- Distinct absorbances near 400–900 cm⁻¹ were associated ther confirmed using XPS analysis on the collected biomass
- EDX spectra. Tables show elemental composition (weight% and stan- CFE from collected biomass. The total protein content was
- After cell digestion using 1% HNO3, Cr was detected was ca. 0.41% in Enterobacter sp. Z11 (insert in Fig. 6D),
- ICP-OES analysis of both forms of Cr in the supernatant, cell reported value of 1.52 U/mg protein for L. casei (Mishra
- CFE from collected biomass of our strains confirmed the potentially mediated by products of microbial metabolites
- Cr(III) inside the cells was indeed validated by ICP-OES
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