Role of rhamnolipids in Pseudomonas aeruginosa RW9 remediation of hexavalent chromium
Chromium hexavalent (Cr(VI)) pollution has become a global issue. Despite the efficiency of various bacterial species reported to treat the pollutant, the removal performance of each species and strains were found to be distinctive. The same phenomenon was also observed in Pseudomonas aeruginosa...
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| Format: | Thesis |
| Language: | en |
| Published: |
2023
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/122731/1/122731.pdf http://psasir.upm.edu.my/id/eprint/122731/ http://ethesis.upm.edu.my/id/eprint/18642 |
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| Summary: | Chromium hexavalent (Cr(VI)) pollution has become a global issue. Despite the
efficiency of various bacterial species reported to treat the pollutant, the removal
performance of each species and strains were found to be distinctive. The same
phenomenon was also observed in Pseudomonas aeruginosa as a well-known
efficient species in Cr(VI) remediation. The discrepancy in performance might be
resulted from the removal mechanisms deployed by the strains. Thus, in this
study the potential of P. aeruginosa RW9 in Cr(VI) remediation was assessed.
The strain was subjected to a range of Cr(VI) to evaluate its tolerance. Results
showed its ability to resist a high concentration of up to 40 mg/L Cr(VI). The
Cr(VI) distribution analysis by a shake flask study at 30 °C, agitation of 150 rpm
for 24 hours incubation with 10 mg/L Cr(VI) was conducted. It was revealed that
the strain developed a complex mechanism to remove Cr(VI), based on surfacebound
(0.46 mg/L), intracellularly accumulated (1.24 mg/L) and extracellular
sequestration (6.74 mg/L), which accounted for approximately 85% of the
removal efficiency. The extracellular sequestration was attributed to the
extracellular reductase and biosurfactant with the latter discovered as the
predominant metabolite. High performance liquid chromatography with a
variable wavelength detector (HPLC-VW) revealed that the strain produced more
biosurfactant in Cr(VI) treated experiment as compared to that of the control (0
mg/L Cr(VI)). The biosurfactant was extracted by acid precipitation coupled with
ethyl acetate extraction and purified by using solid phase extraction method prior
to characterisation analyses by fourier transform infra-red spectroscopy, nuclear
magnetic resonance (NMR) and liquid chromatography mass spectrometry (LCMS)
analyses to confirm the rhamnolipid structure. The LC-MS analysis also
exposed that cells in both control and Cr(VI) treated experiments produced more
di-rhamnolipid with the main congener being Rha-Rha-C10-C10 (m/z 649).
Furthermore, Cr(VI) was found to induce the synthesis of Rha-Rha-C8-C10/
Rha-Rha-C10-C8 congener (m/z 620). The Whole Genome Sequencing (WGS)
verified the presence of the rhamnolipid-encoded genes (rhlA, rhlB and rhlC) and
reductase-encoded gene (chrR) before conducting the one-step quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). It was discovered
that Cr(VI) induced the transcriptional expression of all the target genes with the
highest enhancement observed in rhlB. Later, the performance of the produced
rhamnolipid was tested to assess its efficiency to remediate six chosen elements
(Se, Cd, B, Cr, Zn and Mn) in partially treated leachate considering the metals
concentration in the wastewater. The rhamnolipid, in its crude form of 10 mg/L,
was applied to the wastewater and the remaining concentrations of the elements
were measured using inductively coupled plasma atomic emission spectroscopy
(ICP-OES). Se and Cd were completely removed after a 48-hour treatment.
Meanwhile, B, Zn, Cr and Mn were reduced at 93.45%, 77.37%, 68.04% and
55.42%, respectively. The rhamnolipid exhibited preferential to the respective
elements following Se > Cd > B > Cr > Zn > Mn sequence. The same pattern
was also displayed by the commercial rhamnolipid which served as the control
experiment. Collectively, the findings proved that the strain has a great potential
for application in Cr(VI) bioremediation. In terms of application, it can be
manipulated to produce rhamnolipid which exhibited various downstream
applications especially in remediating metal pollutants. |
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