Novel Insights into Cr(VI)-Induced Rhamnolipid Production and Gene Expression in Pseudomonas aeruginosa RW9 for Potential Bioremediation

Abstract

Rhamnolipid (RL) is renowned for its efficacy in bioremediating several types of organic and metal contaminants. Nevertheless, there has been a scarcity of studies specifically examining the relationship between this substance and metals, especially in terms of their impact on RL formation and the underlying interaction processes. This study addresses this gap by investigating the RL mechanism in Cr (VI) remediation and evaluating its effect on RL production in Pseudomonas aeruginosa RW9. In this study, P. aeruginosa RW9 was grown in the presence of 10 mg l^-1 Cr (VI). We monitored RL yield, congeners distribution, and their ratios, as well as the transcriptional expression of the RL-encoded genes: rhlA, rhlB, and rhlC. Our results revealed that RL effectively reduced Cr (VI) to Cr (III), with RL yield increasing threefold, although with a slight delay in synthesis compared to control cells. Furthermore, Cr (VI) exposure induced the transcriptional expression of the targeted genes, leading to a significant increase in di-RL production. The findings confirm that Cr (VI) significantly impacts RL production, altering its structural compositions and enhancing the transcriptional expression of RL-encoded genes in P. aeruginosa RW9. This study represents a novel exploration of Cr (VI)'s influence on RL production, providing valuable insights into the biochemical pathways involved and supporting the potential of RL in Cr (VI) bioremediation.

Keywords: Bioremediation; Pseudomonas aeruginosa; chromium hexavalent; rhamnolipid.

Fig. 1. Correlation between the growth profiles of P. aeruginosa RW9 and Cr (VI) reduction over 24-h incubation period at 30°C and 150 rpm agitation.

Fig. 2. Cr (VI) removal efficiency by (A) P. aeruginosa RW9 and (B) Bacillus sp. L14 at different Cr (VI) concentrations, under conditions of 30°C and 150 rpm agitation for 24 h in NB.

Fig. 3. Correlation profile between production of RL and reductase by P. aeruginosa RW9 and Cr (VI) removal efficiency over 24 h incubation period at 30°C and 150 rpm agitation.

Fig. 4. (A) proton NMR and (B) carbon NMR spectra of extracted, purified RL produced by P. aeruginosa RW9 after 24 h of cultivation.

Fig. 5. FTIR spectra of extracted, purified RL produced by P. aeruginosa RW9 in NB (control: dark-line) and NB with 10 mg l^-1 of Cr (VI) (treated: grey-line) after 24 h of incubation.

Fig. 6. Gel image displaying PCR products for RL-encoded genes (rhlA, rhlB, and rhlC), reductase-encoded gene (chrR), and housekeeping gene (rpoD).

Fig. 7. Transcriptional expression fold changes of each target gene in P. aeruginosa RW9.

Gray columns represent samples from the 8^th h of incubation, while white columns represent samples from the 24^th h of incubation. Note: Fold-change was calculated from mean Ct values of duplicate reactions from triplicate samples at each sampling time, with RNA template concentration standardized at 200 ng per reaction. Target genes include RL-encoded genes (rhlA, rhlB, and rhlC) and the housekeeping gene rpoD.