Assessment of an internal reference gene in Rhodobacter sphaeroides grown under cobalt exposure

Evolutionary engineering and molecular characterization of cobalt-resistant Rhodobacter sphaeroides

With its versatile metabolism including aerobic and anaerobic respiration, photosynthesis, photo-fermentation and nitrogen fixation, Rhodobacter sphaeroides can adapt to diverse environmental and nutritional conditions, including the presence of various stressors such as heavy metals. Thus, it is an important microorganism to study the molecular mechanisms of bacterial stress response and resistance, and to be used as a microbial cell factory for biotechnological applications or bioremediation. In this study, a highly cobalt-resistant and genetically stable R. sphaeroides strain was obtained by evolutionary engineering, also known as adaptive laboratory evolution (ALE), a powerful strategy to improve and characterize genetically complex, desired microbial phenotypes, such as stress resistance. For this purpose, successive batch selection was performed in the presence of gradually increased cobalt stress levels between 0.1-15 mM CoCl2 for 64 passages and without any mutagenesis of the initial population prior to selection. The mutant individuals were randomly chosen from the last population and analyzed in detail. Among these, a highly cobalt-resistant and genetically stable evolved strain called G7 showed significant cross-resistance against various stressors such as iron, magnesium, nickel, aluminum, and NaCl. Growth profiles and flame atomic absorption spectrometry analysis results revealed that in the presence of 4 mM CoCl2 that significantly inhibited growth of the reference strain, the growth of the evolved strain was unaffected, and higher levels of cobalt ions were associated with G7 cells than the reference strain. This may imply that cobalt ions accumulated in or on G7 cells, indicating the potential of G7 for cobalt bioremediation. Whole genome sequencing of the evolved strain identified 23 single nucleotide polymorphisms in various genes that are associated with transcriptional regulators, NifB family-FeMo cofactor biosynthesis, putative virulence factors, TRAP-T family transporter, sodium/proton antiporter, and also in genes with unknown functions, which may have a potential role in the cobalt resistance of R. sphaeroides.

The periodic table of photosynthetic purple non-sulfur bacteria: intact cell-metal ions interactions

Photosynthetic purple non-sulfur bacteria (PNB) have been widely utilized as model organisms to study bacterial photosynthesis. More recently, the remarkable resistance of these microorganisms to several metals ions called particular interest. As a result, several research efforts were directed toward clarifying the interactions of metal ions with PNB. The mechanisms of metal ions active uptake and bioabsorption have been studied in detail, unveiling that PNB enable harvesting and removing various toxic ions, thus fostering applications in environmental remediation. Herein, we present the most important achievements in the understanding of intact cell-metal ions interactions and the approaches utilized to study such processes. Following, the application of PNB-metal ions interactions toward metal removal from contaminated environments is presented. Finally, the possible coupling of PNB with abiotic electrodes to obtain biohybrid electrochemical systems is proposed as a sustainable pathway to tune and enhance metal removal and monitoring.

Bioremediation of lead contaminated soil with Rhodobacter sphaeroides

Bioremediation with microorganisms is a promising technique for heavy metal contaminated soil. Rhodobacter sphaeroides was previously isolated from oil field injection water and used for bioremediation of lead (Pb) contaminated soil in the present study. Based on the investigation of the optimum culturing conditions and the tolerance to Pb, we employed the microorganism for the remediation of Pb contaminated soil simulated at different contamination levels. It was found that the optimum temperature, pH, and inoculum size for R. sphaeroides is 30-35 °C, 7, and 2 × 10(8) mL(-1), respectively. Rhodobacter sphaeroides did not remove the Pb from soil but did change its speciation. During the bioremediation process, more available fractions were transformed to less accessible and inert fractions; in particular, the exchangeable phase was dramatically decreased while the residual phase was substantially increased. A wheat seedling growing experiment showed that Pb phytoavailability was reduced in amended soils. Results inferred that the main mechanism by which R. sphaeroides treats Pb contaminated soil is the precipitation formation of inert compounds, including lead sulfate and lead sulfide. Although the Pb bioremediation efficiency on wheat was not very high (14.78% root and 24.01% in leaf), R. sphaeroides remains a promising alternative for Pb remediation in contaminated soil.