The photosynthetic membrane proteome of Rhodobacter sphaeroides R-26.1 exposed to cobalt

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.

Interaction between the photosynthetic anoxygenic microorganism Rhodobacter sphaeroides and soluble gold compounds. From toxicity to gold nanoparticle synthesis

Biological processes using microorganisms for nanoparticle synthesis are appealing as eco-friendly nanofactories. The response of the photosynthetic bacterium Rhodobacter sphaeroides to gold exposure and its reducing capability of Au(III) to produce stable gold nanoparticles (AuNPs), using metabolically active bacteria and quiescent biomass, is reported in this study. In the former case, bacterial cells were grown in presence of gold chloride at physiological pH. Gold exposure was found to cause a significant increase of the lag-phase duration at concentrations higher than 10 μM, suggesting the involvement of a resistance mechanism activated by Au(III). Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry (SEM/EDS) analysis of bacterial cells confirmed the extracellular formation of AuNPs. Further studies were carried out on metabolically quiescent biomass incubated with gold chloride solution. The biosynthesized AuNPs were spherical in shape with an average size of 10 ± 3 nm, as analysed by Transmission Electron Microscopy (TEM). The nanoparticles were hydrophilic and stable against aggregation for several months. In order to identify the functional groups responsible for the reduction and stabilization of nanoparticles, AuNPs were analysed by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), X-ray Fluorescence Spectrometry (XRF) and X-ray Absorption Spectroscopy (XAS) measurements. The obtained results indicate that gold ions bind to functional groups of cell membrane and are subsequently reduced by reducing sugars to gold nanoparticles and capped by a protein/peptide coat. Gold nanoparticles demonstrated to be efficient homogeneous catalysts in the degradation of nitroaromatic compounds.

Purple non-sulfur photosynthetic bacteria monitor environmental stresses

Heavy metal ion pollution and oxygen deficiency are major environmental risks for microorganisms in aqueous habitat. The potential of purple non-sulfur photosynthetic bacteria for biomonitoring and bioremediation was assessed by investigating the photosynthetic capacity in heavy metal contaminated environments. Cultures of bacterial strains Rhodobacter sphaeroides, Rhodospirillum rubrum and Rubrivivax gelatinosus were treated with heavy metal ions in micromolar (Hg(2+)), submillimolar (Cr(6+)) and millimolar (Pb(2+)) concentration ranges. Functional assays (flash-induced absorption changes and bacteriochlorophyll fluorescence induction) and electron micrographs were taken to specify the harmful effects of pollution and to correlate to morphological changes of the membrane. The bacterial strains and functional tests showed differentiated responses to environmental stresses, revealing that diverse mechanisms of tolerance and/or resistance are involved. The microorganisms were vulnerable to the prompt effect of Pb(2+), showed weak tolerance to Hg(2+) and proved to be tolerant to Cr(6+). The reaction center controlled electron transfer in Rvx. gelatinosus demonstrated the highest degree of resistance against heavy metal exposure.

The lipidome of the photosynthetic bacterium Rhodobacter sphaeroides R26 is affected by cobalt and chromate ions stress

A detailed characterization of membrane lipids of the photosynthetic bacterium Rhodobacter (R.) sphaeroides was accomplished by thin-layer chromatography coupled with matrix-assisted laser desorption ionization mass spectrometry. Such an approach allowed the identification of the main membrane lipids belonging to different classes, namely cardiolipins (CLs), phosphatidylethanolamines, phosphatidylglycerols (PGs), phosphatidylcholines, and sulfoquinovosyldiacylglycerols (SQDGs). Thus, the lipidomic profile of R. sphaeroides R26 grown in abiotic stressed conditions by exposure to bivalent cobalt cation and chromate oxyanion, was investigated. Compared to bacteria grown under control conditions, significant lipid alterations take place under both stress conditions; cobalt exposure stress results in the relative content increase of CLs and SQDGs, most likely compensating the decrease in PGs content, whereas chromate stress conditions result in the relative content decrease of both PGs and SQDGs, leaving CLs unaltered. For the first time, the response of R. sphaeroides to heavy metals as Co(2+) and CrO4 (2-) is reported and changes in membrane lipid profiles were rationalised.

The reaction center is the sensitive target of the mercury(II) ion in intact cells of photosynthetic bacteria

The sensitivity of intact cells of purple photosynthetic bacterium Rhodobacter sphaeroides wild type to low level (<100 μM) of mercury (Hg²⁺) contamination was evaluated by absorption and fluorescence spectroscopies of the bacteriochlorophyll-protein complexes. All assays related to the function of the reaction center (RC) protein (induction of the bacteriochlorophyll fluorescence, delayed fluorescence and light-induced oxidation and reduction of the bacteriochlorophyll dimer and energization of the photosynthetic membrane) showed prompt and later effects of the mercury ions. The damage expressed by decrease of the magnitude and changes of rates of the electron transfer kinetics followed complex (spatial and temporal) pattern according to the different Hg²⁺ sensitivities of the electron transport (donor/acceptor) sites including the reduced bound and free cytochrome c₂ and the primary reduced quinone. In contrast to the RC, the light harvesting system and the bc₁ complex demonstrated much higher resistance against the mercury pollution. The 850 and 875 nm components of the peripheral and core complexes were particularly insensitive to the mercury(II) ions. The concentration of the photoactive RCs and the connectivity of the photosynthetic units decreased upon mercury treatment. The degree of inhibition of the photosynthetic apparatus was always higher when the cells were kept in the light than in the dark indicating the importance of metabolism in active transport of the mercury ions from outside to the intracytoplasmic membrane. Any of the tests applied in this study can be used for detection of changes in photosynthetic bacteria at the early stages of the action of toxicants.