Heavy metal ions effect on light-harvesting complexes of Rhodobacter sphaeroides studied by derivative spectroscopy

The non-sulfur, purple, facultatively phototrophic bacterium Rhodobacter (R.) sphaeroides represents a unique model for the investigation of the structure, function and biosynthesis of the energy-transducing system in photosynthetic machineries. Photosynthetic units share a basic architecture, composed by an efficient light collecting system, which funnels light to the reaction center, where photons are converted into chemical potential energy with a quantum yield close to unity. In the R. sphaeroides wild type (strain 2.4.1), two distinct antenna complexes are present: the LH-II, absorbing at 800 and 850 nm, and the LH-I, with a maximum at 870 nm, appearing as a shoulder of the 850 nm LH-II band1. Strong dependence of the absorption spectrum on changing growing conditions, i.e light intensity or heavy metal ions concentration, requires to find out a robust, non-destructive instrumental method of investigation which could help to quickly solve the complex structure of the absorption signals in the presence of different stress factors. In this work, we present second-order derivative spectroscopy as the ideal technique to tackle this issue. Peaks that originate from derivation correspond to a maximum in the original spectrum, but with the advantage of a sharpening effect, which enables the precise assessment of relevant wavelengths2,3. Such effect was successfully tested on potassium permanganate solutions, whose composite band, presenting seven overlapped peaks located in the visible region of the electromagnetic spectrum, was separated in well-distinct signals, even at very low concentrations (Figure). Interestingly, Lambert-Beer Law is maintained, allowing an accurate calculation of peak ratios directly on the sharpened second derivative spectrum. This great advantage was exploited to investigate the effect of the presence of heavy metal ions on the LH complexes within cultures of R. sphaeroides 2.4.1 cells. This was possible through the precise individuation of absorption maxima and the evaluation of relative ratios between the three LH-I and LH-II peaks. Preliminary results clearly show a direct influence of tested metals on the biosynthesis of the LH-I complex, thus confirming the potentialities of the proposed technique as a promising tool for the evaluation of the chronic effect of exposure to pollutants during bacterial growth.