Low-Frequency Oscillations of Bacteriochlorophyll Oligomers in Chlorosomes of Photosynthetic Green Bacteria

In green photosynthetic bacteria, light is absorbed by bacteriochlorophyll (BChl) c/d/e oligomers, which are located in chlorosomes - unique structures created by Nature to collect the energy of very weak light fluxes. Using coherent femtosecond spectroscopy at cryogenic temperature, we detected and studied low-frequency vibrational motions of BChl c oligomers in chlorosomes of the green bacteria Chloroflexus (Cfx.) aurantiacus. The objects of the study were chlorosomes isolated from the bacterial cultures grown under different light intensity. It was found that the Fourier spectrum of low-frequency coherent oscillations in the Q_y band of BChl c oligomers depends on the light intensity used for the growth of bacteria. It turned out that the number of low-frequency vibrational modes of chlorosomes increases as illumination under which they were cultivated decreases. Also, the frequency range within which these modes are observed expands, and frequencies of the most modes change. Theoretical modeling of the obtained data and analysis of the literature led to conclusion that the structural basis of Cfx. aurantiacus chlorosomes are short linear chains of BChl c combined into more complex structures. Increase in the length of these chains in chlorosomes grown under weaker light leads to the observed changes in the spectrum of vibrations of BChl c oligomers. This increase is an effective mechanism for bacteria adaptation to changing external conditions.