Among all photosynthetic organisms, green bacteria have evolved one of the most efficient light-harvesting antenna, the chlorosome, that contains hundreds of thousands of bacteriochlorophyll molecules, allowing these bacteria to grow photosynthetically by absorbing only a few photons per bacteriochlorophyll molecule per day. In contrast to other photosynthetic light-harvesting antenna systems, for which a protein scaffold imposes the proper positioning of the chromophores with respect to each other, in chlorosomes, this is accomplished solely by self-assembly. This has aroused enormous interest in the structure-function relations of these assemblies, as they can serve as blueprints for artificial light harvesting systems. In spite of these efforts, conclusive structural information is not available yet, reflecting the sample heterogeneity inherent to the natural system. Here we combine mutagenesis, polarization-resolved single-particle fluorescence-excitation spectroscopy, cryo-electron microscopy, and theoretical modeling to study the chlorosomes of the green sulfur bacterium Chlorobaculum tepidum. We demonstrate that only the combination of these techniques yields unambiguous information on the structure of the bacteriochlorophyll aggregates within the chlorosomes. Moreover, we provide a quantitative estimate of the curvature variation of these aggregates that explains ongoing debates concerning the chlorosome structure.

We performed polarization-resolved fluorescence excitation spectroscopy on individual chlorosomes from the photosynthetic green sulfur bacterium Chlorobaculum tepidum. The experiments were conducted at room temperature and under cryogenic conditions. All spectra showed a strong intensity modulation as a function of the polarization of the incident radiation, and we determined the modulation ratio as a function of the excitation energy. Under ambient conditions this ratio shows only little variation across the absorption band, whereas the low-temperature experiments clearly revealed that the broad absorption band around 740 nm consists of several spectral contributions.

Green-sulfur bacteria have evolved a unique light-harvesting apparatus, the chlorosome, by which it is perfectly adapted to thrive photosynthetically under extremely low light conditions. We have used single-particle, optical spectroscopy to study the structure-function relationship of chlorosomes each of which incorporates hundreds of thousands of self-assembled bacteriochlorophyll (BChl) molecules. The electronically excited states of these molecular assemblies are described as Frenkel excitons whose photophysical properties depend crucially on the mutual arrangement of the pigments. The signature of these Frenkel excitons and its relation to the supramolecular organization of the chlorosome becomes accessible by optical spectroscopy. Because subtle spectral features get obscured by ensemble averaging, we have studied individual chlorosomes from wild-type Chlorobaculum tepidum by polarization-resolved fluorescence-excitation spectroscopy. This approach minimizes the inherent sample heterogeneity and allows us to reveal properties of the exciton states without ensemble averaging. The results are compared with predictions from computer simulations of various models of the supramolecular organization of the BChl monomers. We find that the photophysical properties of individual chlorosomes from wild-type Chlorobaculum tepidum are consistent with a (multiwall) helical arrangement of syn-anti stacked BChl molecules in cylinders and/or spirals of different size.

Figure S6. Bravais-lattice with lattice vectors of length a = 1.25 nm and b = 0.98 nm enclosing an angle of γ = 122°for the arrangement of the BChl molecules as proposed in 1 . The vectors C refer to the rolling vector for the model structures proposed for the wild type (C WT perpendicular to the aaxis), the bchQRU mutant (C QRU parallel to the a-axis) and the bchR mutant (C R enclosing an angle δ= 69°with the a-axis, this work).