Characterization of Chlorobium tepidum chlorosomes: A calculation of bacteriochlorophyll c per chlorosome and oligomer modeling

The bacteriochlorophyll (Bchl) c content and organization was determined for Chlorobium (Cb.) tepidum chlorosomes, the light-harvesting complexes from green photosynthetic bacteria, using fluorescence correlation spectroscopy and atomic force microscopy. Single-chlorosome fluorescence data was analyzed in terms of the correlation of the fluorescence intensity with time. Using this technique, known as fluorescence correlation spectroscopy, chlorosomes were shown to have a hydrodynamic radius (Rh) of 25 ± 3.2 nm. This technique was also used to determine the concentration of chlorosomes in a sample, and pigment extraction and quantitation was used to determine the molar concentration of Bchl c present. From these data, a number of ∼215,000 ± 80,000 Bchl c per chlorosome was determined. Homogeneity of the sample was further characterized by dynamic light scattering, giving a single population of particles with a hydrodynamic radius of 26.8 ± 3.7 nm in the sample. Tapping-mode atomic force microscopy (TMAFM) was used to determine the x, y, z dimensions of chlorosomes present in the sample. The results of the TMAFM studies indicated that the average chlorosome dimensions for Cb. tepidum was 174 ± 8.3 × 91.4 ± 7.7 × 10.9 ± 2.71 nm and an overall average volume 90,800 nm³ for the chlorosomes was determined. The data collected from these experiments as well as a model for Bchl c aggregate dimensions was used to determine possible arrangements of Bchl c oligomers in the chlorosomes. The results obtained in this study have significant implications on chlorosome structure and architecture, and will allow a more thorough investigation of the energetics of photosynthetic light harvesting in green bacteria.