Light intensity effects on pigment composition and organisation in the green sulfur bacterium Chlorobium tepidum

We have investigated the changes in the pigment composition and organisation of the light-harvesting apparatus of the green sulfur bacterium Chlorobium tepidum growing under different light intensities. Cells grown at lower light intensities had lower exponential growth rates and increased amounts of the main light-harvesting pigments, bacteriochlorophyll c and carotenoids, on a cell protein basis. Absorption spectra of chlorosomes isolated from cells grown at low light intensities revealed a red-shift of up to 8 nm in the Q(y) band of bacteriochlorophyll c compared to chlorosomes from high light grown cells. A similar red-shift of up to 4 nm was also observed in the corresponding fluorescence emission peaks. HPLC analysis of pigment extracts showed a correlation between the red-shift and the content of the more alkylated BChl c homologs, which increased as light intensity for growth was lower. Furthermore, analysis of the carotenoid composition in chlorosomes revealed a conspicuous change in the ratio between chlorobactene and 1', 2'-dihydrochlorobactene, which dramatically decreased from 5 to 0.7 in light-limited cultures.