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 organisatio n of the light-harvesting apparatus of the green sulfur bacterium Chlorobiu m tepidum growing under different light intensities. Cells grown at lower l ight intensities had lower exponential growth rates and increased amounts o f 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 li ght grown cells. A similar red-shift of up to 4 nm was also observed in the corresponding fluorescence emission peaks. HPLC analysis of pigment extrac ts showed a correlation between the red-shift and the content of the more a lkylated 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.