In Vivo Energy Transfer from Bacteriochlorophyll c, d, e, or f to Bacteriochlorophyll a in Wild-Type and Mutant Cells of the Green Sulfur Bacterium Chlorobaculum limnaeum

Green sulfur bacteria have light-harvesting antenna systems, called chlorosomes, which usually contain one of bacteriochlorophyll (BChl) c, d, and e molecules depending on the bacterial strain. Additionally, BChl f has never been found in nature but was observed in chlorosomes of the constructed mutant. In this study, we used the brown-colored green sulfur bacterium Chlorobaculum limnaeum RK-j-1 strain possessing BChl e, and constructed its mutants accumulating only either BChl c, d, or f. In the mutant cells, these pigments showed different electronic absorption spectra, and their Qy peaks shifted hypsochromically in the order of BChls c, d, e, and f. The energy transfer from the chlorosomal aggregates to BChl a in these mutant cells was observed at 77 K by using picosecond time-resolved fluorescence measurements. According to the Förster energy-transfer mechanism, the energy-transfer efficiency from the chlorosomal aggregates to BChl a decreased in the order of BChl c, BChl d, BChl e to BChl f.