Fluorescence emission spectroscopy of a single light harvesting chlorosome from green filamentous photosynthetic bacteria

Abstract

The fluorescence emission properties of a light-harvesting antenna, chlorosome, from green filamentous photosynthetic bacteria on a quartz plate immersed in a buffer solution were studied at the single-unit level with a total internal reflection microscope. The independent fluorescence spots from single chlorosomes were observed by an evanescent illumination, and their emission was gradually photobleached at room temperature. Atomic force microscope observation evidenced that most chlorosomes were situated independently on a quartz plate. The fluorescence emission spectra from all the individual chlorosomes exhibited mainly the band from bacteriochlorophyll(BChl)-c aggregates around 750 nm, which were almost identical to that of an ensemble spectrum of chlorosomes in a buffer solution. The fluorescence emission from BChl-a in baseplates was observed in limited chlorosomes and the intensity was weaker than that of the ensemble spectrum. All the fluorescence bands from BChl-c aggregates had a similar peak position and also a similar bandwidth. The conservative spectroscopic properties of each Chloroflexus chlorosome indicate that the presence of stereoisomers at the 31-position of BChl-c molecules (31-R/S=2/1) would not give any heterogeneity of chlorosomal pigment organization among Chloroflexus chlorosomes. The single homolog of BChl-c molecules in Chloroflexus might be a possible factor in the spectral homogeneity among single Cfl. chlorosomes. This work demonstrates that single-molecule spectroscopy could be a powerful tool to reveal in detail the electronic properties of photosynthetic pigment organization as well as the distribution of composed pigments in a single antenna apparatus.