Spectroscopic insights into the decreased efficiency of chlorosomes containing bacteriochlorophyll f

Chlorosomes are light-harvesting antenna complexes that occur in green photosynthetic bacteria which have only been shown naturally to contain bacteriochlorophyll (BChl) c, d, or e as the principal light-harvesting pigments. BChl f has long been thought to be an obvious fourth member of the so-called Chlorobium chlorophylls, because it possesses a C-7 formyl group like BChl e and lacks a methyl group at C-20 like BChl d. In organisms that synthesize BChl c or e, the bchU gene product encodes the enzyme that methylates the C-20 position of these molecules. A bchU null mutant of the green sulfur bacterium Chlorobaculum limnaeum strain 1677(T), which normally synthesizes BChl e, has recently been generated via insertional inactivation, and it produces chlorosomes containing BChl f [Vogl et al., 2012]. In this study, chlorosomes containing BChl f and monomeric BChl f in pyridine were characterized using a variety of spectroscopic techniques, including fluorescence emission and excitation spectroscopy, fluorescence lifetime and quantum yield determinations, and circular dichroism. These spectroscopic measurements, as well as Gaussian simulation of the data, show that chlorosomes containing BChl f are less efficient in energy transfer than those with BChl e. This can primarily be attributed to the decreased spectral overlap between the oligomeric BChl f (energy donor) fluorescence emission and the BChl a (energy acceptor) absorption in the chlorosome baseplate. This study allows us to hypothesize that, if they exist in nature, BChl f-containing organisms most likely live in rare high-light, anoxic conditions devoid of Chl a, d, or BChl e filtering.

K. Vogl, M. Tank, G.S. Orf, R.E. Blankenship, D.A. Bryant, Bacteriochlorophyll f: properties of chlorosomes containing the "forbidden chlorophyll," Front. Microbiol. 3 (2012) 298.