REARRANGEMENT OF LIGHT-HARVESTING BACTERIOCHLOROPHYLL HOMOLOGS AS A RESPONSE OF GREEN SULFUR BACTERIA TO LOW-LIGHT INTENSITIES

The pigment composition of two species of green-colored BChl c-contain ing green sulfur bacteria (Chlorobium limicola and C. chlorovibrioides ) and two species of brown-colored BChl e-containing ones (C. phaeobac teroides and C. phaeovibrioides) incubated at different light intensit ies have been studied. All species responded to the reduction of light intensity from 50 to 1 mu Einstein(E) m(-2) s(-1) by an increase in t he specific content of light harvesting pigments, bacteriochlorophylls and carotenoids. At critical light intensities (0.5 to 0.1 mu E m(-2) s(-1)) only brown-colored chlorobia were able to grow, though at low specific rates (0.002 days(-1) mg prot(-1)). High variations in the re lative content of farnesyl-bacteriochlorophyll homologues were found, in particular BChl e(1) and BChl e(4), which were tentatively identifi ed as [M, E] and [I, E] BChl(F) e, respectively. The former was almost completely lost upon reduction of light intensity from 50 to 0.1 mu E m(-2) s(-1), whereas the latter increased from 7.2 to 38.4% and from 13.6 to 42.0% in C. phaeobacteroides and C. phaeovibrioides, respectiv ely. This increase in the content of highly alkylated pigment molecule s inside the chlorosomes of brown species is interpreted as a physiolo gical mechanism to improve the efficiency of energy transfer towards t he reaction center. This study provides some clues for understanding t he physiological basis of the adaptation of brown species to extremely low light intensities.