Effect of carotenoid deficiency on cells and chlorosomes of Chlorobium phaeobacteroides

The effects of inhibition of carotenoid biosynthesis by 2-hydroxybiphenyl on the photosynthetic growth, pigment composition and chlorosome structure of Chlorobium phaeobacteroides strain CL1401 were examined. At a concentration of 20 mug 2-hydroxybiphenyl.ml(-1), carotenoid synthesis was largely inhibited (85%), but the photosynthetic growth rate was almost unaffected (mu (control)=0.00525+/-0.00007 h(-1) and mu (HBP-treated)=0.00505+/- 0.0005 h(-1)). Cells grown in the presence of the inhibitor were 5 mum-70 mum long, while control cells were between 2-5 mum long. Moreover, 2-hydroxybiphenyl-treated cells contained fewer, unevenly distributed chlorosomes per mum of cytoplasmic membrane with an irregular arrangement (2.5+/-1.5 vs of 9.1+/-1.9). This was concomitant to the 83% decrease in the content of bacteriochlorophyll (BChl) e in 2-hydroxybiphenyl-treated cells. Electron microscopy revealed that the shape of carotenoid-depleted chlorosomes changed from ellipsoidal to spherical, although the mean volume was similar to that of control chlorosomes. SDS-PAGE analysis of the chlorosome polypeptide composition showed that the amount of CsmA protein decreased by 60% in carotenoid-depleted chlorosomes. This was paralleled by a decrease in the baseplate BChl a content. The data suggest that carotenoids are close to the chlorosomal baseplate, where they carry out both structural and photoprotective functions.