Effect of carotenoid deficiency on cells and chlorosomes of Chlorobium phaeobacteroides

Abstract.

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 µg 2-hydroxybiphenyl ·ml^–1, carotenoid synthesis was largely inhibited (85%), but the photosynthetic growth rate was almost unaffected (µ _control=0.00525±0.00007 h^–1 and µ _HBP-treated=0.00505±0.0005 h^–1). Cells grown in the presence of the inhibitor were 5 µm–70 µm long, while control cells were between 2–5 µm long. Moreover, 2-hydroxybiphenyl-treated cells contained fewer, unevenly distributed chlorosomes per µm 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.