Adaptation of intracytoplasmic membranes to altered light intensity in Rhodobacter sphaeroides

Abstract

The model photosynthetic bacterium Rhodobacter sphaeroides uses a network of bacteriochlorophyll (BChl)–protein complexes embedded in spherical intracytoplasmic membranes (ICM) to collect and utilise solar energy. We studied the effects of high- and low-light growth conditions, where BChl levels increased approximately four-fold from 1.6×106 to 6.5×106 molecules per cell. Most of this extra pigment is accommodated in the proliferating ICM system, which increases from approximately 274 to 1468 vesicles per cell. Thus, 16×106nm2 of specialised membrane surface area is made available for harvesting and utilising solar energy compared to 3×106nm2 under high-light conditions. Membrane mapping using atomic force microscopy revealed closely packed dimeric and monomeric reaction centre-light harvesting 1-PufX (RC-LH1-PufX) complexes in high-light ICM with room only for small clusters of LH2, whereas extensive LH2-only domains form during adaptation to low light, with the LH2/RC ratio increasing three-fold. The number of upper pigmented band (UPB) sites where membrane invagination is initiated hardly varied; 704 (5.8×105 BChls/cell) and 829 (4.9×105 BChls/cell) UPB sites per cell were estimated under high- and low-light conditions, respectively. Thus, the lower ICM content in high-light cells is a consequence of fewer ICM invaginations reaching maturity. Taking into account the relatively poor LH2-to-LH1 energy transfer in UPB membranes it is likely that high-light cells are relatively inefficient at energy trapping, but can grow well enough without the need to fully develop their photosynthetic membranes from the relatively inefficient UPB to highly efficient mature ICM.