Light reactions in bacterial photosynthesis

Photosynthetic Bacteria contain bacteriochlorophylls a and b with absorption maxima of 775 and 790 nm respectively. These pigments are contained within sac-like extensions of the plasma membrane called chlorosomes in green sulfur and nonsulfur bacteria and intracytoplasmic vesicles in purple Bacteria. Bacterial photosynthesis is an anoxygenic photosynthesis that relies on photosystem I only and is termed a cyclic phosphorylation in Figure.

*Can grow as chemoorganotrophs aerobically in the dark.

                 Figure  2.  Photosystem I of green sulfur Bacteria. FeS, iron sulfur protein; Fd, ferredoxin.

There is no net change in the numbers of electrons in the system. ATP synthesis occurs during the generation of a protein motive force during photosynthesis that allows ATP synthase to synthesize ATP. The electrons expelled from the reaction center return to the bacteriochlorophyll via the electron transport chain. The photosynthetic apparatus consists of four membrane-bound pigment–protein complexes, plus an ATP synthase. For NADPH synthesis, Bacteria must use electron donors like hydrogen, H2S, sulfur and organic compounds with a more negative reduction potential than water. In this case direct transfer can occur via ferredoxin.

The purple sulfur Bacteria cannot synthesize NADPH directly by photosynthetic electron transport. This is because their acceptor molecules are more positive than the NADP+/ NADPH couple -0.32 volts. In this case electrons enter the cytochromes from the electron donors, and ATP from the light reactions is used to reduce NADP+ to NADPH by energy-dependent reverse electron flow in Figure 3.

               Figure  3.  Energy-dependent reverse electron flow. BChl, bacteriochlorophyll; BPh, Bacterial pheophytin.