Oxygen uncouples light absorption by the chlorosome antenna and photosynthetic electron transfer in the green sulfur bacterium Chlorobium tepidum

In photosynthetic green sulfur bacteria excitation energy is transferred fr om large bacteriochlorophyll (BChl) c chlorosome antennas via small BChl a antennas to the reaction centers which then transfer electrons from cytochr ome c to low-potential iron-sulfur proteins. Under oxidizing conditions a r eversible mechanism is activated in the chlorosomes which quenches excited BChl c. We used flash-induced cytochrome c oxidation to investigate the eff ect of this quenching on photosynthetic electron transfer in whole cells of Chlorobium tepidum. The extent of cytochrome c photooxidation under aerobi c conditions decreased to approx. 3% of that under anaerobic conditions whe n BChl c was excited under light-limiting conditions. Photooxidation obtain ed by excitation of BChl a was similar under aerobic and anaerobic conditio ns. We interpret this drastic decrease in energy transfer from BChl c to th e reaction center as a consequence of the quenching mechanism which is acti vated by O-2. This reversible uncoupling of the chlorosome antenna might pr event formation of toxic reactive oxygen species from photosynthetically pr oduced reductants under aerobic conditions. The green filamentous bacterium Chloroflexus aurantiacus also contains chlorosomes but energy transfer fro m the BChl c and BChl a antennas to the reaction center in this species was not affected by O-2. (C) 1999 Elsevier Science B.V. All rights reserved.