Trapping and charge stabilization in chlorosome containing bacteria: Comparative study on Chloroflexus aurantiacus and Chlorobium limicola

Abstract

Trapping and charge stabilization in intact cells of Chloroflexus aurantiacus and Chlorobium limicola were studied by means of photovoltage kinetics and fluorescence emission spectra upon excitation in the chlorosome. In Chloroflexus two electrogenic phases with time constants of 100 ps for trapping (P+ H- formation) from the core-complexes and 530 ps for charge stabilization (Q(A) reduction) were found. In Chlorobium there were also two electrogenic phases of 105 ps and 650 ps. The later phase most likely represents the charge stabilization on the iron-sulfur center, F-X. A comparison of stationary fluorescence spectra of both species indicates that the trapping time from the core pigments in Chlorobium is significantly faster than in Chloroflexus. This proves the existence of at least two intermediary accepters between the primary donor (P 840) and F-X in the Chlorobium reaction center. In both species the photovoltage signal in the low excitation limit appeared after a lag phase of 55 to 65 ps, which is ascribed to the energy transfer from the chlorosome to the core pigments.