Absorbance changes (Delta A) were studied in reaction centers (RCs) from the photosynthetic bacteria Rhodobacter sphaeroides and Chloroflexus aurantiacus under extremely low Light intensity irradiation. The dependence of Delta A as a function of the intensity of the actinic light was linear for Rb. sphaeroides wild type (Q(B)-free) and o-phenanthroline-inhibited Rb. sphaeroides RCs (strain R-26). Significant nonlinear Delta A changes as a function of the intensity of photoactivation were observed in the RCs from C. aurantiacus and for Q(B)-containing RCs from Rb. sphaeroides. The investigations of Delta A changes upon the history of the photoactivation revealed pronounced hysteresis loop-type dependencies for these RCs under a wide range of illumination intensities. It is proposed that the two branches of the hysteresis loop correspond to different electron-conformational (structural) states of the protein induced by the light variation, which substantially influences the rate constants of certain electron transfer reactions. These experimental data can be explained in terms of nonequilibrium self-organization effects arising due to self-consistent switching effects in the flexible cofactor-protein conformation and controlled by the actinic light. It is concluded that RCs exist in at least two conformational states, i.e. a light-adapted state and a dark-adapted state. These states differ strongly in the rates of the electron transfer to Q(B) and the subsequent recombination rate.