Modulation of fluorescence in Heliobacterium modesticaldum cells

In what appears to be a common theme for all phototrophs, heliobacteria exhibit complex modulations of fluorescence yield when illuminated with actinic light and probed on a time scale of μs to minutes. The fluorescence yield from cells of Heliobacterium modesticaldum remained nearly constant for the first 10-100 ms of illumination and then rose to a maximum level with one or two inflections over the course of many seconds. Fluorescence then declined to a steady-state value within about one minute. In this analysis, the origins of the fluorescence induction in whole cells of heliobacteria are investigated by treating cells with a combination of electron accepters, donors, and inhibitors of the photosynthetic electron transport, as well as varying the temperature. We conclude that fluorescence modulation in H. modesticaldum results from acceptor-side limitation in the reaction center (RC), possibly due to charge recombination between P₈₀₀⁺ and A₀^-.