Millisecond delayed light emission from intact cells of photosynthetic bacteria: proof of radiative charge recombination of the primary stable charge pair P+QA‒

The millisecond delayed fluorescence (DL) of intact purple photosynthetic bacteria was measured following rectangular light excitation of variable duration to assess the role of electron donors to the oxidized reaction-center dimer (P⁺). Strains possessing complete ( Rubrivivax gelatinosus , Blastochloris viridis ), modified ( pufC ), or lacking ( cycA ) cytochrome subunits were examined. Several key findings were obtained: (1) all strains exhibited similar dominant DL decay components (1–10 ms) but markedly different amplitudes—bacteria capable of multiple turnovers showed DL intensities roughly two orders of magnitude higher than cycA (enhancement effect); (2) in cycA , DL decayed much faster than the charge-pair recombination P⁺Q _A ⁻ → PQ _A (≈ 60 ms); (3) DL saturation was delayed relative to prompt fluorescence (PF) by more than an order of magnitude in rise time; (4) the lag phase was longer and the rise was steeper in Rvx. gelatinosus than in cycA ; and (5) both the redox agent ferrocene and the protonophore FCCP functioned as potent quenchers of DL. The experimental results were reproduced by a minimal kinetic model assuming that (i) the precursor of millisecond DL is a small subfraction of P⁺Q _A ⁻ states undergoing radiative recombination, and (ii) the branching ratio of radiative to non-radiative recombination pathways is approximately 2 × 10⁻⁶, as derived from the DL-to-PF intensity ratio in cycA . The origin, mechanisms, and physiological significance of DL in different bacterial species are discussed.