A study of dark luminescence in Chlorella. Background luminescence, 3-(3,4-dichlorophenyl)-1,1-dimethylurea-triggered luminescence and hydrogen peroxide chemiluminescence.

Dark luminescence, defined as the ability of completely relaxed (dark-adapted) photosynthetic systems to emit light, has been studied in Chlorella. Three main effects have been demonstrated. 3-(-3,4-Dichlorophenyl)-1,1-dimethylurea elicits a weak emission LD of very long lifetime (several minutes); it is believed to result from a negative shift of redox potential of the secondary System II electron acceptor B producing in some centers a state Q- (reduced primary acceptor), as postulated by Velthuys and Amesz ((1974 Biochim. Biophys. Acta 333, 85--94), which can recombine with an oxidizing equivalent in a state S2 present in very small amount. As in photoinduced luminescence, this recombination excites chlorophyll which then emits light. A much stronger emission LH is observed after injection of H2O2. Both signals are modified or suppressed by treatments specific of the oxygen emission system, such as: thermal denaturation at 50 degrees C, NH2OH, etc. In addition, a weak, permanent background luminescence L0 has been observed; like LD and and LH, it is a System II property and requires the integrity of the oxygen-evolving system. It is believed to reflect a very slow back flow of electrons from an endogeneous reductant pool to oxygen through part of the photosynthetic chain. Using flash preillumination, it is demonstrated that H2O2 is able to oxidize S0 into S2, the latter giving rise to LH; H2O2 does not act on S1 (or much less). The reactive site of H2O2 seems to be the same as the binding site of NH2OH. Evidence is given that the strong LH signal in particular reveals a stable, low pH of the intrathylakoid phase in Chlorella.