Bicarbonate stimulates the electron donation from Mn²⁺ to P 680 + in isolated D1/D2/cytochrome b559 complex

Influence of bicarbonate on the efficiency of the electron donation from Mn²⁺ to P680+ in isolated D1/D2/cytochrome b559 complex was investigated. All the experiments were carried out in a medium depleted of HCO3-/CO₂. Kinetics of photoinduced absorbance changes (ΔA) at different wavelengths and decrease of chlorophyll fluorescence yield (-ΔF) related to photoaccumulation of reduced pheophytin, the intermediary electron acceptor of photosystem II (PSII), in the presence of Mn²⁺ under anaerobic conditions were measured. Addition of bicarbonate (1 mM) increased the amplitude of these ΔA and -ΔF at least by a factor of 3. Measurements of the photoinduced ΔA, related to photooxidation of the primary electron donor of PSII, chlorophyll P₆₈₀, were done in the presence of silicomolybdate as electron acceptor. These results show that the addition of 0.05 mM Mn²⁺ alone or jointly with 1 mM bicarbonate induces a 20% and 70%-decrease of the magnitude of the ΔA at 680 nm. The effect of Mn²⁺ (in the presence and absence of bicarbonate) was completely eliminated by the addition of 12 mM EDTA. All these bicarbonate effects were not observed if MgCl₂ or formate were used instead of MnCl₂ and bicarbonate, respectively. In the absence of Mn ²⁺, bicarbonate induced none of the mentioned above effects (increase of photoaccumulation of reduced pheophytin and decrease of photooxidation of P₆₈₀). The presented data suggest that bicarbonate stimulates the electron donation from Mn²⁺ to D1/D2/cyt b559 reaction center evidently due to formation of easily oxidizable Mn-bicarbonate complexes.