PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Bicarbonate stabilizes Photosystem II steady-state electron transport to artificial acceptors in Spirulina maxima intact filaments. Javier Fernandez-Velasco^*,1, Joel Freeman¹, Amanda Harms¹, G. Charles Dismukes², Tom Wydrzynski¹, ¹ Research School of Biological Sciences, Canberra, ACT, Australia²

ABSTRACT- The alkalinophile filamentous cyanobacterium Spirulina maxima is a useful organism to study specific interactions of bicarbonate with PSII. We explored the role of bicarbonate in the PSII steady-state photosynthetic electron transport to the artificial electron acceptor pair p-phenyl-benzoquinone/ferricyanide at pH 8, under conditions where the native electron transport to PSI is >90% blocked by DBMIB. In washed intact filaments, with no added bicarbonate, a strong inhibition (75-95%) develops during 10 min of reaction. The presence of bicarbonate (5-20 mM) at time zero stabilizes the reaction, attenuating the inhibition to 40-50%. Similarly, the addition of bicarbonate to the inhibited system partially reverses the inhibition, after a 1 min reactivation phase. The inhibition in the absence of bicarbonate increases with light intensity (400-4000 uE.m^-2.s^-1), with bicarbonate protecting at all intensities. The uncoupler and ADRY reagent FCCP (50-350 M) accelerates the inhibition observed in the absence of bicarbonate. Again, the presence of bicarbonate partially protects against the FCCP-induced inhibition, and addition of bicarbonate after the inhibition is attained partially reverses it. Experiments at various concentrations of FCCP and bicarbonate show that these ligands behave as competitors: FCCP as an inhibitor (Kd 40 M) and bicarbonate as a de-inhibitor (Kd 1-2 mM). Because of the high concentrations needed, FCCP would be acting as an ADRY reagent, inhibiting electron transport by de-stabilizing the higher S states. One hypothesis is that bicarbonate has a native role at the donor site of PSII, in which it protects the stability of the S states, with bicarbonate and FCCP having competing effects. In contrast, although the PSII activity of spinach thylakoids and intact cells of Gloeobacter violaceous are inhibited by FCCP, bicarbonate gives no protection. Thus, this role of bicarbonate is not universal, but may reflect an evolutionary or ecological trait specific to S. maxima.

KEY WORDS: bicarbonate, photosystem II, adry reagent, s-states