Microamperometric measurements of photosynthetic activity in a single algal protoplast

The effects of p-benzoquinone (BQ) on photosynthetic and respiratory electr on transport in a single algal protoplast (radius, 100 mu m) was investigat ed quantitatively by amperometric measurements using microelectrodes. Under light irradiation (25 kLx) in the presence of 1.00 mM BQ, a single protopl ast consumed BQ by (2.9 +/- 0.2) x 10(-13) mol/s and generated p-hydroquino ne (QH(2)) by (2.7 +/- 0.3) x 10(-13) mol/s, suggesting that BQ was quantit atively reduced to QH(2) via the intracellular photosynthetic electron-tran sport chain. The generation of QH(2) increased with light intensity and wit h concentration of BQ added to the outside solution but became saturated wh en the tight intensity was above 15 kLx or the BQ concentration was higher than 0.75 mM. The addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, a ph otosynthetic electron-transport inhibitor, decreased the generation of QH(2 ) upon light irradiation, suggesting that BQ accepts electrons from a site in the photosynthetic electron-transport chain after the photosystem II sit e. The presence of 1.00 mM BQ increased the generation of photosynthetic ox ygen by similar to(2.6 +/- 1.0) x 10(-13) mol/s, which was similar to 1.5-2 times larger than that expected from the consumption of BQ. The electrons produced by the additional generation of oxygen is used to reduce intracell ular species as well as to reduce BQ.