Effect of hydrogen ion buffers on photosynthetic oxygen evolution in the blue-green alga, Agmenellum quadruplicatum.

The photosynthetic oxygen evolution capacity of Agmenelium quadruplication suspended in four hydrogen ion buffers (pH 7.4, 0.05 M) and its synthetic marine growth medium was measured with an oxygen electrode. High rates of oxygen evolution were obtained in the growth medium and N-tris(hydroxymethyl)-methylglycine (Tricine) buffer. Compared to oxygen evolution in the growth medium, rates in phosphate buffer and N-tris(hydroxymethyl)-2-aminoethanesulphonic acid (TES) buffer were sometimes reduced by up to 30% and rates in tris (hydroxymethyl) amino-methane (Tris) were consistently reduced by 50%. An incubation-rinsing procedure caused inhibition of oxygen evolution in TES, phosphate, and Tris by 50 to 100%. Oxygen evolution could be restored to cells rinsed in TES or phosphate by resuspension in growth medium or in buffer plus magnesium and calcium ions. Bezoquinone-supported oxygen evolution was not affected by rinsing with any buffer tested except Tris. Ferricyanide was photoreduced at a low rate by cells rinsed in Tes but at a high rate in TES plus magnesium and calcium ions. We interpreted our results to mean that, in Agmenellum quadruplicatum, inhibition of photosynthetic oxygen evolution by Tris occurs at the level of photosystem 2 while the effects of TES and phosphate are on electron-transport occurring after the rate-limiting reaction.