Inhibition by Proton Buffers of Photosynthetic Utilization of Bicarbonate in Chara corallina

Abstract

A simple model was developed to explain the mechanism by which buffers inhibit HCO*3¯ utilization in Chara corallina. The chief assumption was that an acidic compartment exists in the peiiplasmic space and that exogenous buffer species must cross the apoplast to dissipate this H+ gradient (¿pH). Assuming that HCO*3¯ utilization depends on H+ extrusion, buffer-induced H+ dissipation should result in a reduction in inorganic carbon (C*i) limited photosynthesis. The model predicts that buffers with a pKa midway between the bulk phase pH (pHo) and an assumed periplasmic pH will be most effective in dissipating the periplasmic ¿pH. Experiments conducted at pHo = 9.3 and with 5 mM concentrations of various buffers show that buffers in the range pKa 7.5-9.0 are most inhibitory to HCO*3¯ utilization. The initial slope of the photosynthetic response to C*i (i.e. where C*i availability is rate limiting) is more sensitive to buffers than rates at high C*i levels. Buffer inhibition was reversible. Experimental data correspond well with the model and indicate that HCO*3¯ utilization sites in the periplast are considerably more acidic than the bulk phase during photosynthetic HCO*3¯ utilization. Results suggest that during buffer inhibition experiments ¿pH is around 2 or more units when the bulk phase pH is 9.3.