Effects of O2, pH and DIC on photosynthetic net-O2 evolution by marine macroalgae

Abstract

Net photosynthetic O2 evolution by five marine macroalgae:Ulva lactuca L.,Enteromorpha sp.,Ceramium strictum Harvey,Fucus serratus L., andF. vesiculosus L., collected from Danish waters in the summer of 1983 was followed at increasing O2 and with pH either fixed close to pH 7, 8 or 9, or drifting upwards during photosynthesis in a closed chamber to determine the effects of changing O2, pH and DIC (dissolved inorganic carbon) on photosynthesis. Increasing O2, increasing pH and decreasing DIC together limited O2 evolution. Raising the O2 concentration with pH and DIC held constant resulted in less inhibition of net-O2 evolution than when all three factors acted together. The O2 inhibition of photosynthesis was similar to the reported O2 inhibition of ribulose 1,5-bisphosphate carboxylase isolated from lower and higher plants. Net-O2 evolution as a function of the molar ratio of O2 to HCO−3 + CO2 in solution provided a general, linear relationship (r2 = 0.72 to 0.84), predicting inhibition of photosynthesis based on O2 pH and DIC changing together. Slopes of this relationship, representing competition between O2 and carbon based on external concentrations, were similar for the five taxonomically different algae, suggesting that similar processes act to reduce net-O2 evolution.