Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater

Abstract

With an experimental system developed for aquatic plants using the mass spectrometry technique and infrared gas analysis of CO2, the responses to various O2 concentrations of gas exchanges were studied with the red macroalga C. crispus S. Irrespective of the CO2 concentration, net photosynthesis was O2 sensitive with a 45-70% stimulation at 2% O2. Even with high CO2, a significant Warburg effect was detected. Although photosynthesis was CO2 sensitive, O2 photoconsumption was only weakly affected by CO2 even at high CO2 where it was still photodependent. O2 photoconsumption was always sensitive to O2 concentration whatever the CO2 concentration, but with O2 exceeding 20% the kinetics disagreed with the Michaelis-Menten model, with saturation being reached more rapidly. With various CO2 concentrations, the apparent Km (O2) ranged from 4-16% O2 with a relatively constant Vmax (O2) of .apprx. 1/3 the Vmax (CO2). Dark respiration seemed to be O2 insensitive. These results are discussed in relation to the nature of the processes able to consume O2 in the light, and seem to be consistent with a significant involvement of a Mehler-type reaction.