Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater: Effects of Light and CO(2) Concentration.

With an experimental system using mass spectrometry techniques and infra-red gas analysis of CO(2) developed for aquatic plants, we studied the responses to various light intensities and CO(2) concentrations of photosynthesis and O(2) uptake of the red macroalga Chondrus crispus S. The CO(2) exchange resistance at air-water interface which could limit the photosynthesis was experimentally measured. It allowed the calculation of the free dissolved CO(2) concentration. The response to light showed a small O(2) uptake (37% of net photosynthesis in standard conditions) compared to C(3) plants; it was always higher than dark respiration and probably included a photoindependent part. The response to CO(2) showed: (a) an O(2) uptake relatively insensitive to CO(2) concentration and not completely inhibited with high CO(2), (b) a general inhibition of gas exchanges below 130 microliters CO(2) per liter (gas phase), (c) an absence of an inverse relationship between O(2) and CO(2) uptakes, and (d) a low apparent K(m) of photosynthesis for free CO(2) (1 micromolar). These results suggest that O(2) uptake in the light is the sum of different oxidation processes such as the glycolate pathway, the Mehler reaction, and mitochondrial respiration. The high affinity for CO(2) is discussed in relation to the use of HCO(3) (-) and/or the internal CO(2) accumulation.